1. Homework due Monday December 7 th Chapter 14: 1, 3, 5, 6, 8, 10, 11, 15, 22, 25 No class this Friday, Dec. 4 th ! Elementary Particles CHAPTER 14 Elementary Particles Steven Weinberg (1933 - )

2. Hadrons Hadrons are particles that act through the strong force. Two classes of hadrons: mesons and baryons. Mesons are particles with integral spin having masses greater than that of the muon (106 MeV/c 2 ). They’re unstable and rare. Baryons have masses at least as large as the proton and have half-integral spins. Baryons include the proton and neutron, which make up the atomic nucleus, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles. All baryons decay into protons. Quarks

3. Some Hadrons

4. Energy and momentum in particle decays 1.The energy available for the decay is the difference in the rest energy between the initial decaying particle and the particles that are produced in the decay. We call the Q value: Q=(m i -m f )c 2 Of course Q must be positive for the decay to occur. 2.The available energy Q is shared as kinetic energy of the decay products in such a way as to conserve linear momentum and energy.

5. Example Compute the energies of the proton and the  meson that result from the decay of a  ° at rest. Q=(m  ° -m p -m  )c 2 = 1116MeV - 938MeV - 140MeV = 38MeV So the total kinetic energy of the decay products must be: K p + K  = 38MeV Conservation of relativistic momentum requires p p =p  Substitute into the relativistic formula for Kinetic Energy yields p p =p  = 101MeV/c The kinetic energies are: K  =33MeV and K p =5 MeV

6. Baryon Conservation The number of nucleons (baryons) is always conserved. So we define a new quantum number called baryon number, which has the value B = +1 for baryons and −1 for anti-baryons, and 0 for all other particles (mesons, leptons). The conservation of baryon number requires the same total baryon number before and after an interaction. 1 1 0 0 0

7. Lepton Conservation The leptons are all fundamental particles, and there is conservation of leptons for each of the three kinds (families) of leptons. The number of leptons from each family is the same both before and after a reaction. We let L e = +1 for the electron and the electron neutrino; L e = −1 for their antiparticles; and L e = 0 for all other particles. We assign the quantum numbers L  for the muon and its neutrino and L  for the tau and its neutrino similarly. Thus leptons give us three additional conservation laws.

8. A strange individual. Some strange particles Strangeness The behavior of the K-mesons is very odd.  0 mesons decay quickly into pairs of photons, but K 0 mesons don’t. A new quantum number was defined: Strangeness, S, which is conserved in the strong and electromagnetic interactions, but not in the weak interaction. The kaons ( K ) have S = +1, lambda (  ) and sigmas (  ) have S = −1, the xi (  ) has S = −2, and the omega (  ) has S = −3. When the strange particles are produced by the p + p strong interaction, they must be produced in pairs to conserve strangeness.

9. A Veritable Zoo of Particles! Physicists like to think that the universe is, in the end, simple and elegant. So maybe all these particles are in fact composed of a smaller set of simpler ones. Quarks! First proposed in 1964 by Murray Gell-Mann and George Zweig, quarks have charges of ±1/3 and ±2/3 that of an electron. An up quark has a charge of +2/3, and a down quark has a charge of -1/3. Two ups and a down make a proton. An up and two downs make a neutron. A proton Murray Gell-Mann (1929- )

10. Baryons and Mesons Revisited Mesons are made up of pairs of quarks— a quark and an anti-quark. Baryons are made up of three quarks.

11. Evidence for Quarks Richard E. Taylor (1929-) In 1967, at the Stanford Linear Accelerator Center (SLAC), Jerome Friedman, Henry Kendall, and Richard Taylor scattered 20- GeV electrons off protons, analogous to experiments performed by Rutherford on the nucleus five decades earlier, and found back-scattered electrons and that the proton had internal structure (that is, quarks!). Nevertheless, the quark idea only caught on slowly, and it wasn’t until 1990 that they won the Nobel Prize.

12. Truth, Beauty, and Charm And then a fourth quark called the charmed quark (c) was proposed to explain some additional discrepancies in the lifetimes of some of the known particles. A new quantum number called charm C was introduced so that the new quark would have C = +1 while its anti-quark would have C = −1 and particles without the charmed quark have C = 0. Charm is similar to strangeness in that it is conserved in the strong and electromagnetic interactions, but not in the weak interactions. This behavior was sufficient to explain the particle lifetime difficulties. Two additional quarks, top and bottom (or truth and beauty), were also required to construct some exotic particles (the Upsilon-meson). A strange quark (s) was also required.

13. Quark Properties The spin of all quarks (and anti-quarks) is 1/2.

14. Quark Description of Particles Baryons normally consist of three quarks or anti-quarks. A meson consists of a quark-anti-quark pair, yielding the required baryon number of 0. 1/3e -2/3e 2/3e -1/3e 2/3e 1/3e 2/3e -2/3e -1/3e 2/3e -1/3e 1/3e 2/3e -1/3e -2/3e 2/3e -2/3e

15. But the  − does exist! Quantum Chromodynamics (QCD) This isn’t possible unless some other quantum number distinguishes each of these quarks in one particle. A new quantum number called color circumvents this problem and its properties establish Chromodynamics (QCD). Discovery of the  -

16. Gluons The particle that mediates the very strong interaction between quarks is called a gluon (for the “glue” that holds the quarks together); it’s massless and has spin 1, just like the photon. Computed image of quarks and gluons in a nucleon Like the photon, the gluon has two transverse polarization states. There are eight independent types of gluon (depending on the colors of the quarks involved).

17. Quark-Antiquark Creation No one’s ever measured a free quark. Physicists now believe that free quarks cannot be observed; they can only exist within hadrons. This is called confinement. This occurs because the force between the quarks increases rapidly with distance, and the energy supplied to separate them creates new quark-anti- quark pairs. J/ 

18. Fundamental and Composite Particles We call certain particles fundamental; this means that they aren’t composed of other, smaller particles. We believe leptons, quarks, and gauge bosons are fundamental particles. Other particles are composites, made from the fundamental particles. Some of these fundamental particles (W, Z, ,  ) have short lifetimes and decay, but this is okay.

19. The Families of Matter The three generations (or families) of matter. Note that both quarks and leptons exist in three distinct sets. One of each charge type of quark and lepton make up a generation. All visible matter in the universe is made from the first generation. Second- and third-generation particles are unstable and decay into first- generation particles. Second-generation particles occur in astrophysical objects and cosmic rays. Third-generation particles were probably important in the early universe. All are produced in accelerators.

20. The Four Fundamental Interactions

21. Photons and gravitons are massless. W and Z bosons are heavy. Gluons are also massless and appear to violate our calculation of the inverse relation between the range and mass of such particles, but, because quarks are confined within hadrons, this effectively limits the range of the strong interaction to 10 −15 meters, roughly the size of an atomic nucleus. But why are there four fundamental interactions?

22. The Standard Model Over the latter half of the 20 th century, numerous physicists combined efforts to model the electromagnetic, weak, and strong interactions, which has resulted in The Standard Model. It is currently widely accepted. It is a relatively simple, comprehensive theory that explains hundreds of particles and complex interactions with six quarks, six leptons, and four force-mediating particles. It’s based on three independent interactions, symmetries and coupling constants.

23. The Standard Model of particle physics proposes that there’s a field called the Higgs field that permeates all of space. By interacting with this field, particles acquire mass. Particles that interact strongly with the Higgs field have heavy mass; particles that interact weakly have small mass. The Higgs field requires another boson. It’s called the Higgs particle (or Higgs boson) after Peter Higgs.. The Higgs Boson What about all the particle masses?

24. The Higgs boson was recently detected! Higgs! That little bump? That's where CERN has seen a significant number of unusual events at about 125 GeV, which means that something new is going on. Are we sure it’s the Higgs? All evidence to date suggests yes.

25. In the mid-19th century, Maxwell unified electricity and magnetism into a single force with his now famous equations. James Clerk Maxwell (1831-1879) In free space: where is the electric field, is the magnetic field, and c is the velocity of light. Ampère’s Law Maxwell’s term (displacement current)

26. Unifying All the Interactions Maxwell had unified electricity and magnetism into his electromagnetic theory in the 1860s, so 20 th century physicists have been trying to similarly unify all the newly discovered forces into one force. In the 1950s, it was rumored that Heisenberg had done it, and just the details remained to be sketched in. But nothing ever emerged from Heisenberg. So Wolfgang Pauli responded with the following: “Below is the proof that I am as great an artist as Rembrandt; the details remain to be sketched in.”

27. Unifying the Electromagnetic and Weak Forces: the Electroweak Theory In the 1960s, Sheldon Glashow, Steven Weinberg, and Abdus Salam unified the electromagnetic and weak interactions into what they called the electroweak theory. Sheldon Glashow (1932- ) Steven Weinberg (1933 - )

28. Grand Unifying Theories Several grand unified theories now combine the weak, electromagnetic, and strong interactions into one interaction and predict that, at extremely high energies (>10 14 GeV), the electromagnetic, weak, and strong forces fuse into a single unified field. Currently, they can explain why: Neutrinos have a small, nonzero mass. The proton and electron charges have the same magnitude. Massive magnetic monopoles may exist. Just one anywhere in the universe would explain charge quantization. But current experimental measurements have shown the proton lifetime to be greater than 10 35 years. Current theory has it at 10 29 to 10 31 years.

29. Including Gravity: String Theory For the last two decades there has been a tremendous amount of effort by theorists in string theory, which has had several variations. The addition of super-symmetry resulted in the name theory of super-strings. In super-string theory, elementary particles don’t exist as points, but rather as tiny, wiggling loops that are only 10 −35 m in length. Presently super-string theory is a promising approach to unify the four fundamental forces, including gravity. However, because experiments that can confirm or falsify string theory require orders of magnitude more energetic particles than can currently be produced, these theories are very controversial.

30. Super-Symmetry Super-symmetry is a necessary ingredient in many of the theories trying to unify all four forces of nature. The symmetry relates fermions and bosons. All fermions have a super-partner: a boson of equal mass, and vice versa. These particles include sleptons, squarks, axions, winos, photinos, zinos, gluinos, and preons. The super-partner spins differ by ħ/2. Presently, none of the known leptons, quarks, or gauge bosons can be identified with a super-partner of any other particle type.

31. M-Theory Recently theorists have proposed a successor to super-string theory called M-theory. M-theory has 11 dimensions (ten spatial and one temporal) and predicts that strings coexist with membranes, called “branes” for short. Only through experiments (which no one currently knows how to do, since they require > 1,000,000 times more energy than current accelerators can produce) will scientists be able to wade through the vast number of unifying theories.

32. Another Challenge: Matter vs. Antimatter But it appears that, in our universe now, matter dominates over antimatter, and the reason for this has puzzled physicists and cosmologists for years. Events in the early universe may be responsible for this asymmetry. But explanations go far beyond the standard model. Galaxy Anti-galaxy? According to current theory, matter and antimatter should have been created in exactly equal quantities. And baryon number is conserved!