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 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.
Hadrons
Hadrons interact primarily via the strong nuclear force. Hadrons are extremely small, but not geometric points. Their measured size is about 1e-15 meters. (The notation e-15 indicates times 10 raised to the -15th power.) Hence, particle physicists think that hadrons are not truly elementary particles. Rather they are composed of even more fundamental particles called quarks. Mesons are composed of two quarks, and baryons contain three quarks.
High energy particle physicists further classify hadrons into mesons and baryons. These sub-classifications are based on the mass and spin of the particles. The spin of a particle measures its intrinsic angular momentum in integer or half integer units called the spin quantum number or simply spin.
Mesons
Mesons have spin quantum numbers of either 0 or 1. Mesons also have masses that are between the mass of protons and the mass of electrons. The different types of mesons include pions, kaons or K mesons, and eta mesons.
Mesons do not last very long. They decay in average times of about a hundred millionth of a second or less. When mesons decay the final decay products are electrons, positrons (which are the antimatter particles corresponding to electrons), neutrinos, and photons.
Baryons
Protons and neutrons are the best known baryons, but there are many more types of baryons. They include the lamda, sigma, delta, xi, and omega baryons.
Baryons have odd half integer spin quantum numbers (1/2, 3/2, 5/2, etc). They have masses greater than or equal to the mass of protons.
Protons are very stable subatomic particles, but all other types of baryons decay. Neutrons decay after a mean lifetime measured in minutes, but the other types of baryons have lifetimes less than about a ten billionth of a second. When baryons decay, the final product always includes a proton.
Hadrons are a type of subatomic particles that include the familiar protons and neutrons as well as many more exotic types of particles. The Large Hadron Collider (LHC) is so named because high energy particle physicists will use it to accelerate protons, which are a type of hadrons, to speeds very nearly the speed of light.
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.
 |
