CHAPTER 14 Elementary Particles Homework due Wednesday December 10th Chapter 14: 1, 3, 5, 6, 8, 10, 11, 15, 22, 25 No class this Wednesday and Friday We will finish chapter 14 on Monday Dec. 8th Final Exam: Thursday Dec 18th, 8am to 10am in Physics 203 We will review on Wednesday the 10th and Friday the 12th Steven Weinberg (1933 - )

The Weak Force Becquerel had discovered radioactivity in 1896. 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. Image from www.epa.gov

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. Abdus Salam (1926-1996) Sheldon Glashow (1932- )

Neutrinos Beta decay (which emitted electrons) appeared not to conserve energy, momentum, and spin. All have spin ½! + + Neutron Proton 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. The journal Nature rejected the paper, saying that the theory was “too remote from reality.”

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. http://physicsofscifi.blogspot.com/2012/07/star-trek-neutrinos.html

Neutrino Oscillations Time or Distance Number of neutrinos Neutrino Oscillations ne nm 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. nt http://content.answers.com/main/content/wp/en/thumb/8/83/400px-Electron_neutrino_oscillation_long.png

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. http://www.tcd.ie/Physics/Schools/what/quarks/leptons.gif

Muon and Tau Decay e m ne nm 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: m nm e ne http://www.autodynamics.org/main/images/pagemaster/bubbleInvertedSmall.gif

Hadrons Hadrons are particles that act through the strong force. Two classes of hadrons: mesons and baryons. Quarks Mesons are particles with integral spin having masses greater than that of the muon (106 MeV/c2). 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. http://www.lepp.cornell.edu/public/lab-info/video/images/jpg/mesons.jpg

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. Image from http://pfnicholls.com/physics/particles4.html 1 1