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Chapter 44 Particles Physics & Cosmology Elementary Particles Atoms From the Greek for “indivisible” Were once thought to be the elementary particles Atom constituents Proton, neutron, and electron Were viewed as elementary because they are very stable Quarks Physicists recognize that most particles are made up of quarks Exceptions include photons, electrons and a few others The quark model has reduced the array of particles to a manageable few The quark model has successfully predicted new quark combinations that were subsequently found in many experiments Fundamental Forces All particles in nature are subject to four fundamental forces Strong force Electromagnetic force Weak force Gravitational force Strong Force Is responsible for the tight binding of the quarks to form neutrons and protons Also responsible for the nuclear force binding the neutrons and the protons together in the nucleus Strongest of all the fundamental forces Very short-ranged Less than 10-15 m Electromagnetic Force Is responsible for the binding of atoms and molecules About 10-2 times the strength of the strong force A long-range force that decreases in strength as the inverse square of the separation between interacting particles Weak Force Is responsible for instability in certain nuclei Is responsible for beta decay A short-ranged force Its strength is about 10-6 times that of the strong force Scientists now believe the weak and electromagnetic forces are two manifestations of a single force, the electroweak force Gravitational Force A familiar force that holds the planets, stars and galaxies together Its effect on elementary particles is negligible A long-range force It is about 10-43 times the strength of the strong force Weakest of the four fundamental forces Explanation of Forces Forces between particles are often described in terms of the actions of field particles or quanta For electromagnetic force, the photon is the field particle The electromagnetic force is mediated, or carried, by photons Forces and Mediating Particles (also see table 30.1) Interaction (force) Mediating Field Particle Strong Gluon Electromagnetic Photon Weak W± and Z0 Gravitational Gravitons Richard Feynmann 1918 – 1988 Contributions include Work on the Manhattan Project Invention of diagrams to represent particle interactions Theory of weak interactions Reformation of quantum mechanics Superfluid helium Challenger investigation Shared Nobel Prize in 1965 Feynman Diagrams A graphical representation of the interaction between two particles Feynman diagrams are named for Richard Feynman who developed them Feynman Diagram – Two Electrons The photon is the field particle that mediates the interaction The photon transfers energy and momentum from one electron to the other The photon is called a virtual photon It can never be detected directly because it is absorbed by the second electron very shortly after being emitted by the first electron The Virtual Photon The existence of the virtual photon would be expected to violate the law of conservation of energy But, due to the uncertainty principle and its very short lifetime, the photon’s excess energy is less than the uncertainty in its energy The virtual photon can exist for short time intervals, such that ∆E ∆t ≈ ħ Paul Adrien Maurice Dirac 1902 – 1984 Instrumental in understanding antimatter Aided in the unification of quantum mechanics and relativity Contributions to quantum physics and cosmology Nobel Prize in 1933 Antiparticles For every particle, there is an antiparticle An antiparticle has the same mass as the particle, but the opposite charge The positron (electron’s antiparticle) was discovered by Anderson in 1932 From Dirac’s version of quantum mechanics that incorporated special relativity Since then, it has been observed in numerous experiments Practically every known elementary particle has a distinct antiparticle Exceptions – the photon and the neutral pi particles are their own antiparticles Classification of Particles Two broad categories Classified by interactions Hadrons Interact through strong force Composed of quarks Leptons Interact through weak force Thought to be truly elementary Some suggestions they may have some internal structure Hadrons Interact through the strong force Two subclasses Mesons Baryons Decay finally into electrons, positrons, neutrinos and photons Integer spins Masses equal to or greater than a proton Noninteger spin values Decay into end products that include a proton (except for the proton) Composed of quarks Leptons Interact through weak force All have spin of ½ Leptons appear truly elementary No substructure Point- like particles Scientists currently believe only six leptons exist, along with their antiparticles Electron and electron neutrino Muon and its neutrino Tau and its neutrino Conservation Laws A number of conservation laws are important in the study of elementary particles Two new ones are Conservation of Baryon Number Conservation of Lepton Number Conservation of Baryon Number Whenever a baryon is created in a reaction or a decay, an antibaryon is also created B is the Baryon Number B = +1 for baryons B =- 1for antibaryons B = 0 for all other particles The sum of the baryon numbers before a reaction or a decay must equal the sum of baryon numbers after the process Proton Stability Absolute conservation of baryon number indicates the proton must be absolutely stable Otherwise, it could decay into a positron and a neutral pion Never been observed Currently can say the proton has a halflife of at least 1031 years Some theories indicate the proton can decay Conservation of Lepton Number There are three conservation laws, one for each variety of lepton Law of Conservation of ElectronLepton Number states that the sum of electron-lepton numbers before a reaction or a decay must equal the sum of the electronlepton number after the process Conservation of Lepton Number, cont Assigning electron-lepton numbers Le = 1 for the electron and the electron neutrino Le =- 1for the positron and the electron antineutrino Le = 0 for all other particles Similarly, when a process involves muons, muon-lepton number must be conserved and when a process involves tau particles, tau-lepton numbers must be conserved Muon - and tau - lepton numbers are assigned similarly to electron- lepton numbers Strange Particles Some particles discovered in the 1950’s were found to exhibit unusual properties in their production and decay and were given the name strange particles Peculiar features include Always produced in pairs Although produced by the strong interaction, they do not decay into particles that interact via the strong interaction, but instead into particles that interact via weak interactions They decay much more slowly than particles decaying via strong interactions Strangeness To explain these unusual properties, a new law, conservation of strangeness, was introduced Also needed a new quantum number, S The Law of Conservation of Strangeness states that the sum of strangeness numbers before a reaction or a decay must equal the sum of the strangeness numbers after the process Strong and electromagnetic interactions obey the law of conservation of strangeness, but the weak interactions do not Bubble Chamber Example The dashed lines represent neutral particles At the bottom, π- + p → Λ0 + K0 Then Λ0 → π- + p and K0 → π + µ- + νµ Murray Gell-Mann 1929 – Worked on theoretical studies of subatomic particles Nobel Prize in 1969 The Eightfold Way Many classification schemes have been proposed to group particles into families These schemes are based on spin, baryon number, strangeness, etc. The eightfold way is a symmetric pattern proposed by Gell-Mann and Ne’eman There are many symmetrical patterns that can be developed The patterns of the eightfold way have much in common with the periodic table Including predicting missing particles An Eightfold Way for Baryons A hexagonal pattern for the eight spin ½ baryons Strangeness vs. charge is plotted on a sloping coordinate system Six of the baryons form a hexagon with the other two particles at its center An Eightfold Way for Mesons The mesons with spins of 0 can be plotted Strangeness vs. charge on a sloping coordinate system is plotted A hexagonal pattern emerges The particles and their antiparticles are on opposite sides on the perimeter of the hexagon The remaining three mesons are at the center Quarks Hadrons are complex particles with size and structure Hadrons decay into other hadrons There are many different hadrons Quarks are proposed as the elementary particles that constitute the hadrons Originally proposed independently by GellMann and Zweig Quark Model Three types u – up d – down s – strange c – charmed t – top b – bottom Associated with each quark is an antiquark The antiquark has opposite charge, baryon number and strangeness Quark Model, cont Quarks have fractional electrical charges +1/3 e and –2/3 e All ordinary matter consists of just u and d quarks Quark Model – Rules All the hadrons at the time of the original proposal were explained by three rules Mesons consist of one quark and one antiquark This gives them a baryon number of 0 Baryons consist of three quarks Antibaryons consist of three antiquarks Numbers of Particles At the present, physicists believe the “building blocks” of matter are complete Six quarks with their antiparticles Six leptons with their antiparticles See table 30.3 for quark summary Color Isolated quarks Physicist now believe that quarks are permanently confined inside ordinary particles No isolated quarks have been observed experimentally The explanation is a force called the color force Color force increases with increasing distance This prevents the quarks from becoming isolated particles Colored Quarks Color “charge” occurs in red, blue, or green Antiquarks have colors of antired, antiblue, or antigreen Color obeys the Exclusion Principle A combination of quarks of each color produces white (or colorless) Baryons and mesons are always colorless Quark Structure of a Meson A green quark is attracted to an antigreen quark The quark – antiquark pair forms a meson The resulting meson is colorless Quark Structure of a Baryon Quarks of different colors attract each other The quark triplet forms a baryon The baryon is colorless Quantum Chromodynamics (QCD) QCD gave a new theory of how quarks interact with each other by means of color charge The strong force between quarks is often called the color force The strong force between quarks is carried by gluons Gluons are massless particles There are 8 gluons, all with color charge When a quark emits or absorbs a gluon, its color changes More About Color Charge Like colors repel and opposite colors attract Different colors also attract, but not as strongly as a color and its anticolor The color force between color-neutral hadrons is negligible at large separations The strong color force between the constituent quarks does not exactly cancel at small separations This residual strong force is the nuclear force that binds the protons and neutrons to form nuclei Weak Interaction The weak interaction is an extremely short-ranged force This short range implies the mediating particles are very massive The weak interaction is responsible for the decay of c, s, b, and t quarks into u and d quarks Also responsible for the decay of µ and τ leptons into electrons Weak Interaction, cont The weak interaction is very important because it governs the stability of the basic particles of matter The weak interaction is not symmetrical Not symmetrical under mirror reflection Not symmetrical under charge exchange Electroweak Theory The electroweak theory unifies electromagnetic and weak interactions The theory postulates that the weak and electromagnetic interactions have the strength at very high particle energies Viewed as two different manifestations of a single interaction The Standard Model A combination of the electroweak theory and QCD form the standard model Essential ingredients of the standard model The strong force, mediated by gluons, holds the quarks together to form composite particles Leptons participate only in electromagnetic and weak interactions The electromagnetic force is mediated by photons The weak force is mediated by W and Z bosons The Standard Model – Chart Mediator Masses Why does the photon have no mass while the W and Z bosons do have mass? Not answered by the Standard Model The difference in behavior between low and high energies is called symmetry breaking The Higgs boson has been proposed to account for the masses Large colliders are necessary to achieve the energy needed to find the Higgs boson Grand Unification Theory (GUT) Builds on the success of the electroweak theory Attempted to combine electroweak and strong interactions One version considers leptons and quarks as members of the same family They are able to change into each other by exchanging an appropriate particle The Big Bang This theory of cosmology states that during the first few minutes after the creation of the universe all four interactions were unified All matter was contained in a quark soup As time increased and temperature decreased, the forces broke apart Starting as a radiation dominated universe, as the universe cooled it changed to a matter dominated universe A Brief History of the Universe George Gamow 1904 – 1968 Among the first to look at the first half hour of the universe Predicted: Abundances of hydrogen and helium Radiation should still be present and have an apparent temperature of about 5K Cosmic Background Radiation (CBR) CBR represents the cosmic “glow” left over from the Big Bang The radiation had equal strengths in all directions The curve fits a blackbody at 2.9 K There are small irregularities that allowed for the formation of galaxies and other objects Connection Between Particle Physics and Cosmology Observations of events that occur when two particles collide in an accelerator are essential to understanding the early moments of cosmic history There are many common goals between the two fields Some Questions Why so little antimatter in the Universe? Do neutrinos have mass? How do they contribute to the dark mass in the universe? Explanation of why the expansion of the universe is accelerating? Is there a kind of antigravity force acting between widely separated galaxies? Is it possible to unify electroweak and strong forces? Why do quark and leptons form similar but distinct families? More Questions Are muons the same as electrons, except for their mass? Why are some particles charged and others neutral? Why do quarks carry fractional charge? What determines the masses of fundamental particles? Do leptons and quarks have a substructure?