Particles. Particles Particles
In the 1930s, there were many scientists who thought that the problem of the ultimate structure of matter was well on the way to being solved. The atom could be understood in terms of only three particles-the electron, the proton, and the neutron. Quantum physics accounted well for the structure of the atom and for radioactive alpha decay. The neutrino had been postulated and, although not yet observed, had been incorporated by Enrico Fermi into a successful theory of beta decay. There was hope that quantum theory, applied to protons and neutrons, would soon account for the structure of the nucleus. What else was there?The euphoria did not last. The end of that same decade saw the beginning of a period of discovery of new particles that continues to this day. The new particles have names and symbols such as muon (JL), pion (’77’), kaon (K), and sigma (~). All the new particles are unstable; that is, they spontaneously transform into other types of particles according to the same functions of time that apply to unstable nuclei.Thus, if No particles of anyone type are present in a sample at time t = 0, then the number N of those particles present at some later time t is given by Eq. 43-14,
and the half-life TII2′ decay constant A, and mean life ‘Tare related by Eq. 43-17,
The half-lives o fthe new particles range from about 10-6 s to 10-23 s. Indeed, some of the particles last so briefly that they cannot be detected directly but can only be inferred from indirect evidence. ‘These new particles are commonly produced in head-on collisions between protons or electrons accelerated to high energies in accelerators at places like Fermilab (near Chicago), CERN (near Geneva), SLAC (at Stanford), and DESY (near Hamburg,Germany) hey are discovered with particle detectors that have grown in sophistication lIuil (see Fig. 45-1) they rival the size and complexity of entire accelerators of .only a few decades ago.Today there are several hundred known particles. Naming them has strained the resources of the Greek alphabet, and most are known only by an assigned number in a periodically issued compilation. To make sense of this array of particles, we look for simple physical criteria. We can make a first rough cut among the particles in at least the following three ways.
FermIon or Boson?
All particles have an intrinsic angular momentum called spin, as we discussed for electrons, protons, and neutrons in Section 32-4. Generalizing the notation of that section, we can write the component of spin S in any direction (assume it to be along a z axis) asS, = mil or m, = s, s – I, … , +s, (45-4)
in which Il is h/27T. m, is the spin magnetic quantum number, and s is the spin
quanrum number. The latter can have either positive half-integer values (1. f…. J or nonnegative integer values (0, I, 2, … ). For example, an electron has the value s = !. Hence the spin of an electron (measured along any direction) can have the values S, -_ 1′>’ 2″ (spin up) or S, = -!fI (spin down).Confusingly, the term spin is actually used. in two ways: It properly means a particle’s intrinsic angular momentum S, but it is often used loosely to mean theparticle’s spin quantum number s. In the latter case, for example, an electron is said to be a spin-] particle.Particles with half-integer spin quantum numbers (like electrons) are called fermions, after Fermi, who (simultaneously with Paul Dirac) discovered the statistical laws that govern their behavior. Like electrons, protons and neutrons also have s = !and are fermions. Particles with zero or integer spin quantum numbers are called bosons, after I~dian physicist Satyendra Nath Bose, who (simultaneously with Albert Einstein) discovered the governing statistical laws for those particles. Photons, which have s = I, are bosons; you will soon meet other particles in this class.This may seem a trivial way to classify particles, but it is very important for this reason
on which the strong force does 1101 act, leaving the weak force as the dominant force, are called leptons, Protons. neutrons, and points arc hadrons: electrons and neutrinos are leptons. You will soon meet other members of these classes. We can make a further distinction among the hadrons because some of themare bosons (we call them mesons); the piont is an example. The other hadrons are fermions (we call them baryons); the proton is an example.