The Structure of Quarks and Leptons They have been , considered the elementary particles ofmatter, but instead they may consist of still smaller entities confjned within a volume less than a thousandth the size of a proton by Haim Harari n the past 100 years the search for the the quark model that brought relief. In imagination: they suggest a way of I ultimate constituents of matter has the initial formulation of the model all building a complex world out of a few penetrated four layers of structure. hadrons could be explained as combina­ simple parts. All matter has been shown to consist of tions of just three kinds of quarks. atoms. The atom itself has been found Now it is the quarks and leptons Any theory of the elementary particles to have a dense nucleus surrounded by a themselves whose proliferation is begin­ fl. of matter must also take into ac­ cloud of electrons. The nucleus in turn ning to stir interest in the possibility of a count the forces that act between them has been broken down into its compo­ simpler-scheme. Whereas the original and the laws of nature that govern the nent protons and neutrons. More recent­ model had three quarks, there are now forces. Little would be gained in simpli­ ly it has become apparent that the pro­ thought to be at least 18, as well as six fying the spectrum of particles if the ton and the neutron are also composite leptons and a dozen other particles that number of forces and laws were thereby particles; they are made up of the small­ act as carriers of forces. Three dozen increased. As it happens, there has been er entities called quarks. What comes basic units of matter are too many for a subtle interplay between the list of par­ next? It is entirely possible that the pro­ the taste of some physicists, and there is ticles and the list of forces throughout gression of orbs within orbs has at last no assurance that more quarks and'lep­ the history of physics. reached an end and that quarks cannot tons will not be discovered. Postulating In about 1800 four forces were be more finely divided. The leptons, the a still deeper level of organization is per­ thought to be fundamental: gravitation, class of particles that includes the elec­ haps the most straightforward way to electricity, magnetism and the short­ tron, could also be elementary and indi­ reduce the roster. All the quarks and range force between molecules that is visible. Some physicists, however, are leptons would then be composite ob­ responsible for the cohesion of matter. not at all sure the innermost kernel of jects, just as atoms and hadrons are, and A series of remarkable experimental matter has been exposed. They have be­ would owe their variety to the number and theoretical discoveries then led to gun to wonder whether the quarks and of ways a few smaller constituents can the recognition that electricity and mag­ leptons too might .not have some inter­ be brought together. The currently ob­ netism are actually two manifestations nal composition. served diversity of nature would be not of the same basic force, which was soon The main impetus for considering still intrinsic but combinatorial. given the name electromagnetism. The another layer of structure is the convic­ It should be emphasized that as yet discovery of atomic structure brought tion (or perhaps prej udice) that there there is no evidence q uar ks and leptons a further revision. Although an atom is should be only a few fundamental build­ have an internal structure of any kind. ing blocks of matter. Economy of means In the case of the leptons, experiments ATOM has long been a guiding. principle of have probed to within 10-16 centimeter physics, and it has served well up to and found nothing to contradict the as­ /' now. The list of the basic constituents sumption that leptons are pointlike and /' / of matter first grew implausibly long structureless. As ·for the quarks, it has /' /' toward the end of the 19th century, not been possible to examine a quark in ,.. ,.. when the number of chemical elements, isolation, much less to discern any possi­ ,.. and hence the number of species of at­ ble internal features. Even as a strictly • oms, was approaching 100. The resolu­ theoretical conception, the subparticle NUCLEUS tion of atomic structure solved the prob­ idea has run into difficulty: no one has lem, and in about 1935 the number of been able to devise a consistent descrip­ " elementary particles stood at four: the tion of how the subparticles might move " proton, the' neutron, the electron and inside a quark or a lepton and how they x 10,000 the neutrino. This parsimonious view of might interact with one another. They the world was spoiled iruthe 1950's and _' would have, to_ be. almost unimaginably 1960's;, it turned out that tBe proton and small: if an atom were magnified to the the neutron are representatives ofa very size of the earth, its innermost constitu­ 10-8 CENTIMETER large family of particles, the family now ents could be no larger than a grapefruit. called hadrons. By the mid-1960's the Nevertheless, models of quark and lep­ HIERARCHY OF PARTICLES in the struc­ number of fundamental forms of matter ton substructure make a powerful ap­ ture of matter currently has four levels. All was again roughly 100. This time it was peal to the aesthetic sense and to the matter is made up of atoms; the atom con- 56 © 1983 SCIENTIFIC AMERICAN, INC electrically neutral overall, its constitu­ near, the colored quarks in one proton particles, and it is not understood in ents are charged, and the short-range "see" the color charges in the other pro­ terms of microscopic events. For the molecular force came to be understood ton. The short-range attractions and re­ other three forces successful theories as a complicated residual effect of elec­ pulsions that result have been identified have been developed and are now wide­ tromagnetic interactions of positive nu­ with the effects of the strong force. In ly accepted. The three theories are dis­ clei and negative electrons. When two other words, just as the short-range mo­ tinct, but they are consistent with one neutral atoms are far apart, there are lecular force became a residue of the another; taken together they constitute practically no electromagnetic forces long-range electromagnetic force, so the a comprehensive model of elementary between them. When they are near each short-range strong force has become a particles and their interactions, which I other, however, the charged constitu­ residue of the long-range color force. shall refer to as the standard model. ents of one atom are able to "see" and One more chapter can be added to this In the standard model the indivisible influence the inner charges of the other, abbreviated history of the forces of na­ constituents of matter are the quarks leading to various short-range attrac­ ture. A deep and beautiful connection and the leptons. It is convenient to dis­ tions and repulsions. has been found between electromagnet­ cuss the leptons first. There are six of As a result of these developments ism and the weak force, bringing them them: the electron and its companion physics was left with only two basic almost to the point of full unification. the electron-type neutrino, the muon forces. The unification of electricity and They are clearly related, but the connec­ and the muon-type neutrino and the magnetism had reduced the number by tion is not quite as close as it is in the tau and the tau-type neutrino. The elec­ one, and the molecular interaction had case of electricity and magnetism, and tron, the muon and the tau have an been demoted from the rank of a funda­ so they must still be counted as sepa­ electric charge of -1; the three neu­ mental force to that of a derivative one. rate forces. Therefore the current list trinos are electrically ne u tral. The two remaining fundamental for­ of fundamental forces still has four en­ There are also six basic kinds of ces, gravitation and electromagnetism, tries: the long-range gravitational, elec­ quark, which have been given the names were both long-range. The exploration tromagnetic and color forces and the up, down, charmed, strange, top and of nuclear structure, however, soon in­ short-range weak force. Within the lim­ bottom, or u, d, C, s, t and b. (The top trod uced two new short-range forces. its of present knowledge all natural quark has not yet been detected experi­ The strong force binds protons and neu­ phenomena can be understood through mentally, and neither has the tau-type trons together in the nucleus, and the these forces and their resid ual effects. neutrino, but few theorists doubt their weak force mediates certain transfor­ The evolution of ideas about particles existence.) The u, C and t quarks have an mations of one particle into another, as and that of ideas about forces are clearly electric charge of +2/3, the d, sand b in the beta decay of a radioactive nucle­ interdependent. As new basic particles quarks a charge of -1/3. In addition us. Thus there were again four forces. are found, old ones turn out to be com­ each quark type has three possible col­ The development of the quark model posite objects. As new forces are discov­ ors, which I shall designate red, yellow and the accompanying theory of quark ered, old ones are unified or red uced to and blue.