Quantum Physics

30.12 Quarks 995charm C 1, its antiquark would have a charm C 1, and all other quarkswould have C 0, as indicated in Table 30.3. Charm, like strangeness, would beconserved in strong and electromagnetic interactions but not in weak interactions.In 1974 a new heavy meson called the J/ particle (or simply, ) was discoveredindependently by a group led by Burton Richter at the Stanford Linear Accelerator(SLAC) and another group led by Samuel Ting at the Brookhaven NationalLaboratory. Richter and Ting were awarded the Nobel Prize in 1976 for this work.The J/ particle didn’t fit into the three-quark model, but had the properties of acombination of a charmed quark and its antiquark ( cc). It was much heavier thanthe other known mesons (3 100 MeV/c 2 ) and its lifetime was much longer thanthose of other particles that decay via the strong force. In 1975, researchers atStanford University reported strong evidence for the existence of the tau ()lepton, with a mass of 1 784 MeV/c 2 . Such discoveries led to more elaborate quarkmodels and the proposal of two new quarks, named top (t) and bottom (b). Todistinguish these quarks from the old ones, quantum numbers called topness andbottomness were assigned to these new particles and are included in Table 30.3. In1977 researchers at the Fermi National Laboratory, under the direction of LeonLederman, reported the discovery of a very massive new meson with compositionbb. In March of 1995, researchers at Fermilab announced the discovery of thetop quark (supposedly the last of the quarks to be found) having mass 173 GeV/c 2 .You are probably wondering whether such discoveries will ever end. How many“building blocks” of matter really exist? The numbers of different quarks and leptonshave implications for the primordial abundance of certain elements, so atpresent it appears there may be no further fundamental particles. Some propertiesof quarks and leptons are given in Table 30.5.Despite extensive experimental efforts, no isolated quark has ever been observed.Physicists now believe that quarks are permanently confined inside ordinaryparticles because of an exceptionally strong force that prevents them from escaping.This force, called the color force (which will be discussed in Section 30.13), increaseswith separation distance (similar to the force of a spring). The greatstrength of the force between quarks has been described by one author as follows: 2Mesonsπ+uuK _dsBaryonspACTIVE FIGURE 30.10Quark compositions of two mesonsand two baryons. Note that themesons on the left contain twoquarks, and the baryons on the rightcontain three quarks.Log into PhysicsNow at www.cp7e.comand go to Active Figure 30.10 toobserve the quark compositions forthe mesons and baryons.uunddudQuarks are slaves of their own color charge, . . . bound like prisoners of a chaingang. . . . Any locksmith can break the chain between two prisoners, but no locksmith isexpert enough to break the gluon chains between quarks. Quarks remain slaves forever.TABLE 30.5The Fundamental Particles and Some of Their PropertiesParticle Rest Energy ChargeQuarksudcstb360 MeV360 MeV1500 MeV540 MeV173 GeV5 GeV 2 3 e 1 3 e 2 3 e 1 3 e 2 3 e 1 3 eLeptonse 511 keV e 107 MeV e 1784 MeV e e 30 eV 0 0.5 MeV 0 250 MeV 02 Harald Fritzsch, Quarks: The Stuff of Matter (London: Allen Lane, 1983).