Quarks with Color and Flavor

QUARKS WITH COLOR AND FLAVOR The particles called quarks may be truly elementary. Their "colors" explain why they cannot be isolated; their "flavors" distinguish four basic kinds, including one that has the property called charm by Sheldon Lee Glashow the Greek root of "atom," Several recent experimental findings, in tric charge, they "feel" the electromag omos,means indivisible, and it was once cluding the discovery last fall of the par netic force, which is roughly 100 times K thought that atoms were the ulti ticles called J or psi, can be interpreted weaker than the strong force. The two mate, indivisible constituents of matter, as supporting the charm hypothesis. kinds of neutrino, which have no electric that is, they were regarded as elementary The basic notion that some subatomic charge, feel neither the strong force nor particles. One of the principal achieve particles are made of quarks has gained the electromagnetic force, but interact ments of physics in the 20th century has Widespread acceptance, even in the ab solely through a third kind of force, been the revelation that the atom is not sence of direct observational evidence. weaker by several orders of magnitude indivisible or elementary at all but has a The more elaborate theory incorporating and called the weak force. The strong complex structure. In 1911 Ernest Ruth color and charm remains much more force, the electromagnetic force and the erford showed that the atom consists of speculative. The views presented here weak force, together with gravitation, a small, dense nucleus surrounded by a are my own, and they are far from being are believed to account for all interac cloud of electrons. It was subsequently accepted dogma. On the other hand, a tions of matter. revealed that the nucleus itself can be growing body of evidence argues that The leptons give every indication of broken down into discrete particles, the these novel concepts must play some being elementary particles. The electron, protons and neutrons, and since then a part in the description of nature. They for example, behaves as a point charge, great many related particles have been help to bring together many seemingly and even when it is probed at the ener identified. During the past decade it has unrelated theoretical developments of gies of the largest particle accelerators, become apparent that those particles too the past years to form an elegant pic no internal structure can be detected. 15 are complex rather than elementary. ture of the structure of matter. Indeed, The hadrons, on the other hand, seem They are now thought to be made up of quarks are at once the most rewarding complex. They have a measurable size: the simpler things called quarks. A soli and the most mystifying creation of about centimeter. Moreover, there 10-13 tary quark has never been observed, in modern particle physics. They are re are hundreds of them, all but a handful spite of many atteinpts to isolate one. markably successful in explaining the discovered in the past 25 years. Finally, Nonetheless, there are excellent grounds structure of subatomic particles, but we all the hadrons, with the significant ex for believing they do exist. More impor cannot yet understand why they should ception of the proton and the antiproton, tant, quarks may be the last in the long be so successful. are unstable in isolation. They decay series of progressively finer structures. into stable particles such as protons, elec They seem to be truly elementary. he particles thought to be made up trons, neutrinos or photons. (The photon, When the quark hypothesis was first Tof quarks form the class called the which is the carrier of the electromag proposed more than 10 years ago, there hadrons. They are the only particles that netic force, is in a category apart; it is were supposed to be three kinds of interact through the "strong" force. In neither a lepton nor a hadron.) quark. The revised version of the theory cluded are the protons and neutrons, and The hadrons are subdivided into three I shall describe here requires 12 kinds. indeed it is the strong force that binds families: baryons, antibaryons and me In the whimsical terminology that has protons and neutrons together to form sons. The baryons include the proton evolved for the discussion of quarks atomic nuclei. The strong force is also and the neutron; the mesons include they are said to come in four flavors, and responsible for the rapid decay of many such particles as the pion. Baryons can each flavor is said to come in three colors. hadrons. be neither created nor destroyed except ("Flavor" and "color" are, of course, ar Another class of particles, defined in as pairs of baryons and antibaryons. bitrary labels; they have no relation to distinction to the hadrons, are the lep This principle defines a conservation the usual meanings of those words.) One tons. There are just four of them: the law, and it can be treated most con of the quark flavors is distinguished by electron and the electron neutrino and veniently in the system of bookkeeping the property called charm (another ar the muon and the muon neutrino (and that assigns simple numerical values, bitrary term). The concept of charm was their four antiparticles). The leptons are called quantum numbers, to conserved suggested in 1964, but until last year it not subject to the strong force. Because properties. In this case the quantum had remained an untested conjecture. the electron aI1d the muon bear an elec- number is called baryon number. For 38 © 1975 SCIENTIFIC AMERICAN, INC baryons it is + for antibaryons and crete values of angular momentum. The tistics (and are called fermions). In this 1, -1 for mesons The conservation of bary angular momentum is measured in units context "statistics" refers to the behavior O. on number then reduces to the rule that of h/2."., where h is Planck's constant, of a population of identical particles. in any interaction the sum of the baryon equal to about erg-second. Those that obey Bose-Einstein statistics 6.6 X 10-27 numbers cannot change. Baryons are particles with a spin angu can be brought together without restric Baryon number provides a means of lar momentum measured in half-integral tion; an unlimited number of pions, for distinguishing baryons from mesons, but units, that is, with values of half an odd example, can occupy the same state. The it is an artificial means, and it tells us integer, such as 1/2 or 3/2. Mesons have Fermi-Dirac statistics, on the other hand, nothing about the properties of the two integral values of spin angular momen require that no two particles within a kinds of particle. A more meaningful tum, such as or given system have the same energy and 0 l. distinction can be established by exam The difference in spin angular mo be identical in all their quantum num ining another quantum number, spin mentum has important consequences for bers. This statement is equivalent to the angular momentum. the behavior of the two kinds of hadron. exclusion principle formulated in 1925 Under the rules of quantum mechan Particles with integral spin are said to by Wolfgang Pauli. He applied it in ics a particle or a system of particles can obey Bose-Einstein statistics (and are particular to electrons, which have a assume only certain specified states of therefore called bosons). Those with spin of 1/2 and are therefore fermions. rotation, and hence can have only dis- half-integral spin obey Fermi-Dirac sta- It requires that each energy level an in 2 3 4 d• • CONFINEMENT OF QUARKS in known subnuclear particles can tential energy of the system rapidly increases and would reach be explained if the quarks are assumed to come in three varieties, enormous values if another process did not intervene: from the arbitrarily designated by colors. A baryon (such as a proton or a potential energy a quark and an antiquark are created The (3). neutron) is a bound system of three quarks, including one of each new quark restores the baryon to its original form, and the anti of the three colors so that the baryon as a whole is colorless. quark (open circle) adheres to the dislodged quark, forming an· (1), The quarks are bound together by the strong force, w/lich also other kind of particle, a meson At any one moment the quark (4). holds together atomic nuclei. The force between colored quarks, and the antiquark in a meson are the same color, but the three however, differs in character from that between colorless composite colors are equally represented. Because of the nature of the strong particles: it does not diminish with distance but remains constant. force, solitary quarks cannot be observed; any attempt to isolate As a result, when a quark is separated from a baryon (2), the po- a quark merely results in the creation of a new hadron or hadrons. 39 © 1975 SCIENTIFIC AMERICAN, INC atom contain only two electrons, with states: + 1, 0 and In the strong inter the 19th century by the Norwegian -l. their spins aligned in opposite directions. actions the members of a multiplet are mathematician Sophus Lie. The Lie One of the clues to the complex nature all equivalent, since electric charge plays group that generates the eightfold way of the hadrons is that there are so many no role in the strong interactions. is called SU(3), which stands for the spe of them. Much of the endeavor to un In 1962 a grander order was revealed cial unitary group of matrices of size derstand them has consisted of a search when the charge multiplets were orga 3 3.

Load more