Chapter 30: Particle Physics

1. Chapter 30: Particle Physics • Fundamental Particles (quarks and leptons) • Fundamental Interactions • Unification • Particle Accelerators • 21st Century Particle Physics

2. §30.1 Fundamental Particles Protons and neutrons are not fundamental particles. They are composed of three quarks each.

3. Each quark has an antiquark that has the same mass, but opposite charge.

4. An isolated quark has not been seen, but combinations of quarks make up the particle groups known as mesons and baryons. Both groups fall under the name of hadron. A meson is a bound quark/antiquark pair. A baryon is composed of three bound quarks. Recent data points to the existence of a new type of baryon called a pentaquark which is a five quark system that contains four quarks and one antiquark or four antiquarks and one quark.

5. The proton and neutron are baryons. A proton is made of two up quarks and one down quark (uud) and a neutron is two down quarks and one up quark (udd). A free neutron decays with a half-life of 10.2 minutes, but a neutron in a nucleus can be stable. The proton appears to be stable with a half-life of at least 1029 years.

7. The electron belongs to a group of particles called leptons. No internal structure of an electron has been observed yet.

8. The leptons and quarks are grouped into three generations. Ordinary matter is made up of particles from the first generation. The muon and tau leptons are not stable, but the electron is stable. The three “flavors” of neutrinos are able to change from one flavor to another (a neutrino oscillation).

9. §30.2 Fundamental Interactions Each fundamental force in nature occurs by the exchange of a mediator or an exchange particle. The exchange particle can transfer momentum and energy between particles.

10. Electromagnetic interactions are mediated by the photon. Weak interactions are mediated by the W+, W-, and Z0. Strong interactions are mediated by gluons. Gravity is mediated by the graviton. Photons, gluons, and gravitons have no charge and are massless.

11. The strong interaction holds quarks together to form hadrons. Quarks have a property called color charge that determine their strong interactions. Leptons have no color charge and so do not “feel” the strong force.

12. There are three types of color charges (red, green, and blue). They form colorless combinations. One each of red, green, and blue will form a colorless baryon. For example, one red and one antired quark can form a meson. It is the need to have colorless quark combinations which prevents them from being removed from a colorless group.

13. A gluon mediates a strong interaction. Quarks emit and absorb gluons, which carry a color charge. Absorbing or emitting a gluon changes the color of a quark. The strong interaction keeps both quark systems and atomic nuclei bound together.

14. The weak interaction proceeds by the exchange of the W+, W-, and Z0. These particles have nonzero masses. Quarks and leptons have weak charge and so feel the weak force.

15. The weak interaction allows one flavor of quark to change into any other flavor of quark. In beta-minus decay, a neutron changes into a proton. This occurs when a down quark changes into an up quark by emitting a W-, which then decays into an electron and an electron antineutrino.

16. The quantum mechanical description of the strong, weak, and electromagnetic forces along with the three generations of quarks and leptons is called the standard model. The standard model is not complete.

17. §30.3 Unification Just after the Big Bang, it is believed that all four fundamental forces were unified together as a single force. As the universe expanded and cooled, the force of gravity split off, followed by the strong force, which was followed by the splitting of the weak and electromagnetic forces.

18. Experiments have been done to show that under certain conditions the electromagnetic and weak forces are unified into the electroweak force. So far a quantum theory of gravity has not been developed. General relativity works on large size scales, but fails on the size scale of atoms.

19. Supersymmetry is an attempt at unifying the strong and electroweak interactions. It has been found that including extra dimensions is a way to unify gravity with the other forces.

20. String theories treat the fundamental particles as vibrating loops of energy. These “strings” vibrate in 10 or 11 dimensions. The extra dimensions are very compact and cannot be observed directly.

21. §30.4 Particle Accelerators A particle accelerator is used to give beams of charged particles high kinetic energy before colliding them with each other or with a stationary target. The results of the collision are recorded by various detectors for later study.

22. Two types of particle accelerators are the synchrotron and the linear accelerator.

23. §30.5 Twenty-First Century Particle Physics Some open questions physicists hope to answer: • Are there only three generations of quarks and leptons? • Will the Higgs particle be found? • Are quarks & leptons fundamental particles? • Is the proton stable?

24. Why is the force of gravity so weak in comparison to the other three fundamental forces? • What is dark matter? • What is dark energy? • What happened to all of the antimatter formed in the early universe? • Can gravity be unified with the other three fundamental forces? • Does our universe only have four dimensions? If so, why?

25. Example (text problem 30.5): A proton of mass 0.938 GeV/c2 and an antiproton, at rest relative to an observer, annihilate each other. What are the kinetic energies of the two pions, each of which has mass 0.14 GeV/c2? The reaction is Total energy is conserved: Assuming K1 = K2 then the KE of each pion is 0.8 GeV.

26. Example (text problem 30.9): Two factors that determine the distance over which a force can act are the mass of the exchange particle that carries the force and the Heisenberg Uncertainty Principle. Assume that the uncertainty in the energy of an exchange particle is given by its rest energy and that the particle travels at nearly the speed of light. What is the range of the weak force carried by the Z0 particle that has a mass of 92 GeV/c2? Compare it to the range of the weak force given in table 30.3. Same value as table 30.3

27. Example (text problem 30.21): A proton in Fermilab’s Tevatron is accelerated through a potential difference of 2.5 MV during each revolution around the ring of radius 1.0 km. In order to reach an energy of 1 TeV, how many revolutions must the proton make? How far has it traveled? During each revolution, the proton is given kinetic energy of qV= 2.5 MeV.

28. Example continued: The distance traveled is

29. Summary • Fundamental Particles (quarks & leptons) • Fundamental Interactions (strong, weak, EM, gravity) • Exchange Particles • Unification • Particle Accelerators