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An overview of elementary particle physics, covering topics such as the nature of elementary particles, the basic forces and gauge bosons, the lepton and quark families, and experimental techniques. It also includes definitions of key terms such as gluon, hadron, lepton, meson, photon, quantum chromodynamics, and quarks. likely to be useful as study notes or a summary for university students in physics or related fields.
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Article to appear in The Encyclopedia of Physical Sciences and Technology, Academic Press, 1987
I. The Nature of Elementary Particles
I?. Size of Elementary Particles C. High-Energy and Elementary Particles
E. Particles and Antiparticles F. Hadrons G. Reactions of Elementary Particles and Hadrons
B. The Force-Carrying Particles and Gauge Bosons
B. Lepton Conservation C. Muon and Tau Lepton Decays
B. Quark Conservation Laws
C. Particle Detectors
Antiparticle Each particle has a partner, called an antiparticle, which is identical except that all charge-like properties (electric charge, strangeness, charm, etc.) are opposite to those of the particle. When a particle and its antiparticle meet, these properties can- cel out in a process called annihilation. The particle and an- tiparticle can then disappear and other particles be produced. Asymptotic freedom The concept that the strong force between quarks gets weaker as the quarks get very close together. Baryon A type of hadron. The baryon family includes the proton, neu- tron, and those other particles whose eventual decay products include the proton. Baryons are composed of 3-quark combi- nations. BNL Brookhaven^ National^ Laboratory,^ near^ New^ York^ City,^ U.S.A. CERN The European Center for Nuclear Research, located near Geneva, Switzerland, and supported by most of the nations of Western Europe. Color A^ property^ of quarks^ and^ gluons,^ analogous^ to^ electric^ charge, which describes how the strong force acts on a quark or gluon. Electromagnetic force The long-range force and interaction associated with the elec- or interaction tric and magnetic properties of particles. This force is inter- mediate in strength between the weak and strong force. The carrier of the electromagnetic force is the photon. Electron volt The^ amount^ of^ energy^ of^ motion^ acquired^ by^ an^ electron^ ac- celerated by an electric potential of one volt: MeV = million electron volts; GeV = billion electron volts; TeV = trillion electron volts. Electroweak force or interaction
The force and interaction which represents the unification of the electromagnetic force and the weak force. Flavor A general name for the various kinds of quarks, such as up, down, and strange. Also^ sometimes^ applied^ to^ the^ various kinds of leptons.
Generation
Gluon Hadron
Lepton
Luminosity
Meson
Photon Quantum Chromo- dynamics (QCD)
Quantum Electra dynamics (QED)
Quarks
The classification of the leptons and quarks into families ac- cording to a mass progression. The first generation consists of the electron and its neutrino, and of the up and down quarks. The second generation consists of the muon and its neutrino, and of the charm and strange quarks. The third generation consists of the tau and its neutrino, and of the bottom and expected top quarks. A massless particle which carries the strong force. A subnuclear, but not elementary, particle composed of quarks. The hadron family of particles consists of baryons and mesons. These particles all have the capability of interacting with each other via the strong force. A circular electron-positron collider with a maximum design energy of about 200 GeV at CERN, Switzerland. A member of the family of weakly interacting particles, which includes the electron, muon, tau, and their associated neutrinos and antiparticles. Leptons are not acted upon by the strong force, but are acted upon by the electroweak and gravitational forces. A measure of the rate at which particles in a collider interact. The larger the luminosity the greater the rate of interaction. Any strongly interacting particle that is not a baryon. Mesons are composed of quark-antiquark combinations. A massless particle that carries the electromagnetic force. A theory that describes the strong force among quarks in a manner similar to the description of the electromagnetic force by quantum electrodynamics. The theory that describes the electromagnetic interaction in the framework of quantum mechanics. The quantum of the electromagnetic force is the photon. The family of elementary particles that make up the hadrons. The quarks are acted upon by the strong, electroweak, and gravitational force Five are known, called up, down, strange, charm, and bottom. A sixth, called top, is expected to exist.
v I. THE^ NATURE^ OF^ ELEMENTARY^ PARTICLES
A. Definition of an Elementary Particle An elementary particle is the simplest and most basic form of matter; it is very small, much smaller than atoms or nuclei. There are three kinds of elementary particles: leptons, quarks, and force-carrying particles also called gauge bosons. The best known example of an elementary particle is the electron which is a lepton. The best known example of a force-carrying particle or gauge boson is the photon which carries the electromagnetic field. A piece of matter is called an elementary particle when it has no other kinds of particles inside of it, and no sub-parts that can be identified. How does one know whether a particle is elementary^?^ Only^ by^ experimenting^ with^ it^ to^ see if it can be broken up, or by studying it to determine if it has an internal structure or parts. This is illustrated in Fig. 1. Molecules^ are^ not^ elementary^ because they can be broken up into atoms by chemical reactions or by heating or by other means. Nor are atoms elementary: they can be broken up into electrons and nuclei by bombarding the atom with other atoms or with light rays. Nor is the nucleus elementary: by bombarding nuclei with high energy particles or with high-energy light rays called gamma rays, the nucleus can also be broken up into protons and neutrons. For about fifty years physicists considered the neutron and proton to be elementary, but in the last two decades it was found that they are made up of yet simpler particles called quarks. Hence the neutron and proton are not elementary particles, but to the best of our knowledge the quarks are elementary. With respect to the other constituent of ordinary matter, the electron, no experiment has succeeded in breaking it up or in finding an internal structure for it. Hence the electron is an elementary particle. B. Size of Elementary Particles -As one proceeds down through the sequence, Fig. 1, of molecule, atom, nu- cleus, proton and neutron, and finally quark and electron, the size of the particles gets smaller and smaller. The size of atoms is of the order of 10s8 centimeters
E=Mc2. (^) (1.1)
velocity of light. Therefore to produce new particles, a large amount of energy,
The second reason for needing high-energy particles is that one investigates a particle size and structure by bombarding it with other particles. And the deeper one wishes to penetrate into a particle, the higher must be the energy of the bombarding particles.
The Heisenberg uncertainty principle also leads to the conclusion that the in- vestigation of small distances requires high-energies. To measure small distances very precisely, there must be a large uncertainty in the momentum associated with that measurement. A large uncertainty in momentum can only be accom- modated by a large initial momentum which requires high-energies. The principal way in which we give high-energy to a particle is to accelerate it by the force of an electric field on the particle’s charge, in a large device called an accelerator (Sec. VI.). A convenient unit for measuring both energy and mass is the electron-volt (eV). Th is is the energy acquired by an electron or proton passing through an electric potential with a total voltage of 1 volt. Larger units are
The significance of these energy units can be appreciated by looking at some particle masses expressed in electron-volts: (1) The electron mass is about 0.5 MeV. (2) The proton mass is about 1 GeV.
(3) The heaviest known particle, the Z”, has a mass of about 100 GeV = 0.1 TeV. The highest effective particle energies produced by existing accelerators are in the range of several hundred GeV. In this article, to simplify the units, we express mass in terms of the equivalent from Eq. 1.1, rather than use the conventional unit eV/c2.
D. The Properties and Types of Elementary Particles Each kind of elementary particle is distinguished by the intrinsic properties of the particle, mass and spin; and by how the particle connects with the basic forces. Table I gives the maseee and spins of the well established elementary particles. The masses have a vast range of values, the photon’s mass is 0, the Z”‘s mass is about 93 GeV. More massive elementary particles may exist; there have not yet been comprehensive searches for these more massive particles because of the energy limitations of existing accelerators. Some particles have a permanent angular momentum called spin angular momentum. In classical physics one can think of spin angular momentum as being due to a particle permanently rotating or spinning about an axis through its center. However this picture can’t be used quantitatively in elementary particle physics because such particles may have zero size. The spin angular momentum
and s is called the spin of the particle. General quantum mechanical principles limit s to the values 0, l/2, 1, 3/2, 2,... (Particles having half-integer spin l/2,
The known elementary particles have spin l/2 or 1. Besides its mass and spin each kind of particle is distinguished by how it connects with the four basic forces: gravitation, electromagnetism, strong force, and weak force. (These forces are described in the next section). For example, particles can interact with the electromagnetic force through their electric charge. Table I gives the electric charge in units of the magnitude of the charge of the
s charge.^ An^ antiparticle^ is indicated^ by^ a bar^ over^ the^ particle^ symbol.^ Thus e = electron , E = positron 6 = bottom quark , i = bottom antiquark
Sometimes e- is used for the electron, e+ is used for the positron and so forth. With respect to neutral particles, theory and experiment show that a particle may have a different neutral antiparticle or it may be its own antiparticle. For example each neutrino has an antiparticle which is different from itself, but the photon is its own antiparticle. The operation which transforms a particle into its antiparticle and vice-versa is called charge conjugation, and is denoted by C.
F. Hadrons Hadrons are subnuclear particles, but they are not elementary particles. To the best of our knowledge, hadrons (Sec. VII) are made of either three quarks or one quark and one antiquark bound together by the strong force. Table II lists a few of the known hadrons. The first hadrons to be discovered were the proton and neutron, now more than a hundred types of hadrons are known. Although hadrons are not in themselves elementary particles, they are never- theless very important in elementary particle physics research. First, we do not know how to isolate quarks, so to do experiments on quarks one must use the quarks in hadrons. Second, hadrons are a fascinating form of matter, and it is interesting to study them in their own right. G. Reactions of Elementary Particles and Hadrons Reactions involving elementary particles or hadrons are represented in the same fashion as are chemical reactions. For example
neutrino and a neutron. In
an electron and proton interact to form four particles. When the two particles in the final state are the same as those in the initial state,
e-.+p+e-+P and e-+p--be--t-P-
for example, it is called an elastic reaction or elastic scattering. All other reactions are called inelastic. The decay of a single particle is similarly represented:
p- + up + e- + Pe
shows how the muon decays; and
is the most frequent way the r- hadron decays. Reaction and decay processes occur through the various basic forces discussed in Sec. II. The reaction equation does not directly indicate which forces are taking part in the reaction. H. Conservation Laws All reactions and decays of elementary particles and hadrons obey a set of laws called conservation laws. The term conservation here means that some quantity does not change, that is, this quantity is conserved, in a reaction. The simplest example of a conserved quantity is the total energy.
A. The Four Basic Forces Elementary particles interact with each other through four basic forces, Table III. Two of these have been known for hundreds of years: the forces of electromag- netism and of gravitation. The other two forces were discovered in the twentieth century. One is the strong or nuclear force that holds the atomic nucleus together, and the other is the weak force that operates in many forms of radioactivity. The forces are described and distinguished from each other by their strengths, by the distance or range over which they act, and by the precise manner in which they act on particles. Here their strength and range properties are described; the precise manner in which the forces act is discussed later in the sections devoted to each force. The most powerful of the four forces is called the strong force (Sec. VII). However, the strong force is not felt directly in everyday phenomena, since it does not extend beyond a distance of about lo-r3 centimeters from the elementary particle. Quarks connect with each other through the strong force, in fact the force is so powerful that the quarks are held together forming small, but non- elementary, particles called hadrons. The proton and neutron are the best known examples. The distance limitation or range of about lo-l3 cm determines the size of the proton or neutron, their radii are about lo-r3 cm. Protons and neutrons
Thus the strong force is sometimes called the nuclear force, but it is more useful to think of the nuclear force as being a manifestation of the strong force. Atoms and molecules are lo-* cm or larger in size, and electrons are not affected by the strong force, hence there is no direct effect of the strong force on the level
the.size and structure of nuclei. The present theory of the strong force is called quantum chromodynamics. The electromagnetic force (Sec.VIII) between elementary particles follows
s the^ same laws^ as the^ electromagnetic^ force^ used in modern^ technology,^ such as in motors, generators, and electronic equipment. The^ elementary^ particles^ simply behave as very small particles with electric charge. If the particle has non-zero spin it will also behave as a very small magnet in relation to the electromagnetic force. (The strength of the equivalent magnetic properties of a particle is ex- pressed by its magnetic moment.) Unlike the strong force, the electromagnetic force extends out to very large distances. The^ strength^ of^ the^ electromagnetic
from the particle increases..
But there is no sharp cutoff as in the strong force. Hence the range of the elec- tromagnetic force is said to be infinite. The electrodynamics force determines the structure and behavior of atoms and molecules. The theory of the electro- magnetic force is called quantum electromagnetics. The weak force (Sec. VIII) acts over very small distances - less than about lo-l6 centimeters - and it is much less powerful than the strong force. Yet the weak force is not negligible. In, a certain sense it is more pervasive than the strong force. Some elementary particles such as leptons and the W* and Z”, are not affected by the strong force but are affected by the weak force. The radioactive decay of the neutron and of nuclei, as well as the decays of many of
weak force is so small, the force is not seen directly in atomic or human scale phenomena. The gravitational force is important in human-scale and astronomical phe- nomena because of the immense mass of the earth, the planets and stars. But the gravitational force exerted by one elementary particle is very small compared with the three other forces that can be exerted by that particle. Indeed, present day experimental methods in particle physics are not sufficiently sensitive to detect the gravitational force exerted by one elementary particle.
The weak force is carried by three different, recently discovered, particles
electric charge, hence they interact with the electromagnetic force; the 2’ being
with the gravitational force, but not with the strong force. Experiment and theory strongly indicate that the strong force is carried by particles called gluons (Sec. VII) and symbolized by 8. However, unlike the
directly studied. Indeed, most forms of the current theory of the strong force, quantum chromodynamics, state that gluons cannot be isolated. Gluons like quarks are said to be confined to being inside hadrons, a subject discussed later. Indirect experimental evidence and current theory give the gluon a zero mass, a spin 1, and zero electric charge. It does not interact with the electromagnetic or weak forces. Inside a hadron, the gluon carries some part of the hadron’s energy,
The particle conjectured to carry the gravitational force has been called the graviton, and ascribed a spin of 2; but such a particle has not yet been discovered, and there is no experimental evidence for its existence. Because of the feebleness of the gravitational interaction among elementary particles, its detection would be extraordinarily difficult. Furthermore there is no successful application of quantum theory to general relativity at present. Therefore the nature of the particle carrying the gravitational force, or even if there is such a particle, is an open question. The mathematical theories which describe the strong, electromagnetic and weak forces obey a general principle called a gauge symmetry, hence they are
intrinsic to those theories and are called gauge bosons; the boson term indicating that their spins are integers.
III. THE LEPTON FAMILY OF ELEMENTARY PARTICLES
A. Definition of a Lepton
The lepton family of elementary particles is defined by two properties: (1) Leptons are affected by the gravitational, electromagnetic, and weak forces, but not by the strong force. (2) Leptons cannot be arbitrarily created or destroyed; all reactions in- volving leptons follow a principal called lepton generation conservation or just lepton conservation. Table IV shows the six known leptons. The tau neutrmo,. u,, has not been detected but there is a great deal of-indirect evidence for its existence and prop- erties. The leptons come in pairs formed according to the lepton conservation principle; each pair consisting of one charged lepton and one neutral lepton, called a neutrino. Each pair is called a generation, and in each generation the mass of the neutrino is much less than the mass of the charged lepton. The generation pairs are formed according to the lepton conservation principle. The usual representation for the lepton pairs is
Each pair has an antiparticle pair, respectively
B. Lepton Conservation The charged lepton and neutrino in each pair show a unique property, called
Their antiparticles, r+ and & have
