Homework due next Monday, Nov. 30 th Chapter 11: 13, 14, 15, 16, 17, 23, 28, 33, 34, 35 No class this Wednesday and Friday Elementary Particles CHAPTER 14 Elementary Particles Steven Weinberg ( )

Accelerators There are several types of accelerators used presently in particle physics experiments: cyclotrons, linear accelerators, and colliders. They’re all based on the same idea: as the particles move, apply a voltage that accelerates them to higher a speed.

Cyclotrons and Synchrotrons electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in highly curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain. Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research. A charged particle in a mag- netic field travels in a circle. Accelerating it with voltage yields a cyclotron. A problem with cyclotrons, however, is that, when charged particles are accelerated, they radiate

Linear Accelerators Linear accelerators or linacs typically have straight electric-field-free regions between gaps of RF voltage boosts. The particles gain speed with each boost, and the voltage boost is on for a fixed period of time, so the distance between gaps becomes increasingly larger as the particles accelerate. Linacs are sometimes used as pre-acceleration device for large circular accelerators.

Colliders If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles. Head-on collisions are twice as energetic as those involving hitting an object at rest, so physicists began building colliding- beam accelerators, in which the particles meet head-on.

Large Hadron Collider Counter- propagating protons each have an energy of ~7 TeV, giving a total collision energy of 14 TeV. The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.

The Weak Force Becquerel had discovered radioactivity in But why does it occur? In 1934, Enrico Fermi proposed an additional fundamental interaction or force, called the Weak Interaction, which is responsible for radioactivity and initiates nuclear fusion and fission. It is felt by all fermions and has an even shorter range than the strong force. The weak interaction is actually much stronger than gravity. It played a key role in the early universe in the creation of matter.

The Weak Interaction In the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam predicted that particles that they called W (for weak) and Z should exist that mediate the weak interaction. They have been observed in accelerators. Sheldon Glashow (1932- ) Abdus Salam ( )

Neutrinos Beta decay (which emitted electrons) appeared not to conserve energy, momentum, and spin. The journal Nature rejected the paper, saying that the theory was “too remote from reality.” NeutronProton + Electron + Neutrino So, in 1930, Wolfgang Pauli proposed the neutrino to explain the discrepancy. It has zero charge and spin ½. Alas, it was so light and interacted so weakly with other particles that it could not be detected. All have spin ½!

Neutrino Detectors Neutrinos have been identified experimentally in gigantic underground (to filter out other cosmic rays) mine-shaft detectors filled with liquids. Super-Kamiokande neutrino detector in Japan. It holds 50,000 tons of ultra- pure water when in operation. The small golden dots are photomultipliers.

Neutrino Oscillations One of the most perplexing problems over the last three decades was the solar neutrino problem: the number of neutrinos reaching Earth from the sun was a factor of 2 to 3 too small if our understanding of nuclear fusion was correct. This problem was solved when it was realized that neutrinos come in three varieties or flavors, electron, muon, and tau, and researchers saw neutrinos changing or “oscillating” into another flavor (the sun only emits electron neutrinos). Also, this could only happen if neutrinos have mass. Time or Distance Number of neutrinos e  

Classifying Elementary Particles Particles with half-integral spin are called fermions and those with integral spin are called bosons. This is a particularly useful way to classify elementary particles because all stable matter in the universe appears to be composed, at some level, of constituent fermions. Fermions obey the Pauli Exclusion Principle. Bosons don’t. Photons, W ±, and the Z are called gauge bosons and are responsible for the various forces. Fermions exert attractive or repulsive forces on each other by exchanging gauge bosons, which are the force carriers.

Leptons: Electrons, Muons, Taus & Neutrinos The leptons are perhaps the simplest of the elementary particles. They appear to be point-like, that is, with no apparent internal structure, and seem to be truly elementary. Thus far there has been no plausible suggestion that they are formed from some more fundamental particles. Each of the leptons has an associated neutrino, named after its charged partner (for example, muon neutrino). There are only six leptons plus their six antiparticles.

Muon and Tau Decay Even though they’re fundamental, leptons can decay into each other! For example, the muon decays into an electron, and the tau can decay into an electron, a muon, or even hadrons. The muon decay (by the weak interaction) is:   e e

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

Conservation Laws Physicists like to have clear rules or laws that determine whether a certain process can occur or not. It seems that everything that is not forbidden occurs in nature. Certain conservation laws are already familiar from our study of classical physics. These include mass-energy, charge, linear momentum, and angular momentum. These are absolute conservation laws: they are always obeyed. Some conservation laws are absolute, but other conservation laws may be valid for only one or two of the fundamental interactions and not for others.

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