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Transcript
Lesson 30: Particle Physics! ! Part 1: Antimatter! ! For every kind of particle in the universe, there is a corresponding kind of antimatter particle. Antimatter particles have the same mass as their corresponding matter particle but opposite charge. ! Particle Symbol Antiparticle Proton Antiproton Neutron Antineutron Electron Positron Neutrino Antineutrino ! Symbol When a matter particle and a its corresponding antimatter particle collide, they annihilate each other.The total mass of the particles is converted into energy. ! ! ! When a matter particle comes into contact with an antimatter particle of another type, they do not annihilate. ! ! ! Example: Calculate the energy released in the annihilation of a positron-electron pair. ! ! ! ! ! ! ! ! ! Pair Production:! When a gamma ray with enough energy hits a nucleus, it can turn into a matter/antimatter pair. ! ! This picture of pair production was taken in a bubble chamber. A bubble chamber is a tool used to observe the movement of particles. It consists of a vessel filled with a superheated transparent liquid (most often liquid hydrogen) and can be used to take pictures of the paths of electrically charged particles moving through it. If a magnetic field is applied in the bubble chamber, scientists can use the curved paths of the particles to determine their charge and mass. ! ! Example: In the picture to the right, a magnetic field is directed into the page. Determine which particle is the electron and which is the positron. ! ! ! Part 2: The four fundamental forces! ! Physicists believe that four fundamental forces govern all interactions in the universe:! Gravitational !! !! ! Weak Force !! !! ! Electromagnetic !! !! ! Strong Force !! !! ! ! Force mediating particles:! In quantum physics, energy is carried by particles! ! Example:! ! Modern physics extends this idea to say that the energy of each of the four forces is carried by its own unique particle.! ! The force mediating particles are:! Gravitational Force !! ! ! Weak Force Electromagnetic Force Strong Force Part 3: The standard model of matter! ! ! ! ! ! ! ! ! ! ! ! ! ! Particles Fermions Quarks ! ! ! ! ! Quarks ! ! ! ! ! ! ! Antiquarks ! ! ! ! ! ! ! ! Bosons ! ! ! ! ! ! Force Mediating Particles Gravitational weak Electromagnetic Strong Leptons leptons ! ! ! ! ! Hadrons ! Mesons Antileptons ! Baryons ! ! ! ! ! Proton Neutron Example: The pion is a meson made from the up quark and the antidown antiquark. What is the charge of the pion?! ! ! ! ! ! ! Example: The proton is a baryon made of two up quarks and a down quark. What is the charge of the proton?! ! ! ! ! ! Example: The neutron is a baryon made from two down quarks and an up quark. What is the charge of the neutron?! ! ! ! ! ! ! ! Part 4: Standard Model and Radioactive Decay! ! Radioactive decay processes happen as a result of failure of the forces that hold the nucleus together: The strong force and the weak force. ! ! Strong force:! ! ! ! ! ! ! ! ! ! The strong force is much stronger than the electromagnetic force. However, it has a much shorter range and can’t reach across larger nuclei. ! When the strong force fails, the nucleus breaks into pieces:! ! ! ! ! ! ! Alpha decay:! Fission:! ! Weak Force:! ! ! ! ! ! ! When the weak force fails, a quark can change identity. ! ! Up turns to down:! ! ! ! ! ! Down turns to up:! ! ! ! ! Beta decay is a failure of the weak force.! ! ! Beta Positive Decay:! ! ! Proton —> Neutron + positron + neutrino! ! ! ! ! ! ! ! ! ! ! ! Beta Negative Decay:! ! ! Neutron —> Proton + electron + antineutrino! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Practice Problems:! ! 1.! ! 2.! ! 3.! ! 4.! ! 5.! ! 6.! ! 7.! ! 8.! ! 9.! ! Find the energy equivalent of the mass of a neutron. [939 MeV/c2]! What kinds of subatomic particles will leave tracks in a bubble chamber and what kinds will not leave tracks in a bubble chamber.! Why is a magnetic field often applied across a bubble chamber? What can the curvature of a particle's track in a magnetic field reveal about the particle?! What is the wavelength of the photons produced in electron-positron pair annihilation? (2.4 x 10-12 m)! Describe and explain the differences in the tracks made in a bubble chamber by the particles in each pair (assume the particles travel the same velocity):! (a)! protons and alpha particles ! ! (b)! protons and electrons! Which fundamental force is the strongest over large distances? What is the strongest over short distances? What is the weakest fundamental force?! The diagram shows a particle track recorded in a bubble chamber at the CERN particle accelerator. The magnetic field in the bubble chamber was 1.2 T directed out of the page.! (a)! Does the particle have a positive or negative charge? Explain your reasoning.! (b)! Estimate the initial radius of the particle's path. [10 cm]! (c)! Determine the initial momentum of the particle. Assume the particle is an electron or a positron. [1.9 x 10-20 Ns]! (d)! Why does the particle's path spiral inward?! (e)! What could cause the short tracks that branch off from the large spiral track?! The mass of a psi particle is 3.097 GeV/c2. Express this mass in kilograms. [5.506 x 10-27 kg]! An X-ray photon with a frequency of 1.8 x 1018 Hz collides with a nucleus. Will electron-positron pair production occur? Explain your answer.! 10.! In the β+ decay of nitrogen-12 (12.01864 u) into carbon-12 (12.00000 u), a positron with energy of 11.0 MeV is emitted. What is the energy of the electron neutrino? Assume the mass of the positron is 5.485799 x 10-4 u (same as an electron) (5.8 MeV)! ! 11.! Protons injected into the Tevatron synchrotron at the Fermilab travel around a circumference of 6.4 km. When they are injected into the synchrotron, they have an energy of 8.0 GeV, but when they leave they can have an energy of 1.0 TeV. If the protons gain 3.0 MeV in each rotation, how many rotations are required and how far do the protons travel? (3.3 x 105 rotations, 2.1 x 109 m)! ! 12.! The tracks in this diagram show the creation of two particles in a bubble chamber. Initially, the two particles have the same speed.! (a)! What evidence suggests that a photon created the two particles?! (b)! Describe the path of this photon.! (c)! Which of the tracks shows the path of a positively charged particle?! (d)! Give two reasons why the other track must show the path of a negatively charged particle.! (e)! How are the mass and charge of the two particles related?! (f)! Why is it likely that the interaction involves an antiparticle?! ! 13.! The dark tracks in this diagram show a high speed proton colliding with a hydrogen atom in a bubble chamber, deflecting downward, and then colliding with another hydrogen atom. These tracks curve clockwise slightly.! (a)! In which direction is the magnetic field oriented?! (b)! What conclusions can you make about the mass, speed, and charge of the particles involved in the first collision?! ! 14. Doubly charged magnesium-25 ions are deflected by a uniform magnetic field (B = 0.725 T) in the ion separation region of a mass spectrometer. If these ions have kinetic energy of 8.35 x 10-16 J as they enter the ion separation region, what is the radius of curvature of their path? [3.60 x 10-2 m]! ! 15. What is the difference between Hadrons and Leptons? What type of particle is an electron? How about a positron? How about a proton? How about an anti-proton?! ! 16.! Identify a major difference that distinguishes ! (a)! leptons from hadrons ! (b)! mesons from baryons! (c)! fermions and bosons! ! 17.! ! 18.! ! 19.! ! 20.! ! ! ! ! ! Compare the quark composition of a proton to that of a neutron.! List the 12 fundamental particles of matter in the standard model.! Determine the charge on a particle having the quark composition uus. [+1]! Describe the difference between mesons and baryons in terms of quarks.!
