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03 Perimeter Explorations Beyond the Atom: Remodelling Particle Physics T E A C H E R ’ S G U I D E Contents 3 Opening Information Introduction to Particle Physics Curriculum Links Suggested Ways to Use this Resource Teacher Tips 7 Particle Physics in a Nutshell A one-page summary of the video’s contents 8 Student Activities These activities can be used individually or in combination with others. The material can be adapted to a variety of classroom levels and courses. Student activity sheets are included on the DVD-ROM in an editable electronic format so you can adapt them to your classroom. 8 Activity 1: Video Summary A set of discussion questions that review the content of the video. 10 Activity 2: Scattering Experiment Students use balloons, marbles, and a wine glass to explore Rutherford’s scattering experiment. 12 Activity 3: Bubble Chamber Detective Students use conservation of charge and momentum to analyze two historic images from CERN and Brookhaven. Requires a prior knowledge of moving charges in a magnetic field. 16 Activity 4: Taming the Particle Zoo Students sort particle zoo cards and use the patterns they find to explore an early quark model. 18 Activity 5: Finding the Top Quark Students use conservation of momentum and energy to analyze data from Fermilab’s D-Zero detector. 21 Video Chapter Summaries This section provides additional background information on the content of the video, as well as further topics to extend the video. Questions are anticipated and clear, concise, and accurate information is provided. 32 Activity Solutions 37 Closing Information Who are the people in the video? Appendix A: Particle Zoo Cards Appendix B: Particle Physics Equations and Constants Credits 1 Beyond the Atom: Remodelling Particle Physics Perimeter Institute PERIMETER EXPLORATIONS PERIMETER INSTITUTE This series of in-class educational resources is designed to help teachers explain a range of important topics in physics. Perimeter Explorations is the product of extensive collaboration between international researchers, Perimeter Institute’s outreach staff and science educators. Each module has been designed with both, expert and less experienced teachers in mind, and thoroughly classroom tested. Perimeter Institute for Theoretical Physics is an independent, non-profit, research institute whose mission is to make breakthroughs in our understanding of our universe and the forces that govern it. Such breakthroughs drive advances across the sciences and the development of transformative new technologies. Located in Waterloo, Ontario, Canada, Perimeter also provides a wide array of research training and educational outreach activities to nurture scientific talent and share the importance of discovery and innovation with students, teachers, and the general public. In partnership with the Governments of Canada and Ontario, Perimeter is a successful example of public-private collaboration in scientific research, training, and outreach. 2 Beyond the Atom: Remodelling Particle Physics Introduction What is everything made of? Humans have asked this question for thousands of years and are still actively searching for the answer today. Ernest Rutherford began a new chapter in this pursuit over a century ago when he fired alpha particles at gold foil, uncovering the atomic nucleus. The Large Hadron Collider (LHC) at CERN is the latest effort to probe deeper into nature. The LHC smashes protons together at the unprecedented energy levels of 14 TeV (1 TeV is the amount of energy an electron gains when accelerated through a potential difference of 1 000 000 000 000 V). Our current model of matter is the Standard Model and while it is extremely good at describing how matter behaves at low energies it is expected to fail at energies higher than a few TeV. The LHC has been designed to push the limits of the Standard Model—discoveries are going to be made. The Standard Model states that all matter is made of fundamental particles called quarks and leptons. There are six quarks and six leptons arranged in three families, or generations. In reality, the answer to the question “What is everything made of?” is found in the first generation. The atoms and molecules of our everyday world are all made of up quarks, down quarks and electrons. The other generations are needed to explain the structure of particles created in high-energy physics labs, cosmic rays or various exotic astronomical objects. The Standard Model also states that matter interacts through bosons—force-mediating particles exchanged between similar particles of matter. For example, when two charged particles interact, they will exchange photons, which will either cause the charged particles to be attracted or repelled. There are bosons for each of the atomic forces: the photon for the electromagnetic force, the gluon for the strong nuclear force, and the W and Z bosons for the weak nuclear force. Each of these bosons has been observed and the theories describing these forces are very well developed. Any new model will have to surpass one of the strongest theories ever constructed. mechanism, the universe is saturated with a field called the Higgs field that interacts with some particles more than others. Any attempt to accelerate these particles is met by a resistance called inertia. The LHC is testing for the existence of this field by putting so much energy into a small space that the Higgs field will produce a Higgs boson. If the Higgs boson exists, it will be created during collisions in the LHC. The challenge for researchers will be to detect the Higgs boson if it is created. One of the basic concepts in particle physics is that if there is enough energy to create a set of particles (via E = mc2), then the particles will be created (within the constraints of other parameters). At the energy level of the LHC, every particle in the Standard Model will be created in the collisions. The number of Higgs bosons produced is expected to be very small, however, so the LHC has been designed to produce over 600 million collisions per second. Even with this enormous collision rate, the production rate of Higgs events is expected to be one every few hours—that is a lot of data to sift through to find a single event! After two and a half years of operation the LHC announced the discovery of a new boson consistent with the Higgs model. Further analysis will reveal whether this boson is the Higgs or something else. Particle physics and the LHC may represent the epitome of advanced physics but many of the fundamental concepts involved are accessible to a high school physics audience. In this Perimeter Institute Exploration, students review how electric and magnetic fields, conservation of momentum, and special relativity led to the development of the Standard Model. The video features state-of-the-art animations, researcher interviews, and narration to introduce students to the Higgs mechanism and other concepts being explored at the LHC. Through the video and accompanying activities students will see that the physics they are studying in high school is is at the centre of the largest particle physics experiment ever. You may notice the absence of gravity from the previous list of forces. The Standard Model does not include gravity, although some theorists have proposed the existence of a graviton as a mediating particle. Another feature missing from earlier versions of the Standard Model is an explanation for mass—not the mass of atoms (that is largely explained using E = mc2), but the mass of fundamental particles. In the Standard Model, fundamental particles would be massless —if not for the Higgs mechanism. According to the Higgs Section of the LHC's 27km long tunnel 3 Beyond the Atom: Remodelling Particle Physics Curriculum Links Topic Connection to Particle Physics Relevant Materials Nature of Science Scientists use models to explain complex phenomena. These models are often based on patterns and make testable predictions. Experiments are designed to test models. Activity 2: Scattering Experiment Activity 4: Taming the Particle Zoo Video: Chapter 1, 3 and 5 Models of Matter Experiments have refined our models of matter from Dalton to Thomson to Rutherford. Increasing energies have allowed scientists to probe deeper into matter, leading to the Standard Model. Activity 1: Video Summary Activity 2: Scattering Experiment Activity 4: Taming the Particle Zoo Video: Chapter 1, 3 and 4 Rutherford Scattering Rutherford used alpha particles to probe the structure of matter. He concluded that the positive charge and almost all the mass were concentrated in a very small volume. Activity 1: Video Summary Activity 2: Scattering Experiment Video: Chapter 1 Fields Forces act at a distance through fields. Electromagnetic fields are used to accelerate and guide beams of charged particles. Magnetic fields are used in particle detectors to identify the charges and momenta of particles. Activity 3: Bubble Chamber Detective Activity 5: Finding the Top Quark Video: Chapter 1 and 2 Standard Model The Standard Model of matter uses six quarks, six leptons, and four bosons to give a complete description of matter and forces. The Large Hadron Collider is testing the Standard Model. Activity 1: Video Summary Activity 4: Taming the Particle Zoo Activity 5: Finding the Top Quark Video: Chapter 3 and 4 Energy-Mass Equivalence Particle accelerators put enough energy into a small volume that matter is produced, according to E = mc2. Activity 1: Video Summary Activity 5: Finding the Top Quark Video: Chapter 2, 3 and 4 Conservation Laws Particle collisions obey the laws of conservation of energy, of momentum, and of charge. Activity 3: Bubble Chamber Detective Activity 5: Finding the Top Quark Video: Chapter 2, 3 and 4 4 Beyond the Atom: Remodelling Particle Physics Suggested Ways to Use this Resource This resource has been developed by a team of experienced educators in collaboration with Perimeter Institute researchers and outreach staff. The resource consists of a 30-minute video, five classroom activities and background material. The activities can be adapted to a variety of classroom levels and courses. The activity sheets are available in editable form on the DVDROM. Intermediate Level Science (1+ Lesson) Most intermediate Science courses include models of the atom and the Periodic Table. This resource will supplement these lessons and show students how the process of science that led to the Periodic Table is still ongoing. Activity 2: Scattering Experiment (30 minutes) introduces students to Rutherford’s gold foil experiment. Video (30 minutes) can be paused at several points to encourage interactive discussions. Activity 1: Video Summary (15 minutes) can be used as a discussion tool or as a homework assignment. Activity 4: Taming the Particle Zoo can be used as an extension activity when studying the Periodic Table. Senior Level Physics (2 Lessons) This resource can be used in several places in the senior Physics curriculum. Particle physics is a fantastic application of conservation of energy and momentum, electric and magnetic fields, and special relativity. The activities allow students to gain a deeper appreciation for the Standard Model as well as experiencing the process of science involved in developing models. First Class: Activity 4: Taming the Particle Zoo (30 minutes) uses patterns to predict the omega-minus particle. This activity can be adapted to use only spin-3/2 cards to predict the omegaminus particle. Activity 3: Bubble Chamber Detective (45 minutes) guides students through the analysis of two bubble chamber images, including the image that resulted in the discovery of the omega-minus particle. Second Class: Video (30 minutes) can be paused at several points to encourage interactive discussions. Activity 5: Finding the Top Quark (45 minutes) shows how conservation laws and energy-mass equivalence are used to analyze data from a particle detector. Teacher Tips The 30-minute video has been divided into scenes based on curriculum topics. The video can be viewed in one continuous showing or in parts, according to your preferences. Activity 2: Scattering Experiment This activity is appropriate for an intermediate level class that is studying the model of the atom or for a senior level class that is studying fields. Students will explore Rutherford’s scatThe following suggestions are the product of many workshops tering experiment using several approaches: physical models, role playing, and scale diagrams. with experienced teachers and their students. We are confident that these activities will challenge and inspire your students to discover the amazing world of particle physics. Each In the first activity, students need to put as large a charge as activity can be used individually or in combination with others. possible on the balloons. The charge on the suspended balloon must cover the entire surface or the balloon will simply rotate instead of swing. A longer thread will produce a bigger Activity 1: Video Summary This question sheet has been designed to encourage student deflection. dialogue. Students can work independently, in pairs, or participate in a larger group discussion. 5 Beyond the Atom: Remodelling Particle Physics Teacher Tips continued In the second activity, a little bit of modeling clay is needed to create a smooth lip for the ball to roll up. The launching ramp should be kept at least 15 cm from the glass in order to make it challenging. An effective track can be fashioned from shelving support cut into 25 cm lengths. Encourage students to test a variety of scenarios (e.g., what happens when you miss the glass altogether?). These two activities can be supplemented with a computer simulation such as: http://phet.colorado.edu/en/simulation/ rutherford-scattering Activity 3: Bubble Chamber Detective The photograph in Figure 1, a tutorial, and more examples can be found at the CERN High School Teacher program website: http://teachers.web.cern.ch/teachers/archiv/HST2005/ bubble_chambers/BCwebsite/index.html. This activity gives students who are familiar with conservation laws and charged particles moving in a magnetic field the chance to apply that knowledge. Conservation of charge is used to identify whether an event is a collision or a decay. Conservation of momentum is used to infer the trajectory of neutral, and therefore unseen particles. These unseen particules are vital to the discovery of the omega-minus particle in Part 2. Activity 4: Taming the Particle Zoo Remove the eta-prime (η′), omega-minus (Ω–), and Nobel Prize cards before giving the cards to the students. An intermediate Science class can do Part 1 as a modelling activity. Allow enough time for the students to attempt several sorting methods before steering them in the right direction (group the particles by spin). Once the particles are separated by spin, they can be organized by charge and strangeness to produce patterns. Part 1 can be simplified by only using the spin-3/2 cards to predict the existence of the omega-minus particle. The first group to use the pattern to give a correct description of the omega-minus particle can be awarded the Nobel Prize. Students in a senior class can go on to complete Part 2, which uses the quark model to explain the pattern in the particles. Students are encouraged to look for problems raised by the quark model and to suggest solutions. Note: The solution that works for the mesons does not work for the baryons. The explanation for this involves a superposition of states and is probably too advanced for a high school class. 6 Activity 5: Finding the Top Quark Students in a senior level class will use conservation of momentum and energy to determine the mass of the top quark. Role playing helps students picture this three-dimensional collision more clearly and reinforces that it does not involve large objects breaking apart into smaller pieces. Starting with a Think-Pair-Share activity before moving on to the enactment will help students conceptualize the collision. The numerical analysis starts with inferring the momentum of the undetected neutrino by finding the total momentum of the detected particles and asserting that total momentum should be zero. Once the momentum of the neutrino is determined, the total energy of the collision can be calculated by adding the magnitudes of the momentum vectors together (do not forget the neutrino). The total energy comes from the mass of the top-antitop pair according to E = mc2. The Higgs simulation gives students the chance to get a feel for the statistical nature of the data analysis used in modern detectors. Students will find that a cutoff of around 50 GeV seems to give good results. The simulation will not let you use Auto events until you have done three by hand. If you want students to get a better feel for the experiment, do not mention Auto events. Scientists wait until they accumulate enough data to generate a significance of 5 before announcing a discovery. Even with the Auto function on, students will find that it takes a long time to achieve a significant result. Beyond the Atom: Remodelling Particle Physics Particle Physics in a Nutshell • All matter is made out of atoms. Atoms are organized into the Periodic Table. • More exploration leads to more particles, requiring more quarks. The current model has six quarks (and six antiquarks), six leptons (and six antileptons), and four bosons. • Rutherford scattering reveals the internal structure of atoms. • Charged particles interact through fields. Fields convert the kinetic energy of moving particles into potential energy. In particle collisions, more kinetic energy means a closer approach, which means more detail. • Focusing a lot of energy into a small volume can create new particles (E = mc2). When matter is created out of energy, we also create antimatter. • Technological improvements lead to increases in energy and rapid discovery of many new particles. The lack of an organizing structure leads to the “particle zoo”. • Patterns found in the particle zoo points to deeper structure— quarks. Quarks are fundamental particles that have fractional charge (e.g., ⅓ or ⅔) and new characteristics such as strangeness and colour. • The quark model introduces a new force to hold quarks together—the strong force. The strong force acts on colour charge and gets stronger as distance increases. ATLAS simulated Higgs event Bubble chamber image • The mass of the proton is mostly due to the binding energy of the quarks that make it up, not to the mass of the quarks. Why do quarks even have mass? This question leads to the Higgs field. • Particles interact with the Higgs field, which impedes acceleration, giving inertia (mass). Massless particles like the photon do not couple to the Higgs field. • Every field has a boson associated with it. The Higgs boson is like a ripple in the Higgs field. The Large Hadron Collider (LHC) will operate at high enough energy levels to generate Higgs bosons. • The LHC will produce over 600 million collisions per second. Researchers expect to produce one Higgs event every three hours (talk about a needle in a haystack!). • Four large detectors at the LHC will study the collisions for many different things: the Higgs boson, dark matter, quark-gluon plasma, matter-antimatter asymmetry, and supersymmetry. The Compact Muon Solenoid (CMS) detector at CERN 7 Beyond the Atom: Remodelling Particle Physics Activity 1 Video Summary NAME : 1. The model of the atom has changed over the past 150 years as new evidence has been found. Draw labelled diagrams for a helium atom using each of these models. Dalton’s model of indivisible balls of matter Thomson’s model using electrons in a positive mass Rutherford’s model using electrons and nuclei 2. Rutherford fired alpha particles at gold foil and was surprised by the results. He developed the nuclear model of the atom because (a) most of the alpha particles went through (b) most of the alpha particles bounced back (c) a few of the alpha particles went through (d) a few of the alpha particles bounced back Explain your choice and make a labelled diagram of Rutherford’s experiment. 3. The Large Hadron Collider (LHC) accelerates particles to unprecedented energy levels. Higher-energy particles are used at the LHC because they can (a) get closer to each other and probe smaller distances (b) create massive particles from the energy (c) both of the above Explain your choice. 4. The most famous physics equation is Einstein’s E = mc2. (a) Explain what each letter in the equation represents, and what the equation means. (b) Draw a picture of what happens when an electron and a positron are created in a bubble chamber. Where does the mass of the electron and positron come from? 5. Science was much simpler 80 years ago. Everything appeared to be made of just electron, protons, and neutrons. Then physicists probed deeper into matter and detected new particles. (a) Why was the discovery of new particles a problem? (b) How was it solved? 8 | Activity 1 Beyond the Atom: Remodelling Particle Physics 6. The electromagnetic force pulls opposite charges together and pushes like charges apart. The strong force is different from the electromagnetic force in that the strong force is only attractive and it has (a) three types of charge and gets weaker with distance (b) three types of charge and gets stronger with distance (c) two types of charge and gets weaker with distance (d) two types of charge and gets stronger with distance 7. The Standard Model describes what is needed to make matter and the forces that hold it together. (a) Fill in the table for the Standard Model. 1st Generation 2nd Generation 3rd Generation Bosons Quarks Leptons (b) What is special about the first column? (c) How are the next two columns different from the first? (d) How is the last column different from the other three? (e) Compare the periodic table of chemistry and the Standard Model of physics. 8. What particle in the Standard Model is the LHC designed to find? Why is it important? 9. What else might the LHC find? Make a list of all the possibilities mentioned. Activity 1 | 9 Beyond the Atom: Remodelling Particle Physics Activity 2 Scattering Experiment NAME : Over one hundred years ago, J.J. Thomson showed that all atoms contained smaller negatively charged particles called electrons. This implied that most of the mass and all of the positive charge in the atom had to be made of something else. Physicist Ernest Rutherford explored the distribution of positive charge within the atom by directing alpha particles at a thin gold foil. Most of them went through, but about 1 in every 8000 was repelled backwards. 1. The positively charged alpha particles were repelled by the positive charge of the gold atoms. We can model this repulsion with two charged balloons. Hang one balloon from a thread. Use the other balloon to see how far away from vertical you can repel the first balloon. The balloons cannot touch. Predict and Explain: What should you do to get the greatest deflection? Observe and Explain: What conditions give the greatest deflection? Extend and Explain: How is the balloon activity similar to Rutherford’s experiment? How is it different? How could you change the activity to make it more similar? 2. In this activity, you will use a ball, a wine glass and a metal track to model how alpha particles are scattered by the gold foil. The ball represents the alpha particles and the wine glass stem represents the nucleus of the gold atom. The track is used to launch the ball at the glass (see figure at right). Spread a bit of modeling clay around the edge of the base of the glass to make a smooth ramp for the ball to roll up. Position the track 15 cm away from the glass. Place the ball at the top of the track and let go. Predict and Explain: What should you do to get the ball to roll up onto the base and roll straight back down, without bouncing off the stem? 10 | Activity 2 15 cm Beyond the Atom: Remodelling Particle Physics Observe and Explain: What happens to the ball if it is not aimed directly at the stem? Extend and Explain: How is the wine glass activity similar to Rutherford’s experiment? How is it different? How could you change the activity to make it more similar? 3. The alpha particles in Rutherford’s experiment had similar speeds and trajectories and yet they were scattered by different amounts. Most travelled straight through the gold without any deflection, some slowed down and were slightly deflected and a few stopped and were repelled back. Get into groups of four or five. Plan and perform a skit explaining these results. You may use props and sound but no words. What are the key ideas that you need to communicate? 4. Rutherford calculated that the alpha particles got to within 2.7 x 10-14 m of the centre of the gold nucleus. Later experiments have shown that the radius of the gold nucleus is 0.75 x 10-14 m. (a) Let a centimetre represent 10-14 m. Draw a scale model of the nucleus and the path of an alpha particle that makes a head-on approach and gets as close as possible. (b) Alpha particles that approach the nucleus off to one side are deflected as they go by. Add the path of one of these particles to your scale model. Will it get as close to the nucleus as one that approaches head-on? (c) The gold atoms are separated by 10-10 m. Where would the next gold nucleus be in your diagram? Activity 2 | 11 Beyond the Atom: Remodelling Particle Physics Activity 3 Bubble Chamber Detective NAME : Physicists discovered dozens of different ‘elementary’ particles using bubble chambers. Bubble chambers are large vessels of super heated liquids (usually hydrogen) in a uniform magnetic field. Identical charged particles are injected into the chamber where they collide inelastically with protons in the liquid to form new particles which may or may not decay. The following principles will allow you to analyze the events photographed in a bubble chamber: Charge is always conserved. • Only moving charged particles leave a trail. Neutral and stationary particles do not leave trails. • The charged particles have a charge of either +1e or -1e, where e = 1.6 x 10-19 C • Charge is determined by the direction the particle curves in a magnetic field. Momentum is always conserved. • The magnetic field bends the path of charged particles. • The radius of a curved trail is proportional to particle’s momentum. Changes in the trails are evidence of an interaction • A particle can collide with the protons in the liquid hydrogen to form new particles, or • A particle can decay into new particles. The radius of a curved trail is proportional to momentum Part 1: CERN’s Two Metre Hydrogen Bubble Chamber Figure 1 is a photograph showing seven kaons entering a bubble chamber filled with liquid hydrogen. Kaons are unstable subatomic particles that can be produced in large quantities, making them useful in bubble chamber experiments.There is a uniform magnetic field directed into the page. Answer each question and provide a brief written justification for your answer. 1. The kaon trails are curving slightly to the right. What is the charge of the kaons? (a) –1 (b) +1 (c) 0 (d) not enough information 2. At point P the single trail of a kaon branches into two trails. What is the charge of the particle on the right? (a) –1 (b) +1 (c) 0 (d) not enough information 3. Compare the total charge going into point P with the total charge going out. The single charged kaon (a) has decayed into two oppositely charged particles. (b) has decayed into two identically charged particles. (c) has interacted with a proton and produced two oppositely charged particles. (d) has interacted with a proton and produced two identically charged particles. 4. Compare the tracks going into point P with the tracks going out. What can you infer? (a) A charged particle has been produced that moves up and to the left. (b) A charged particle has been produced that moves up and to the right. (c) A neutral particle has been produced that moves up and to the left. (d) A neutral particle has been produced that moves up and to the right. 5. There is a kink in the track at point T. The particle making the track has (a) interacted with a neutral particle. (b) interacted with a positively charged particle. (c) decayed into a positively charged particle and a neutral particle. (d) decayed into a negatively charged particle and a neutral particle. 12 | Activity 3 Beyond the Atom: Remodelling Particle Physics 6. There is a very slight kink in the track at point R where the curvature increases. The particle making the track has a) interacted with a neutral particle. b) interacted with a positively charged particle. c) decayed into a positively charged particle and a neutral particle. d) decayed into a negatively charged particle and a neutral particle. 7. The two charged particles that appear at point S pass by each other at point U. Draw a straight line from this intersection to point S, where the particles were created from a neutral particle. This line gives the direction of the original neutral particle. Extend the line back down the page. Where did the neutral particle originate? a) at point P b) at point Q c) at point R d) not enough information 8. Two new particles appear at point Q. What can we infer about the event happening at point Q? a) A stationary charged particle has decayed into two new particles. b) A stationary neutral particle has decayed into two new particles. c) A charged particle moving up and to the left has decayed into two new particles. d) A neutral particle moving up and to the left has decayed into two new particles. 9. Extend the curved trails from point Q to find their intersection point by tracing the existing curve onto another piece of paper and using this to extend the path. Where did the neutral particle that decayed to form these particles originate? a) at point P b) at point Q c) at point R d) not enough information 10. Put it all together and give a complete description of the events that happen in this photograph. Part 2: Brookhaven National Laboratory’s Bubble Chamber Figure 2 is an historic photograph. It provided the first evidence for the omega-minus particle which had been predicted two years earlier by Murray Gell-Mann. Negative kaons enter at the bottom. A uniform magnetic field is directed into the page. The essential trails have been darkened. 1. Identify the charges of the particles interacting at point V. 2. Identify the charges of the particles interacting at point W. 3. Two oppositely charged particles have been created from a neutral particle at point Y. The curves have been extended until they intersect. Draw a line to show where the neutral particle originated. 4. Oppositely charged particles have been produced from neutral particles at points X and Z. Find the origin of these neutral particles by extending the line that bisects the ‘vee’ formed by the charged particles. These lines should intersect with the line you drew in question 3 at a single point. What can you infer from this? Refer back to Question 2 in your explanation. 5. Reconstruct the interactions by labeling the visible and invisible particle trails on Figure 2. Begin at point V where a kaon (K–) – collides with a proton to produce the omega-minus (Ω ), a neutral kaon (K0) and a positive kaon (K+). The omega decays 0 at point W into a neutral xion (Ξ ) and a negative pion (π–). The xion then decays into a neutral lambda (Λ0) and a neutral pion (π0). The neutral pion decayed almost immediately into two photons (γ) which decay at points X and V into electronpositron (e–, e+) pairs. The lambda decays at point Y into a proton and a negative pion. 6. Dusan Radojicic was the researcher who first analyzed this photograph in 1964. Imagine that it is three in the morning when he realizes that he has evidence for the omega-minus particle predicted by Gell-Mann in 1962. He wants to phone his boss with the news right away but his analysis is based on four invisible trails. What would you do? How would you explain your analysis over the phone? Activity 3 | 13 Beyond the Atom: Remodelling Particle Physics U T S R Q P Figure 1 Photograph of CERN bubble chamber. Arrows indicate kaons moving up the page. There is a constant magnetic field into the page. 14 | Activity 3 Beyond the Atom: Remodelling Particle Physics Z Y X W V Figure 2 Photograph from Brookhaven National Laboratory. Negative kaons enter from the bottom of the image. There is a constant magnetic field directed into the page. Activity 3 | 15 Beyond the Atom: Remodelling Particle Physics Activity 4 Taming the Particle Zoo NAME : We have come a long way from Dalton and the indivisible atom. First, Thomson found the electron, Rutherford discovered the proton, and in 1932 Chadwick found the neutron. Then, the list expanded over the next 30 years to include over 90 different particles. Particle physics in the 50s and 60s was much like chemistry in the 1880s: a tremendous amount of data but no widely accepted theory to provide an organizing structure. In this activity we will examine some of these particles, identify a pattern, and explore a theory that will help us tame the particle zoo, just as Mendeleev did for the elements when he built the first Periodic Table. Part 1: Finding Patterns 1. Take a deck of particle cards and inspect the information on the cards. [Note: S is a new property called “strangeness”.] 2. Sort the particles into three distinct groups based on information on the cards. Which characteristic is the best choice for this? Why? 3. Take one of the three groups. Organize its particles into rows and columns based on two of the other characteristics. Repeat for the other two groups. 4. Two of the groups should have eight members and look similar. The third group should look different and have nine members. Describe the geometric patterns that emerge from your arrangement of the cards. Patterns are often a clue to deeper structure. 5. Inspect the larger group. The pattern seems incomplete. You can complete the pattern by adding one more particle to the group. On a blank card, write down the characteristics (mass, spin, Q, S) you expect the missing piece will have. Show this prediction to your teacher. Murray Gell-Mann won the Nobel Prize in 1969 for his theory explaining all of the known particles and predicting the omegaminus particle. Gell-Mann arranged the known particles into groups, much as you did in Part 1 of this activity. His theory was motivated by the geometric patterns that he found. He recognized that these patterns pointed to a deeper structure within matter. Just as the patterns in the Periodic Table can be explained using protons and electrons to build atoms, GellMann’s patterns suggested that the particles were made of smaller, more fundamental, particles which he called quarks. Each quark has a characteristic charge and strangeness. It also has a “mirror image” antiquark with the opposite charge and strangeness. Quark Characteristics Flavour up (u) Charge Strangeness down (d) 0 strange (s) 0 -1 antiup ( u ) antidown ( d ) antistrange ( s ) 0 1 0 All of the known particles can be constructed by arranging quarks according to a simple set of rules: • Baryons are made of three quarks. (Antibaryons are made of three antiquarks) • Mesons are made of one quark and one antiquark. • Quarks have fractional charge and combine to produce integer charge (e.g., + - = +1). • Quarks have two distinct spin states: (½ or –½). When we combine quarks, the spins either add or subtract, just like the charges do to produce either spin-0, spin-½, or spin- particles. 16 | Activity 4 Beyond the Atom: Remodelling Particle Physics Part 2: Understanding Patterns 1. Mesons are a combination of one quark and one antiquark. The table below has all of the possible combinations of u, d, and s quarks and antiquarks. Determine the combined Q and S values for each combination. Then, match these values with the mesons in the spin-0 group that you built in Part 1. Spin-0 Mesons qq uu ud Q +1 S 0 us du dd ds su sd ss Particle Symbol What problems do you see in your results? How could you resolve them? 2. Baryons are a combination of three quarks. The table below has all of the possible combinations of u, d, and s quarks. Determine the combined Q and S values for each combination. Then, match these values with the baryons in the spingroup and the spin-½ group that you built in Part 1. Baryons qqq uuu uud Q +1 S 0 udd ddd uus uds dds uss dss sss Spin-3/2 Baryons Spin-1/2 Baryons p What problems do you see in your results? How could you resolve them? Activity 4 | 17 Beyond the Atom: Remodelling Particle Physics Activity 5 Finding the Top Quark NAME : In 1995, Fermilab discovered evidence for the sixth and final quark of the Standard Model. This was done by accelerating and colliding protons and antiprotons together to form a top quark and an antitop quark. The energy of the protons was converted into the mass of the quarks via E = mc2. The two quarks cannot be detected directly because they decay immediately into other particles. In this activity, you will use the mometum and energy of the decay particles to determine the mass of the original pair. Part 1: Top Quark 1. Protons and antiprotons were accelerated to equal and opposite speeds of over 99% of the speed of light. When they collided, as shown in Figure 1, the following interactions occurred: • • • • They annihilated and produced a top-antitop pair with almost no momentum. This quark pair decayed almost immediately into lighter particles with lots of momentum which moved approximately at right angles to the original proton beams. Three of these particles continued to decay into other particles, producing four “jets” that were measured in the inner sections of the detector. The fourth particle decayed into a muon that was detected in the outer part of the detector and a neutrino that was not detected. Figure 1 A Top Antitop Quark Event Model these interactions with beach balls, tennis balls, and marbles. Be sure that momentum is conserved at each step. from the D-Zero Detector at Fermilab 2. Figure 2 shows an event display from one of these collisions. Draw the horizontal and vertical components for each of the five labelled momentum vectors. Measure the length of each vector and each component. Record these lengths in the table below (the first one has been done as an example). Add the numbers in each direction and then convert these lengths to momentum units, using the scale: 1 mm = 1 GeV/c. Magnitude (mm) 95.5 Horizontal (mm) -94 Vertical (mm) -14 Total (mm) Total (GeV/c) 3. The momentum vectors do not add up to zero. Conservation of momentum demands that the initial and final momenta must be equal. D-Zero detects most particles, but neutrinos slip through unobserved. Use conservation of momentum to determine the momentum of the missing neutrino. Draw this momentum on the event display. 4. The equation E = mc2 is for a particle at rest. The full equation is E2 = (pc)2 + (mc2)2, where p is the relativistic momentum of the particle. The top and antitop quarks decay and produce jets of high-momentum particles. The momenta of these particles is so large that we can ignore the (mc2)2 term here and the equation can be simplified to E = pc. This means that a small particle with 95.5 GeV/c of momentum has 95.5 GeV of energy. Find the total energy of all of the particles, including the neutrino, by adding their energies. (Remember that energy is added as a scalar, not a vector.) 18 | Activity 5 Beyond the Atom: Remodelling Particle Physics 5. The energy released by the collision of the proton and antiproton is just enough to produce a top-antitop pair that is at rest in this particular, carefully-chosen event. The momentum of the top-antitop pair is extremely small compared to their rest-mass energy. The first term in E2 = (pc)2 + (mc2)2 vanishes and the equation simplifies to E = mc2. Use this to find the mass of a top quark. 6. In order to find the top quark, Fermilab collided a proton and an antiproton together at very high speeds. You have a friend who studies biology. Your friend thinks that this is a rather sloppy way to dissect protons to see what is inside them. Explain to your friend how this collision is not like a dissection. Be sure to refer to the masses of protons (0.938 GeV) and top quarks (172 GeV). Part 2: Higgs Particle Simulation In 2010 the LHC began colliding protons at the unprecedented energy of 7 TeV. This ambitious project was designed to detect the only particle in the standard model that had not yet been found—the Higgs particle. Based on previous experiments, it was expected to have a mass of between 115 and 185 GeV. Fermilab had hoped to discover it, but reached the limits of its energy without finding the elusive particle. The LHC is now the only accelerator with enough energy to produce the Higgs. If the Higgs is found at the lower end of possible energies, it will most likely be detected through a decay into two photons. Lancaster University in England has a simulation that lets you look at data to find evidence for a Higgs particle. Your job is to find collisions that produce two photons, measure their energies, and use the energies and the angle between them to determine the mass of the particle that formed them. There are a lot of other interactions taking place, so there is a lot of noise in the data. You need to look at many examples for your results to be statistically significant. 1. Go to http://www.lppp.lancs.ac.uk/higgs/higgs.html. Go to Measurement and scroll to the bottom of the page. Press Fire. You should see an image similar to the top quark data in Figure 2 Look for two towers that do not have any lines leading to them. That is the signature of two photons. You will probably not find them at first. Go to Options and select an energy cutoff of 20 GeV, then press Fire. Try other energy cut-offs. What energy cut-off lets you find photon events most rapidly? What happens when the energy cut-off is too high or too low? 2. Find an event with two photon towers. To calculate the mass of the particle that created the two photons, select Measure energies and click on the two photon towers. Then select Measure angle and click on the angle. Finally, select Calculate mass. The mass will be displayed on a histogram on the bottom left. To confirm the detection of a Higgs particle you need to get a lot of events. Your challenge is to collect as many events as possible in 10 minutes and to develop the largest significance value. How many did you find? Click on the Fit button in the bottom right when it is red. What was the significance of your findings? You are aiming for a significance greater than 5. Activity 5 | 19 Beyond the Atom: Remodelling Particle Physics 54.8 GeV c 17.0 GeV c 95.5 GeV c . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2 GeV c muon muon +y +x 65.9 GeV c Figure 2 D-Zero Detector at Fermi National Accelerator Laboratory 20 | Activity 5 Beyond the Atom: Remodelling Particle Physics Video Chapter 1 Models This chapter of the video: • introduces the Large Hadron Collider (LHC) and particle physics • reviews how scientific models are developed • describes Rutherford's gold foil experiment Introduction Science is a process of learning about our universe through observation and explanation. We build models to explain our observations and then test these models using logic and experiment. Experiments are designed to test specific predictions made by the models and to provide new observations that the models must explain. Models that fail to predict or explain observations are modified or discarded. As observations improve, so do the models. Particle physics is the study of the elementary building blocks of the universe and the forces through which they interact. Its origins go back to ancient times when philosophers considered what would happen if an object were cut into smaller and smaller pieces. Some felt that matter could be continuously divided into smaller and smaller pieces forever. Others concluded the logical answer was that eventually one would reach a point where the object could no longer be divided—our word atom comes from the Greek word for “uncuttable.” This idea stood untested for almost two thousand years, until developments in chemistry and physics allowed scientists to probe deeper into matter. One of the earliest developments was electrolysis: the process of applying an electric potential across a solution to separate the constituent parts. As researchers developed this tool, they modified their experiments to pull gases apart. This led to the development of the cathode ray tube (CRT). In a CRT, even when an electric potential is applied across a vacuum, something is still observed moving from one end of the tube to another: a flow of electrons. In a sense, the CRT acts like a very sharp knife, allowing scientists to “cut the uncuttable.” Figure 1 Early cathode ray tube. The discovery of the electron by J. J. Thomson in 1897 showed that atoms were made of even smaller parts. This raised the question: What is the structure of the atom? Thomson knew that atoms were electrically neutral and that electrons were negatively charged. He imagined the atom looking like a “raisin bun,” with the “bun” being a positive substance that held the tiny negatively charged electrons or “raisins” in place. It was this model that Ernest Rutherford set out to examine. Figure 2 Ernest Rutherford set up an experiment to test the "rasin bun" model of the atom. Rutherford’s scattering experiment overthrew the raisin bun model and demonstrated that the positively charged part of the atom must be concentrated into a very small volume at the centre of the atom—the nucleus. The significance of Rutherford’s discovery does not stop at the knowledge gained in the experiment. The technique that Rutherford used was revolutionary. Instead of pulling atoms apart by applying an electric potential across them, he probed the atom by firing particles at it—a technique that is still proving useful today. Figure 3 The "raisin bun" model fails to explain the observations 21 Beyond the Atom: Remodelling Particle Physics Rutherford’s Model and Fields Electrically charged objects exert forces over a distance—a seemingly mysterious phenomenon. Somehow, the objects are able to reach out through space and affect each other without touching (as the students observe in the Scattering Experiment). This behaviour can be explained using fields. An electric field is a region of space surrounding charged objects in which other charged objects feel a force. Since fields exert forces, they are capable of doing work and transferring energy. [ [ “It was as though you had fired a 15-inch shell at a piece of tissue paper and it had bounced straight back and hit you.” – Ernest Rutherford surrounding the target flashed whenever struck by an alpha particle. Rutherford and his students observed the individual flashes through low-power microscopes. Eventually they accumulated enough data to determine that 1 out of every 8000 alpha particles was being deflected straight back. This was a huge surprise and a puzzle that took Rutherford over a year to solve. Rutherford knew that the strength of the electric field surrounding the gold nucleus decreases with distance. Alpha particles that travel straight through the gold must be far from any nuclei. Particles that are deflected slightly must have been closer to a nucleus and experienced a small force. Particles that bounce back must have traveled directly at a nucleus, slowed down, stopped and then reversed direction. The strength of the field depends on the charge distribution of the object producing the field. In the raisin bun model, the atom was a lump of positive matter with little negative bits suspended in it. Rutherford knew that the field generated by such a dilute positive charge would barely disturb the trajectory of an alpha particle passing through the atom. To test this idea he fired positively charged alpha particles at a thin gold foil. He chose gold foil as his target because it can be made very thin, giving clean results. A scintillating screen Figure 4 Flashes of light are produced when an alpha particle hits the screen. 22 Beyond the Atom: Remodelling Particle Physics Video Chapter 2 Fields and E=mc2 This chapter of the video: • reviews how energy is transferred in fields • explains how E= mc2 leads to particle production • describes how particles are detected in bubble chambers The kinetic energy of the particle is stored up by the field as potential energy and then transformed back into kinetic energy as the alpha particle moves away. The initial energy of the alpha particle determines how close it gets to the gold nucleus, as the equation below shows. Rutherford’s original experiment shows that the minimum separation distance between the alpha particles and the gold nucleus was 2.7 × 10–14 m, which is about 10 000 times smaller than the atom. This is not the actual radius of the nucleus, just the distance of closest approach for the particles that collided with the nucleus head-on. collider. Conservation of energy dictates that the total energy before a collision must equal the total energy after a collision, but it does not say how that energy is distributed between rest-mass energy and momentum. The particles produced in a collision are constrained by various conservation laws (charge, spin, etc) but within those parameters, anything that can happen, will happen with a certain probability. Due to quantum randomness, we cannot predict what particles will be produced in any one collision. We can, however, predict the probability of a given result. As long as the initial collision has enough energy to produce the total rest mass of a certain set of particles, then it will produce those particles some of the time. Figure 1 Equations for distance of closest approach In order to get closer to the nucleus, the alpha particle must start with more kinetic energy. To increase the kinetic energy of particles, we accelerate them. The increased energy allows them to penetrate into more intense regions of the electric field, giving researchers a more precise description of the nucleus. As the energy of the particles increases, we reach a point where there is so much energy in such a small region of space that new particles appear. In order to understand where these new particles come from, we must examine the nature of energy and mass. Energy-Mass Equivalence One result of Einstein’s special theory of relativity is the unification of energy and mass. Einstein showed that energy and mass are equivalent and interchangeable. This can be seen in the general form of the relativistic energy-momentum expression E2 = (E_o )2 + (pc)2, where E is the total energy, E_o is the rest mass energy (E_o = mc2, where m is the rest mass and c is the speed of light), and p is the relativistic momentum (p = γmv, where γ is the relativistic factor due to the velocity v). For objects that are at rest (p = 0), this expression simplifies to the more familiar E = mc2. Figure 2 Particles created in a detector seem to appear out of nowhere. Note that the rest mass of a particle does not change when it is accelerated—even to relativistic speeds. As energy is added to the system the first term in the energymomentum expression will not change, so at relativistic speeds the contribution of rest mass to the total energy becomes negligible and the expression simplifies to E = pc. It is sometimes said that as objects approach the speed of light they get heavier and heavier, but that is misleading. As objects approach the speed of light they get harder to accelerate, which is the same effect as would occur due to an increase in mass, but does not actually mean that there is a change in the rest mass of the particle. Particle physicists usually refer to E_0 as "rest energy" and rarely use the term "rest mass". There is no need to qualify mass because there is only one mass and it does not change. We use the term "rest-mass energy" to reflect common pedagogical usage. The relativistic energy-momentum expression encapsulates the fundamental physics behind the creation of particles in a 23 Beyond the Atom: Remodelling Particle Physics Consider the collision that produced the top quark at Fermilab in the United States, a discovery announced in 1995 (students analyze these results in the Activity 5: Finding the Top Quark). A proton and antiproton each having rest mass energy of 0.938 GeV were accelerated to energies of 900 GeV before colliding with a total energy of 1.8 TeV. With so much energy available there were many possible combinations of rest mass energy and momentum in the products of the collision. The combination that researchers were looking for was a spray of particles whose total momentum and energy pointed to a large rest mass. This rest mass was that of a top quark–antitop quark pair. The energy needed to produce this pair came from the momentum of the proton and antiproton. Notice how the analysis moves fluidly from momentum to energy to mass without really differentiating between them. Researchers are not being sloppy in their treatment of units—they are making use of deep connections between these three quantities. Figure 3 A Top Antitop Quark Event from the D-Zero Dectector at Fermilab Antimatter One of the more exotic results from particle physics is the discovery and production of antimatter. British physicist Paul Dirac predicted the existence of a positively charged version of the electron while deriving a relativistic version of the Schrodinger wave equation for electrons. The equation had two possible solutions: one solution was the electron, the other was a particle with the same mass as an electron but opposite charge—the positron. This new antimatter particle was discovered within four years of Dirac’s prediction and antimatter has played a major role in particle physics ever since. 24 Antiparticles are fundamental particles that have the same mass as their counterparts but opposite properties (such as charge, colour, and strangeness). Thus, the positron is the antiparticle of the electron. When particles and antiparticles meet, they can annihilate, releasing all of their rest mass energy as other particles according to E = mc2. Science-fiction writers have long admired antimatter and routinely invoke it as a source of energy or as a weapon. In reality, however, it can be very difficult to produce. One of the largest producers of antimatter is CERN and they estimate that it would take 100 billion years and over 1000 trillion dollars to produce even a single gram of antiprotons. Antimatter does, however, have many applications in medicine. For example, positrons are used routinely in hospitals for positron electron topography (PET) scans. Figure 4 A PET scan of a brain Beyond the Atom: Remodelling Particle Physics Video Chapter 3 Particle Zoo and Standard Model This chapter of the video: • describes how scientists found patterns in the particle zoo that gave it order • introduces quarks and the strong nuclear force • gives an overview of the Standard Model The Eightfold Way The first half of the 20th century was a very productive time for particle physics. At the beginning of the century, the electron was the only subatomic particle that had been discovered. By the mid 1960s, there were dozens of subatomic particles, but no underlying theory to explain this “particle zoo.” The particles were grouped by behaviour into two categories: leptons and hadrons. Hadrons were further divided into mesons and baryons. Murray Gell-Mann won the Nobel Prize in 1969 for his “contributions and discoveries concerning the classification of elementary particles and their interactions”. What Gell-Mann and several of his contemporaries did was to organize the known hadrons by their spin, charge, and strangeness (just as students do in the Activity 4: Taming the Particle Zoo). The pattern that emerged revealed a special symmetry among mesons and baryons, which Gell-Mann coined the “Eightfold Way”. success of the model in predicting the existence of the omega-minus particle was further bolstered by the ability of the model to adapt to the discovery of new particles by incorporating three more quarks. Figure 2 Quarks bring order to the particle zoo. The Standard Model The early quark model brought order to the hadrons in the particle zoo of the early 1960s, but as technology improved the number of particles increased. The quark model grew to four, then five, and finally six flavours of quarks organized into three generations. According to the Standard Model of particle physics, these six quarks (and their antiquark counterparts) produce all the observed hadrons: mesons are quark-antiquark pairs, baryons are quark triplets. Quark Figure 1 The Eightfold way brought order to the particle zoo. Two things were apparent to Gell-Mann: first, one of the groups was missing a particle (the omega-minus), whose existence he was able to successfully predict; second, the basic structure of the patterns seemed to be built out of triangles. This pointed to the existence of three fundamental particles: the up, down, and strange quarks. Thus, the quark model was Gell-Mann’s explanation for the Eightfold Way symmetry he had observed. One of the compelling aspects of the Eightfold Way and the quark model behind it was the way they revealed order within the particle zoo via a simple set of rules. Two requirements of a scientific model are that it makes testable predictions and that it can accommodate new observations. The quark model has proven to be very robust in this regard. The early First generation Second generation Third generation Flavour Down Up Strange Charm Bottom Top Symbol d u s c b t Charge (e) Notice that quarks have fractional charges. In nature we do not find free particles with fractional charge; the smallest charge we find is the elementary charge e. The only quark combinations allowed in the Standard Model produce particles with an integer charge. Quarks are spin-½ particles, which means they have two distinct spin states (up and down). The Pauli Exclusion Principle says that two identical particles of this type cannot occupy the same state. There are particles, like the delta- 25 Beyond the Atom: Remodelling Particle Physics plus-plus, that have three apparently identical quarks in them which seems to violate the Pauli Exclusion Principle, so an additional quantum property, colour charge, was added to the quark model to further distinguish one quark from another. Quarks can have one of three “colours”: red, blue, and green. Of course, quarks do not actually have colour in the conventional sense; colour is used here to describe a property of quarks with three variations that combine to produce a neutral result, just as actual primary colours do. Colour charge is not observed in baryons and mesons, so quarks must combine in a way that produces only colourneutral particles. In baryons, this is accomplished by adding a red quark, a blue quark, and a green quark together to get a white (colourless) particle. In mesons, this is accomplished by adding a quark that has colour with an antiquark that has the corresponding anticolour. The Standard Model might appear quirky, with its language of quarks, flavour, colour, strangeness, and spin, but it is described by two very precise mathematical theories: quantum chromodynamics (QCD) and electroweak theory. These are two of the most successful and far-reaching theories ever produced by science. QCD gives a complete description of how the strong nuclear force works. It sets out the rules that govern how quarks combine to give hadrons. Electroweak theory unifies two of the fundamental forces (electromagnetic and weak nuclear). It describes how electrically charged particles behave and sets out the rules for the behaviour of leptons. Leptons are fundamental particles that do not interact with the strong force (i.e. they do not have colour). Like quarks, they come in three generations, with each generation having a negatively charged particle, such as an electron, and a neutrino counterpart. Neutrinos are particles that are produced in prodigious numbers during fusion reactions inside stars. They are incredibly difficult to study because they interact so weakly with matter. There are many experiments being conducted around the world to learn more about these elusive particles. Lepton First generation Name electron Symbol e νe Charge (e) –1 0 26 electron neutrino Second generation muon Third generation muon neutrino tau tau neutrino μ νμ τ ντ –1 0 –1 0 The Standard Model is not just about particles. It also gives a complete description of the strong, weak and electromagnetic forces. In the Standard Model, objects exert forces by exchanging particles called bosons. Bosons carry information about how strong the force is and whether it is attractive or repulsive. The electromagnetic force is mediated by photons, (i.e., electrostatic repulsion occurs when photons are exchanged between two like charges). The strong force is a short-range force that acts between coloured particles (i.e., quarks) by exchanging gluons, massless bosons that carry colour. The weak force is involved in radioactive decay and is mediated by the W and Z bosons. Mediator Electromagnetic Force Strong Force Weak Force Name photon gluon W Symbol γ g W Charge (e) 0 0 ±1 0 Mass (MeV/c2) 0 0 81 800 92 600 ± ± Z0 Z0 The final particle in the Standard Model is the Higgs boson. The Higgs boson was introduced into the Standard Model to explain why fundamental particles have mass. On July 4, 2012 researchers at CERN announced the discovery of a Higgs-like particle. Further analysis is needed to confirm if it is the actual boson predicted by Peter Higgs. Beyond the Atom: Remodelling Particle Physics Video Chapter 4 Higgs and the LHC This chapter of the video: • uses inertia and mass to introduce the Higgs field • builds an analogy for the Higgs field • describes how particles are accelerated at the LHC • discusses the LHC's detectors and highlights the immense complexity of data analysis at the LHC The Higgs Mechanism The Higgs mechanism is a concept that was first introduced into electroweak theory to solve a serious problem. The problem was that there was a theory of the weak interactions—a precursor of the Standard Model—that got many features right, but seemed to predict that all elementary particles were massless. But if they were massless they would all have to travel at precisely the speed of light, which they do not. What made this such a serious problem was that the same feature of the theory that made it successful—a particular symmetry of its equations—was also precisely what seemed to require particles to be massless. The technical details of this symmetry use ideas that go beyond the scope of this resource, but the idea of the Higgs mechanism can be more simply understood. The idea is that the universe is permeated by a Higgs field, which breaks the symmetry that would otherwise make particles massless. The Higgs field permeates all of space even in the absence of any particles; it exists everywhere, even in a vacuum. that couple more strongly are harder to accelerate and so have more inertial mass. Photons are massless despite the Higgs field because they do not couple to it at all. Why some particles couple to the field more strongly than others is still an open question: the coupling strength for each particle is an empirically derived parameter (i.e., we start with observed masses and work backwards to get the coupling strength, rather than predicting the coupling strength theoretically). Because the Higgs mechanism determines every particle’s mass, it plays a vital role in all physical processes. In particular it makes the weak force very short-ranged and this, in turn, is what makes the weak interactions experienced by nuclei so weak. Since the weak force is responsible for all reactions that change protons and neutrons into one another, it is responsible for the nuclear fusion reaction that releases energy by converting hydrogen into helium—a process that requires turning protons into neutrons. Since it is the strength of this force that controls the rate of nuclear fusion, the Higgs mechanism is partly responsible for the reactions that make stars shine. If the Higgs mechanism were slightly different, stars would either not shine at all or burn up rapidly. The Higgs mechanism also ultimately determines the fact that the proton is lighter than the neutron, which is why free neutrons decay into protons (plus other particles) rather than protons decaying into neutrons. Thus, the Higgs mechanism is a very important concept, and finding evidence for or against it will be an important step forward for science. In quantum mechanics, every field has a particle associated with it. In the case of the Higgs field, this particle is the Higgs boson. This boson is like a small ripple in the Higgs field. If we sufficiently disturb the Higgs field in the vacuum, we can create a ripple in it– a Higgs boson. This boson would then decay rapidly into lighter particles, and it is these lighter particles that the detectors at the Large Hadron Collider record and analyze. Figure 1 The Standard Model Particles experience an energy of interaction with this ambient field, and it is this energy of interaction that we interpret as their mass (we are able to do this because energy and mass are interchangeable). Not all particles interact with, or couple to, the Higgs field with the same strength. Particles The Large Hadron Collider (LHC) The Large Hadron Collider or LHC is the world’s largest particle accelerator—a 27 km-long ring buried 100 m below the Swiss-French border near Geneva. When running at full power, it will produce over 600 million collisions per second at unprecedented energies: 14 TeV for protons, 1150 TeV for lead ions. The collisions occur at four different locations on the ring where enormously complicated and sensitive 27 Beyond the Atom: Remodelling Particle Physics detectors analyze the particles produced by the collisions. This data is quickly filtered by several layers of computer analysis to about 100 events of interest per second. These events are then recorded for further analysis at a rate of about 700 MB/s. At this rate the LHC creates enough data to fill a stack of CDs 20 km high every year! Even with this huge amount of data it is has taken two to three years to accumulate enough data to produce reliable conclusions. In the LHC’s typical operating mode, two proton beams circulate in opposite directions. The protons are made by stripping the electrons from hydrogen. The protons start at LINAC-2, a linear accelerator that gives them a kinetic energy of 50 MeV. From LINAC-2 the protons pass through three accelerating rings: the Proton Synchrotron Booster, Proton Synchrotron, and Super Proton Synchrotron. At this point the protons are travelling at a speed of 0.999998c—this is before they have even entered the LHC! The protons are then injected into the LHC in two counter-circulating beams, and for the next 20 minutes they accelerate to their top speed of 0.999999991c. Each proton beam consists of 2808 bunches that are a few centimetres long by a millimetre wide and contain about 100 billion protons apiece. As a bunch approaches a collision point, it gets squeezed into a 16 μm-wide beam by focussing magnets. It is then deflected by a kicking magnet to collide with a bunch of protons travelling in the opposite direction. Figure 2 CERN accelerator complex (simplified) 28 Protons are charged, so they respond to both electric and magnetic fields. Acceleration is achieved by exerting forces on the protons with radio waves. As the protons approach the speed of light they begin to display relativistic effects, making them increasingly difficult to steer. At the LHC the protons become so hard to steer that superconducting magnets with fields in excess of 8 T are used. The magnets have to be superconducting because it takes such a huge electric current (11 700 A) to generate this field that it would be impossible to do so with conventional conductors. There are four large detectors attached to the LHC. Each detector records and examines the collisions in slightly different ways. The two largest detectors, ATLAS and CMS, are general-purpose detectors that are looking for the all kinds of new physics (including the Higgs boson). Another detector, ALICE, uses heavy ion collisions in an effort to study the behaviour of matter at very high temperatures and densities. LHCb is a specialized detector that is studying rare decays involving the bottom quark in an effort to test the Standard Model and to gain ATLAS: A Toroidal LHC ApparatuS CMS: Compact Muon Solenoid ALICE: A Large Ion Collider Experiment LHCb: Large Hadron Collider beauty Beyond the Atom: Remodelling Particle Physics insight into the problem of matter-antimatter asymmetry: why there appears to be so much more matter than antimatter in the universe. Each detector has unique features but they are all trying to do the same basic task—measuring the position, speed, mass, charge, momentum, and energy of the particles created by collisions. To gather all this information, the detectors are built in layers that measure specific properties of the particles produced by the collision (except for the neutrinos). Tracking devices, positioned both near the collision point and in the outer layers, record the trajectories of charged particles. Powerful magnetic fields bend the trajectories of charged particles as they pass through tracking chambers, revealing the charge and momentum of the particles. As the charged particles pass through the tracking chamber, electric signals are sent to a computer for analysis. Tracking chamber Electromagnetic calorimeter Hadron calorimeter Muon chamber photons e± muons π±, p n Innermost Layer... Calorimeters stop the particles and record the amount of energy deposited. Electromagnetic calorimeters (ECALs) measure particles like electrons, positrons, and photons. As these particles pass through an ECAL, they strike the atoms in the calorimeter and create an avalanche of lowerenergy electrons, positrons, and photons that are detected by light-sensitive phototubes. Hadronic calorimeters (HCALs) stop the strongly interacting particles, such as protons, by having them collide with the atoms in a dense substance like iron. These collisions rip electrons out of the iron, and these electrons then radiate photons that in turn produce more electrons and positrons, in a cascading shower of particles that can be detected. A typical HCAL will alternate layers of iron with a less dense substance that will ionize or scintillate as the particles pass through, allowing the computer to track them. Modern detectors produce copious amounts of data for each collision. Most of this data is filtered out by the computers in the detector hall. The LHC creates 600 million collisions every second, so the data analysis challenge is huge. One of the techniques being developed by CERN to cope with this challenge is called the Grid. The Grid is a huge global network of computing centres that share the workload using new data sharing and analysis techniques. CERN has already revolutionized the world through the development of the worldwide web. Who can predict what impact the Grid will have? ...Outermost Layer Figure 3 Layers of ATLAS detector 29 Beyond the Atom: Remodelling Particle Physics Video Chapter 5 Impact of the LHC 0m 1m 2m 3m 4m 5m 6m 7m Magnetic Field Directed Out of Page 4T 2T Silicon Tracker ECAL Electromagnetic Calorimeter HCAL - Hadron Calorimeter Superconducting Solenoid Transverse slice through CMS Iron return yoke interspersed with Muon Chambers Figure 1 Layers of CMS detector This chapter of the video: • reflects on the significance of the LHC • mentions discoveries the LHC might make What Else Might the LHC Find? At peak energy the LHC will be seven times more powerful than any previous particle accelerator. One goal of the LHC is to detect the Higgs boson, but at the energy levels the LHC reaches, there are some other very important phenomena that it is exploring: • Dark matter particles – Invisible particles that make up at least 90% of every galaxy in the universe • Supersymmetry – A symmetry that predicts the existence of heavier partner particles for every particle in the Standard Model • Matter-antimatter asymmetry – Why does the universe have more matter than antimatter? • Quark-gluon plasma – An exotic state of matter that may shed light on the Big Bang 30 Dark Matter Particles When astronomers look at the night sky, they see stars, planets, gas clouds, and other objects that emit light. Until a few decades ago, it was thought that these objects made up the bulk of the universe. However, recent observations, such as the rotation rates of galaxies, have revealed that light-emitting objects account for just a small fraction of the universe’s contents. Physicists now think that most of the matter and energy in the universe is unseen. Ninety percent of the mass of every galaxy is thought to be made of an invisible substance called dark matter. The leading candidate for this dark matter is a new type of subatomic particle called a weakly interacting massive particle (WIMP). Scientists are currently trying to directly detect WIMPs in a number of experiments. The LHC could potentially create and detect particles that would be good candidates for dark matter. Supersymmetry Supersymmetry proposes that each particle in the Standard Model has a partner particle with the same charge but different spin, and possibly with a much higher mass. Beyond the Atom: Remodelling Particle Physics For example, the Particles electron would have a supersymmetric partner called a selectron that is spinless. The LHC should be able to detect the lowest-energy supersymmetric Supersymmetric "shadow" particles particles—if they exist. Note that Figure 2 Schemetic diagram of supersymmetric particles some of these new particles could also fit the description for WIMPs and so supersymmetric theories might also shed light on the origin of dark matter. In Summary The Standard Model provides a very good description of matter and forces using a small number of fundamental particles. Scientists are aware of the limitations of this model, however, and are actively testing it so they can produce a better model. The Large Hadron Collider represents the latest test of the Standard Model. The LHC operates at such high energy that it is expected to exceed the limits of the Standard Model. New discoveries will undoubtedly be made. Results from the LHC are giving us insight into why particles have mass, why the universe is made of matter, and even what the early universe was like. The LHC is moving science forward to a new deeper understanding of the universe at the most fundamental level. Matter-Antimatter Asymmetry Everything around us—buildings, trees, the Earth, and the entire solar system—is made of matter, not antimatter. From the perspective of fundamental physics, this is very puzzling. Matter and antimatter should have been created in equal amounts during Big Bang, but detailed observations and calculations show that there must have been an excess of matter particles. Why there is more matter than antimatter is an important open question in science today. The LHCb detector will provide new insight into this problem by carefully observing reactions involving the b-quark that are slightly different for matter and antimatter. Facts and Mysteries in Elementary Particle Physics by Martinus Veltman (World Scientific, 2003) Quark-Gluon Plasma Ordinarily, quarks and gluons are bound inside the nuclei of atoms. But, in situations with extremely high temperatures or densities, there can be enough energy for them to be free. The gluons and quarks then move around freely in a “soup” called a quark-gluon plasma. According to current Big Bang theories, the universe went through a quark-gluon plasma stage before cooling to a point where the quarks and gluons became confined inside composite particles such as protons and neutrons. The LHC smashes lead ions together at such high energies that a quark-gluon plasma should be created. Physicists can then observe how matter behaved in the earliest stages of the universe. Further Reading Understanding the Universe from Quarks to the Cosmos by Don Lincoln (World Scientific, 2004) The New Cosmic Onion by Frank Close (Taylor & Francis, 2007) Introduction to Elementary Particles by David Griffiths (Harper & Row, 1987) The Quantum World: Quantum Physics for Everyone by Kenneth Ford (Harvard Press, 2004) Figure 3 A lead-lead collision producing a quark-gluon plasma 31 Beyond the Atom: Remodelling Particle Physics Activity Solutions Activity 1: Video Summary 1. Dalton’s model of indivisible balls of matter Thomson’s model using electrons in a positive mass Rutherford’s model using electrons and nuclei Solid ball of matter negative electrons positive substance positive nucleus negative electrons 6. (b) has three types of charge and gets stronger with distance. The strong force acts on the colour charge. There are three colours (red, blue, green). The strong force increases with distance, leading to the strange phenomenon of quark confinement. Quarks are never found in isolation. If you try to pull two quarks apart, the force holding them increases until you add so much energy that a new quark pair is produced. 7. (a) 2. (d) A few of the alpha particles bounced back. The lump of positive matter in Thomson’s model was too diffuse to generate a strong electric field. Rutherford expected the alpha particles would go right through the foil. When 1 out of 8000 alpha particles was deflected straight back, he realized that the positive matter must be concentrated into a very small space. 3. (c) both get closer to probe smaller distances and create massive particles from the energy. Higher-energy particles penetrate deeper into matter, so we get a clearer picture of what is happening at smaller scales. Higher-energy particles will also create more massive particles during collisions. 4. (a) E = energy, m = mass, c = the speed of light. This equation shows that energy and mass are equivalent and interchangeable. Mass is a form of energy and energy has mass. (b) The mass of the electron-positron pair comes from the energy of the photon. Quarks Leptons 1st Generation 2nd Generation 3rd Generation Bosons u c t γ d s b g e μ τ Z0 νe νμ ντ W ± (b) The particles in the first column are fundamental to the periodic table—the world around us is made of this 1st generation. (c) The 2nd and 3rd generations are heavier and are make exotic particles that are not common to everyday experience. (d) The bosons are the force mediators. They are involved in the interactions between matter. The photon and gluon are massless, while the W and Z bosons are very heavy. (e) In the periodic table, over 100 elements organized according to a pattern in the arrangement of the protons, neutrons, and electrons. In the Standard Model, all the subatomic particles organized by the arrangement of quarks and leptons. 8. The one particle that is still being looked for is the Higgs boson. It is needed to make the Standard Model work at higher energies, and if detected will give evidence for the Higgs field which is responsible for giving fundamental particles mass. 5. (a) The discovery of new particles was a problem because they were not made of protons and electrons. For example, pions were too big to be electrons but too small to be protons. There was no model to explain what they were made of. (b) The problem required new fundamental particles—quarks. All known hadron particles can be created using quarks. 32 9. The LHCb experiment is studying the matter-antimatter asymmetry problem by looking at events involving the bottom quark. The ALICE detector is looking for quark-gluon plasmas by colliding lead ions together. The LHC might also find evidence for or against dark matter, supersymmetry, and even extra dimensions. Beyond the Atom: Remodelling Particle Physics Activity 2: Scattering Experiment 1. The hanging balloon should be on as long a thread as possible. This gives maximum deflection for a given amount of force. The electrostatic force needs to be made as large as possible. The balloons need to be rubbed vigorously with fur, hair, or a wool sweater to get as large a charge as possible. The hanging balloon should be rubbed all over—not just on one side. If the charge is just on one side, the hanging balloon will rotate to put this charge farther away. The balloons need to be brought as close to each other as possible. The students will probably not predict the difficulty of preventing the hanging balloon from slipping off to one side or the other. Similar: The objects have the same electrical charge and repel each other. The strength of the repulsive force increases as the separation decreases. The objects are roughly spherical. It is hard to get a straight-line, head-on repulsion in both cases. Different: The gold nucleus is 40 times more massive than the alpha particle and has about 40 times the charge as well. The balloons are almost identical. The hanging balloon is like the gold nucleus because it is the target, but it is like the alpha particle because it was the one that was deflected. The model involves gravity and tension. The real experiment does not involve tension and the gravitational force is negligible. More similar: Throw the balloon at a balloon that is fixed in place. 2. The ball should be launched with enough speed to get on the base, but not to hit the stem. If the balls have too much energy they will bounce off the stem. The track needs to be aimed directly at the stem. If it is aimed slightly to the side, the balls will be deflected. Similar: The wine glass acts like the gold nucleus by not moving. The balls are like the alpha particles being fired at the target. Very few alpha particles bounce straight back. Different: The force of repulsion is not electrostatic. The balls could be launched at a variety of speeds. More similar: Make the ramp a fixed height to produce the same speed for the balls. 3. Planning the performance requires that students say what they know and consider how to represent it accurately. The bulk of the learning takes place during this planning phase. The performances should show how only the direct approach will stop the alpha particle completely and send it straight back. The more the approach differs from this path, the less it is slowed and the smaller the deflection. 4. (a) The nucleus should have a diameter of 1.5 cm and the closest approach should be 2.7 cm away from its centre. (b) The particles off to one side do not get as close. This is because they don’t stop completely, but begin to deflect while still moving. (c) The next nucleus will be 10–10 m (10 000 × 10–14 m) away. This means that the next nucleus would be 10 000 cm = 100 m or about a residential block away! Most of matter is empty space. It only appears solid due to the electrostatic repulsion of the electrons buzzing around in this space. Activity 3: Bubble Chamber Detective Part 1: CERN’s Two Metre Hydrogen Bubble Chamber 1. (a) –1 The kaons curve very slightly to the right. The right hand (or left hand) rule indicates that they are negative. 2. (b) +1 The particle curves to the left so according to the right hand (or left hand) rule it must be positive. 3. (c) has interacted with a proton to produce two oppositely charged particles. Charge is always conserved. A single negative kaon cannot decay into two charged particles so (a) and (b) are eliminated. We know that a negative kaon entered the point and that two charged particles have left it. The one on the right is positive. The other track doesn’t curve enough to indicate its charge. However, it must be negative in order to conserve the kaon’s charge. Where did the positive charge come from? The kaon must have interacted with one of the protons in the hydrogen. 4. (c) A neutral particle has been produced that moves up and to the left. Momentum is always conserved. We see one track continuing in the original direction and another track moving up and to the right. There must be another particle moving somewhat to the left. We can’t see it, so it must be neutral. 5. (d) Decayed into a negatively charged particle and a neutral particle. Charge is always conserved. The total charge before and after the event is the same so a decay has occurred. Conservation of momentum infers the existence of the neutral particle. 33 Beyond the Atom: Remodelling Particle Physics 6. (d) decayed into a negatively charged particle and a neutral particle. Charge is always conserved. The total charge before and after the event is the same so a decay has occurred. Conservation of momentum infers the existence of the neutral particle. This the same type of event as question 05 but in this case the neutral particle does not take away as much momentum so the track only changes slightly. 7. (c) point R. This is the neutral particle from question 06. (Note: The negative particle that decays at R is the omegaminus particle. This particle was predicted by Murray GellMann in 1962 and found in 1964 using the next bubble chamber photograph. The Taming the Particle Zoo activity has students analyze patterns to make the same prediction that Gell-Mann did. The second bubble chamber photograph is the one that first detected the omega-minus.) significant component up the page because of conservation of the original momentum. 3. When you join the intersection points and extend the line, you find that the neutral particle did not come from the kink at point W as you might expect. There is not enough evidence to determine where along this line the particle was produced. 4. The three lines meet at one point. From this we can infer that the three neutral particles had the same origin. No track is visible so we can infer that a neutral particle must have decayed into three other neutral particles that then proceeded to decay at points X, Y, and Z. 5. 8. (d) A neutral particle moving up and to the left has decayed into two new particles. Conservation of charge eliminates answers (a) and (c). Momentum is always conserved. The two particles created at Q have a net momentum toward the top left. These particles were either produced by the decay of a neutral particle moving up and to the left or by the decay of a stationary particle along with a neutral particle moving down and to the right. This second option is impossible since the chamber is filled with liquid hydrogen and hydrogen is made of protons and electron which are stable, so there is nothing that could decay into three particles. 9. (c) point P. This is the neutral particle from question 04. 10. A negative kaon collides with a proton at point P. This collision produces three particles: a neutral particle moves off to the top left where it decays at Q into two oppositely charged particles, a positive particle that exits the chamber, and a negative particle that decays at R into a negatively charged particle and a neutral particle. The neutral particle decays into two oppositely charged particles at S. The negative particle from R moves up and to the right until it decays into a negative particle and a neutral particle Part 2: Brookhaven National Laboratory’s Bubble Chamber 1. There is a charged particle moving in almost the same direction as the original kaon. It curves to the left, so it must be positive. This positive charge indicates that the negatively charged kaon interacted with one of the protons of liquid hydrogen. The charged particle that branches to the right must be negative to conserve charge from the kaon. There must also be a neutral particle going somewhat to the left to conserve momentum. 2. There is a negative particle going to the right. It must be negative to conserve charge and because it curves clockwise. There must also be a neutral particle to conserve the original momentum. Its momentum will ultimately have a 34 6. This question requires that the students clearly explain the steps of the analysis using the four neutral trails. The full analysis requires an understanding of what some of the hundreds of particles are and how they can decay, which the students can’t be expected to know. This question also lets the students reflect on science as a human endeavour with bosses, gambles, and payoffs. Go to the TeacherGuide folder on the DVD to read an email from Dusan Radojicic, recounting the night he discovered the first omega-minus. Beyond the Atom: Remodelling Particle Physics Students who recognize this and suggest the possibility of another particle can be given the eta-prime card–the missing meson. Second, there are three quark-antiquark pairs that have the same Q and S values. Students might suggest that there must be some other property that would further distinguish the particles. This is partly true: there is another quantum number called isospin which eliminates the strange-antistrange solution for π0, but it also demonstrates a very important aspect of quantum mechanics—one particle can be the superposition of two (or more) different states. The quark content of the π0 can be either up-antiup or down-antidown which means that if you could pull a π0 apart, half of the time you would get an up-antiup pair and half of the time you would have a down-antidown pair. The η and η’ are a more complicated mixture of upantiup, down-antidown and strange-antistrange quarks. The difference between η and η’ is very subtle but extremely important to the development of our understanding of symmetry in the Standard Model. Activity 4: Taming the Particle Zoo Part 1: Finding Patterns 2. Students will find that sorting the cards by their spin will produce three groups of roughly equal size. 3. Organizing these groups by charge and strangeness will lead to patterns emerging. 4. Two groups (spin-0 and spin-½) will have eight cards in them arranged in three rows. The top row will contain two cards, the middle row will contain four cards (two in the same place) and the bottom row will contain two cards. The more familiar pattern emerges if the top and bottom rows are shifted half a card toward the centre by changing the columns to diagonals. 2. 5. The third group (spin- ) will have nine cards arranged in three rows. One row will have 4 cards, the next 3, the next 2. From the pattern, it is pretty obvious that the last row should have 1 card. The more familiar pattern emerges if the cards are shifted over by changing the columns to diagonals. By inspecting the cards, students should be able to propose that the missing card will have spin- , Q = –1, S = –3, mass ≈ 1680 MeV. The omega-minus card can be given to each group when they give you the description of the missing particle. A Nobel Prize card is included to award to the first group to propose this particle (they don’t have to get the name right, just the description). Murray Gell-Mann was awarded the Nobel Prize in 1969 for his model that predicted the existence of the omegaminus. Part 2: Understanding Patterns 1. qq uu ud us du dd ds su sd ss Q 0 +1 +1 -1 0 0 -1 0 0 S 0 0 +1 0 0 +1 -1 -1 0 qqq uuu uud udd ddd uus uds dds uss dss sss Q +2 +1 0 -1 +1 0 -1 0 -1 -1 S 0 0 0 0 -1 -1 -1 -2 -2 -3 p n Spin-3/2 Baryons Spin-1/2 Baryons Same basic problems: missing particles and more than one particle in same spot. Instead of pointing to missing particles (although that is reasonable and students who suggest that should be congratulated), the empty boxes are pointing to deeper structural rules. In this activity we have chosen to only look at the charge and strangeness of the quarks, but quarks also have spin of ½. When spin-½ quarks are combined they will either produce a symmetric spin state of or a mixed symmetry spin state of ½. There are ten quark combinations that will produce a spin of but only eight that will produce a spin of ½. This has to do with the total angular momentum of the baryon and the possible spin configurations that will produce symmetric or antisymmetric wave functions. There is no way to produce a spin-½ particle with three similar quarks. Particle Symbol [NOTE: In this activity the students will identify π0 with ss, which is incorrect but for reasons that are beyond this simplified activity] There are two basic problems: First, there are nine combinations and the students have only created an octet. 35 Beyond the Atom: Remodelling Particle Physics Activity 5: Finding the Top Quark Part 1: Top Quark 1. Role playing helps students picture this three-dimensional collision more clearly and reinforces that it does not involve large objects breaking apart into smaller pieces. Have the students consider the question first as a Think-Pair-Share activity. Then give two volunteers two identical small balls marked + and – for the proton and antiproton. Have them run toward each other at equal speeds, make the balls collide and then disappear. Next give eight or so other students basketballs, ping pong balls, tennis balls, and so on. Have them hold their particles at the collision point and then count down to zero. The particles should move off in all directions so that momentum is conserved in all three directions. Ask those sitting down if the collision broke any laws of physics. Ask them if it is possible for the particles to move just in the horizontal plane. (It is possible, but not very likely.) Have the participating students demonstrate this motion. In the example that they are going to look at, the particles did just happen to emerge with almost all the momentum in a 2-D plane. This makes the problem much easier to analyze. 2. Students’ values may differ by ±1 mm, which is not significant. They should notice the fact that it appears that momentum is not conserved. Magnitude (mm) Horizontal (mm) Vertical (mm) Total Total (mm) (GeV/c) 22 –35 35 left –62 1 1 up 95 55 17 61 66 –94 –32 11 58 –15 44 14 20 3. The momentum of the neutrino is equal and opposite to the net momentum from question 2. Using the given scale (1 mm = 1 GeV/c) the momentum of the neutrino will be 35 mm long pointing right. 4. The sum of the magnitude of the momentum vectors (plus the momentum of the neutrino) is equal to the total energy. The total energy will be around 330 GeV. 5. The total energy of 330 GeV has to be divided by two because a top-antitop pair were created by the collision. This yields a mass of 165 GeV, which is only 4% different from the accepted value of 172 GeV. 36 6. The top quark is not a particle that already existed inside the proton. The top quark’s mass is 180 times greater than a proton’s mass of 0.938 GeV! When the fast-moving particles collide, all of their energy—from mass and from motion—is turned into pure energy. The collision was just a method to pack a lot of energy into a tiny space. Part 2: Higgs Particle Simulation 1. Setting a high energy cutoff simplifies the diagram greatly by removing the lower energy towers. However, if it is set too high, it may cut off the photons that you are trying to detect. A value of around 50 GeV seems to give the best results. 2. A typical result is 29 events with a significance of 1.6, using an energy cutoff of 50 GeV. If you select Auto events, you can quickly get 71 events with a significance of 2.8. Many (approximatley two thirds) of the generated events were discarded because the photon energies were below the cutoff. Beyond the Atom: Remodelling Particle Physics Who are the people in the video? The status of the contributors below reflect their positions at the time the video was filmed. BRIAN BATELL Postdoctoral Researcher, University of Chicago, Perimeter Institute Batell is a particle physicist who focuses on theories beyond the Standard Model. He is particularly interested in alternative theories of symmetry breaking and dark matter. He obtained his PhD from the University of Minnesota in 2008. NATALIA TORO Faculty Member, Perimeter Institute Toro works in particle physics and is interested in physics beyond the Standard Model. This includes dark matter, other new particles, and understanding the hierarchy problem. She collaborates heavily with experimentalists. Toro obtained her PhD from Harvard in 2007. CLIFF BURGESS Professor, McMaster University Associate Faculty Member, Perimeter Institute Burgess is a physicist with a broad range of interests including string theory, cosmology and particle physics. He also has a passionate interest in outreach. He obtained his PhD from the University of Texas. BRIGITTE VACHON Canada Research Chair in Particle Physics. Professor, McGill University Vachon is an experimentalist particle physicist involved in CERN's ATLAS experiment. She works on trying to understand the nature of subatomic particles at the scale of energies created by the LHC. She obtained her PhD from the University of Victoria in 2002. GHAZAL GESHNIZJANI Postdoctoral Researcher, Perimeter Institute Geshnizjani works in the field of cosmology and focuses on researching the early universe. She obtained her PhD from Brown University in 2005. ANDREAS WARBURTON Professor, McGill University Warburton works in experimental particle physicist and is a member of CERN's ATLAS experiment. One of his research areas is searching for substructure to quarks. He obtained his PhD from the University of Toronto in 1998. PHILIP SCHUSTER Faculty Member, Perimeter Institute Schuster is a particle physicist interested in the nature of dark matter and the weak interaction. His work intersects both theory and experiment. He obtained his PhD from Harvard University in 2007. 37 Beyond the Atom: Remodelling Particle Physics Appendix A Particle Zoo Cards p PROTON mass: 938 MeV spin-½ Q = +1 S=0 Σ *- Δ DELTA MINUS mass: 1232 MeV spinQ = -1 S=0 Σ SIGMA MINUS mass: 1197 MeV spin-½ Q = -1 S = -1 Ξ 0 Ξ XI STAR ZERO mass: 1532 MeV spinQ=0 S = -2 Σ SIGMA PLUS mass: 1189 MeV spin-½ Q = +1 S = -1 discovered: 1953 38 mass: 1315 MeV Κ 0 KAON (K ZERO) mass: 498 MeV spin-0 Q=0 S = +1 Σ Κ - SIGMA ZERO mass: 1193 MeV mass: 135 MeV discovered: 1949 Ξ *- XI STAR MINUS mass: 1535 MeV spinQ = -1 S = -2 discovered: 1962 π + discovered: 1947 0 PION (PI ZERO) spin-0 Q=0 S=0 discovered: 1959 discovered: 1962 + XI ZERO spin-½ Q=0 S = -2 discovered: 1953 *0 π 0 discovered: 1960 discovered: 1954 - mass: 1387 MeV spinQ = -1 S = -1 discovered: 1919 - SIGMA STAR MINUS PION (PI PLUS) mass: 140 MeV spin-0 Q = +1 S=0 discovered: 1947 n NEUTRON mass: 940 MeV spin-½ Q=0 S = -1 spin-½ Q=0 S=0 discovered: 1956 discovered: 1932 KAON (K MINUS) mass: 494 MeV η ETA mass: 548 MeV spin-0 Q = -1 S = -1 spin-0 Q=0 S=0 discovered: 1947 discovered: 1961 Beyond the Atom: Remodelling Particle Physics Appendix A Particle Zoo Cards continued - Ξ XI MINUS mass: 1322 MeV spin-½ Q = -1 S = -2 0 Δ *+ Σ spinQ = +1 S = -1 discovered: 1960 DELTA ZERO KAON (KBAR ZERO) mass: 1231 MeV Κ 0 KAON (K PLUS) mass: 494 MeV spin-0 Q = +1 S = +1 Λ discovered: 1947 Δ ++ mass: 498 MeV spin-0 Q=0 S = -1 π - LAMBDA mass: 1116 MeV spin-½ Q=0 S = -1 PION (PI MINUS) mass: 140 MeV spin-0 Q = -1 S=0 discovered: 1947 Δ + discovered: 1947 discovered: 1954 Κ mass: 1383 MeV discovered: 1952 spinQ=0 S=0 + SIGMA STAR PLUS DELTA PLUS mass: 1235 MeV spinQ = +1 S=0 discovered: 1954 Σ *0 SIGMA STAR ZERO mass: 1384 MeV spinQ=0 S = -1 discovered: 1951 discovered: 1960 ETA PRIME NOBEL PRIZE mass: 958 MeV Murray Gell-Mann DELTA DOUBLE PLUS mass: 1231 MeV spinQ = +2 S=0 discovered: 1954 Ω - OMEGA MINUS mass: 1672 MeV spinQ = -1 S = -3 discovered: 1964 η’ spin-0 Q=0 S=0 discovered: 1964 39 Beyond the Atom: Remodelling Particle Physics Appendix B Particle Physics Equations and Constants Description Equation Variables Si Unit FM – force acting on charged object moving in magnetic field q – quantity of charge on object v – speed of charged object B – magnetic field strength N Newton's 2nd Law Fnet – net force acting on object m – mass of object a – acceleration of object N kg m/s2 Centripetal Acceleration aC – centripetal acceleration v – linear speed of orbiting object R – radius of orbit m/s2 m/s m Kinetic Energy EK – kinetic energy m – mass of object v – linear speed of object J kg m/s Electric Field Strength EQ – magnitude of electric field k – Coulomb’s constant Q1 – charge on object one Q2 – charge on object two r – radial distance between objects J Nm2/C2 C C m Relativistic EnergyMomentum Relation E – total energy E0 – rest-mass energy p – relativistic momentum c – speed of light J J kgm/s m/s Rest-Mass Energy E0 – rest energy m – mass of object (aka “rest mass”) c – speed of light J kg m/s Relativistic Momentum p – relativistic momentum γ – Lorentz factor m – mass of object v – speed of object kgm/s Lorentz Force kg m/s γ – Lorentz factor Lorentz Factor v – speed of object c – speed of light Name C m/s T Symbol Value m/s m/s Si Unit -19 elementary charge e 1.602x10 Coulomb's constant k 9.00x109 Nm2/C2 Speed of Light c 3.00x108 m/s 40 C Beyond the Atom: Remodelling Particle Physics 03 Perimeter Explorations Beyond the Atom: Remodelling Particle Physics Teacher's Guide AUTHOR TEAM Dave Fish Physics Teacher, Sir John A Macdonald Secondary School and Educational Consultant, Perimeter Institute for Theoretical Physics Roberta Tevlin Physics Teacher, Danforth Collegiate and Technical Institute and Teacher Network Coordinator, Perimeter Institute for Theoretical Physics Damian Pope Senior Manager of Educational Outreach, Perimeter Institute for Theoretical Physics SCIENCE ADVISORS Cliff Burgess McMaster University and Perimeter Institute for Theoretical Physics Rolf Landua Head of CERN Education Group Eric Mazur Harvard University Balkanski Professor of Physics and of Applied Physics TEACHER CONTRIBUTORS John Atherton, Alpha II Alternative School Toronto, Ontario James Ball, John F. Ross Collegiate Vocational Institute, Guelph, Ontario Peter Dobias, O'Gorman High School, Timmins, Ontario Dave Doucette, Richmond Hill High School, Richmond Hill, Ontario Dwight Dunfield, Fredericton High School, Fredericton, New Brunswick Darlene Fitzner, Sir Winston Churchill High School, Calgary, Alberta Philip Freeman, Richmond Secondary School, Vancouver, British Columbia Patrick Kossmann, Greenall School, Balgonie, Saskatchewan Lisa Lim-Cole, Uxbridge Secondary School, Uxbridge, Ontario Chris Nichols, Castle View High School, Castle Rock, Colorado, USA Barry Panas, St. John's-Ravenscourt School, Winnipeg, Manitoba Nanouk Pare, John Abbott College, Montreal, Quebec David Vrolyk, Sir John A. Macdonald Secondary School, Waterloo, Ontario EDITORIAL PROJECT MANAGEMENT Kevin Martindale John Yip-Chuck New Leaf Media COVER DESIGN Gabriela Secara 3D IMAGES Steve Kelly PRINTER Denison Print VIDEO PRODUCER Damian Pope Senior Manager of Educational Outreach Perimeter Institute for Theoretical Physics VIDEO DEVELOPMENT, PRODUCTION, AND POST PRODUCTION Show Communications EXECUTIVE PRODUCER Greg Dick Director of Educational Outreach Perimeter Institute for Theoretical Physics COPYRIGHT Published by Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, Ontario, Canada, N2L 2Y5. Copyright © 2013 by Perimeter Institute for Theoretical Physics. All rights reserved. No part of this work covered by the copyright herein, except for any reproducible pages including in this work, may be reproduced, transcribed, or used in any form or by any means—graphic, electronic, or mechanical, including photocopying, recording, taping, Web distribution, or information storage and retrieval systems—without the written permission of Perimeter Institute for Theoretical Physics. For permission to use material from this text or product, submit a request online to Perimeter Institute. The information and activities presented in this book have been carefully edited and reviewed for accuracy and are intended for their instructional value. However, the publisher makes no representation or warranties of any kind, nor are any representations implied with respect to the material set forth herein, and the publisher takes no responsibility with respect to such material. The Publisher shall not be liable for any general, special, consequential or exemplary damages resulting, in whole or in part, from the readers' use of, or reliance upon, this material. EDITOR Tom Moss Gamblin INTERIOR DESIGN AND COMPOSITION Anna-Marie Hatayer Elizabeth Goheen 41 Beyond the Atom: Remodelling Particle Physics ACKNOWLEDGMENTS Special thanks to the teachers who attended: • Einstein Plus 2011, Perimeter Institute • The 2011 PTRA summer institute, Omaha, Nebraska • The OTF Physics Camp in August 2011, Sudbury • The September 2011 meeting of the Physics Teachers Alliance (GTA) The QuarkNet program at Fermilab for permission to modify their activity on the top quark. In particular, Marjorie Bardeen, Rob Grimm (William Fremd High School, Palatine, Illinois, USA), and Tom Jordan. The physics department at Lancaster University for their computer simulation of Higgs boson detection Mick Storr and Goronowy Tudor-Jones and the High School Teachers program at CERN. IMAGE CREDITS CERN including the ALTAS and CMS detectors for use of LHC and detector images on pp. 1, 3, 7, 14, 28, 29, 30, 31 Brookhaven National Laboratory for use of omega-minus image on p. 15 Fermilab for top quark image on p. 20 and Standard Model image on p. 27 Particle Data Group, Lawrence Berkeley National Laboratory of use of supersymmetry image on p. 31 National Museum of Science and Industry (UK) for use of cathode ray tube image on p. 21 iStockphoto/Banks Photos for the PET (Positron Emission Tomography) scan of a human brain on p. 24. Perimeter Institute for Theoretical Physics gratefully acknowledges the support of the Government of Ontario and the Government of Canada. 42 Perimeter Institute Perimeter Institute for Theoretical Physics is an independent, nonprofit, research institute whose mission is to make breakthroughs in our understanding of our universe and the forces that govern it. Such breakthroughs drive advances across the sciences and the development of transformative new technologies. Located in Waterloo, Ontario, Canada, Perimeter also provides a wide array of research training and educational outreach activities to nurture scientific talent and share the importance of discovery and innovation with students, teachers, and the general public. In partnership with the Governments of Canada and Ontario, Perimeter is a successful example of public-private collaboration in scientific research, training, and outreach. Perimeter Explorations This series of in-class educational resources is designed to help teachers explain a range of important topics in physics. Perimeter Explorations is the product of extensive collaboration between international researchers, Perimeter Institute’s outreach staff and science educators. Each module has been designed with both, expert and less experienced teachers in mind, and thoroughly classroom tested. Perimeter Explorations 03 Beyond the Atom: Remodelling Particle Physics Teacher’s Guide includes: - Teacher’s Guide in printed form - Teacher’s Support Material CD-ROM with the teacher’s guide in PDF format and modifiable versions of the student activities To order other Perimeter Institute Educational Outreach Resources, please visit our online store at www.perimeterinstitute.ca Follow us on Twitter @Perimeter Perimeter Institute for Theoretical Physics 31 Caroline Street North Waterloo, Ontario Canada N2L 2Y5 Tel: +1 519 569 7600 Fax: +1 519 569 7611 Made in Canada © 2013 Perimeter Institute for Theoretical Physics ISBN 978-1-927633-00-7 9 781927 633007