Advanced Modern Physics

Kolena - 6th

Leptons and Quarks: The Fundamental Particles

From past courses you may have taken, you may have a false idea of the world around you. For example, years of chemistry class have lied to you. Whereas the primary focus of those classes dealt with neutrons, protons and electrons, we will explore beyond these general particles, to the composition of those particles themselves. Our focus will be on what are believed to be the fundamental constituents of matter, leptons and quarks.

The advent of higher energy particle accelerators is what made the discovery of these elementary particles possible. The most familiar lepton is the electron. Commonly thought of as the most elementary particles in nature, leptons, along with baryons and mesons are the three subdivisions for all atomic particles. Leptons have half-integral spin, which in mechanical terms, constitutes the property of intrinsic angular momentum. Leptons are believed to be fundamental, i.e. not composed of anything smaller. They include particles which do not obey the strong nuclear force, for example, electrons. Leptons can only have an integral amount of charge (e.g. -1 for electron). Other charged leptons are muons and taus. Each lepton has an associated neutral neutrino partner (electron-, mu-, and tau-neutrino) of practically zero rest mass. Furthermore, like every particle, each lepton has an anti-lepton, which has the same mass as the lepton, but is converse in all other properties. It is important to note that lepton number (the number of leptons minus antileptons before and after any reaction) is conserved.

In a variety of experiments in the 1960s, more and more subatomic particles became evident. This made scientists consider the possibility that subatomic particles are composed of yet smaller building blocks. In 1964 Murray Gell-Mann and Yuval Ne'eman proposed the theory of quarks. Quarks are the constituents of all the hadrons. Hadrons are a group of subatomic particles including baryons and mesons--all particles which obey the strong nuclear force. An interesting aspect of the quarks is their fractional charge. There are believed to be three quarks and three anti-quarks. Their charges are +2/3, -1/3, and -1/3. Two problems appeared with the theory of quarks: 1) quarks had to gave half-integral spin for the model to work, however, in experiments they appeared to violate the Pauli Exclusion Principle, which forbids any two particles with the same spin to occupy the same quantum state--often two or even three quarks would be in the same state; 2) Quarks cannot be separated from the particles which they make up--even when bombarded by high energy electrons and neutrinos in particle accelerators, the forces between them, believed to be quite weak, prevailed. To resolve these problems the concept of color was introduced. The colors characteristic of quarks are red, green, and blue, and those of anti-quarks and anti-red, anti-green, and anti-blue. In any independent particle, the colors of the quarks need to add up in such a way that the net color is theoretically white. For example, a proton is composed of a red quark, a green quark, and a blue quark; furthermore, the charges on two of those quarks are +2/3 and on the other one is -1/3.

A remarkable property of matter particles is that they exhibit "family affiliation". They come in three families, each consisting of two quarks and two leptons. In many ways the three families behave as copies of one another. The question of whether a unified theory which justifies the existence of just three families exists is still in debate. The quarks of the first family are "up quarks" and `down quarks", with lepton members being the electron and the electron-neutrino. The two quarks are the building blocks of protons and neutrons, which in turn form atomic nuclei and hence over 99% of all the earth's matter.

The 1995 Nobel Prize in the field of Physics was awarded to Martin Perl of Stanford, California, and to Frederick Raines, University of California, Irvine, California. Perl's contribution was the detection of the tau lepton, which was the first step in determining that a third "family" of fundamental building blocks existed. Raines's first observation of neutrinos is what made supposedly "impossible" neutrino experiments realistic.

The study of the basic building blocks of all matter is constantly changing. As accelerators are made bigger and with more energy, more and more research can be done on the various particles. Now all the hoopla is about leptons and quarks. But where does it end? How do we know that there aren't even smaller particles, particles which compose quarks and lepton? We don't. Which is why we pursue these studies so studiously, in hopes that some day all of our questions about the universe will be someday answered.