Many people are familiar with the basic subatomic particles, such as protons, electrons, and neutrons. However the more exotic elementary particles often leave people confused. Particle physicists classify the hundreds of different types of subatomic particles on the basis of which forces act on the particles.
Four Fundamental Forces
Physicists know of four fundamental forces in nature. All the different types of forces scientists observe are some combination of these fundamental forces. The four fundamental forces are the:
- gravitational force,
- electromagnetic force,
- strong nuclear force, and
- weak nuclear force.
The gravitational force acts on all particles, and the electromagnetic force acts on all particles having either a positive or negative electric charge. Hence, neither of these forces are useful in classifying elementary particles.
The strong and weak nuclear forces however are used to classify elementary particles. There are some subatomic particles, called hadrons, that respond to both the strong and weak nuclear force. Others, called leptons, respond only to the weak nuclear force.
Leptons
Leptons interact via the weak nuclear force, but are not affected by the strong nuclear force. Charged leptons are also significantly affected by the electromagnetic force.
Elementary particle physicists describe subatomic particles by their spin, among other properties. The spin of a particle measures its intrinsic angular momentum in integer or half integer units which are called the spin quantum number or simply spin. Unlike hadrons, which have various values for their spin, all leptons have a spin of 1/2.
Physicists think that leptons are point like particles and therefore truly elementary. In contrast to hadrons, they have no structure and are not composed of more fundamental particles.
Families of Leptons
The known leptons are the electron, electron-neutrino, muon, muon-neutrino, tau, and tau-neutrino. Each of these particles also has its corresponding antimatter particle. Particle physicists divide the known leptons into three families. One family contains the electron and electron-neutrino, another the muon and muon-neutrino, and the third contains the tau and tau-neutrino.
Decay of Leptons
Electrons and the three types of neutrinos are stable particles. Muons and tau particles however decay very rapidly. Muons decay into electrons and neutrinos. Tau particles decay into muons and neutrinos. The muons in turn decay into electrons and additional neutrinos.
Being stable allows electrons to be basic constituents of atoms, and they are therefore well known. Muons and tau particles on the other hand decay too rapidly to become important constituents in larger structures.
Along with hadrons, leptons are one of the basic classifications of elementary subatomic particles.
Further Reading
Beiser, A., Concepts of Modern Physics, 3rd ed. McGraw-Hill, 1981.
Serway, R.A., Moses, C.J., and Moyer, C.A., Modern Physics, 3rd ed. Thomson, 2005.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
