Quantum Physics

994 Chapter 30 Nuclear Energy and Elementary ParticlesTABLE 30.4Quark Compositionof Several HadronsParticle K K K 0pn 0 0 0 QuarkCompositionMesonsBaryonsduudsuussduududdudsuusudsddsussdsssssTABLE 30.3Properties of Quarks and AntiquarksQuarksBaryonName Symbol Spin Charge Number Strangeness Charm Bottomness Topness11Up u 2 2 0 0 0 01Down d 1 3 e 3120 0 0 01Strange s 1 3 e 3121 0 0 01Charmed c 2 3 e 31230 1 0 011Bottom b2 1 3 e30 0 1 011Top t 2 3 e3 e0 0 0 12AntiquarksBaryonName Symbol Spin Charge Number Strangeness Charm Bottomness TopnessAnti-up1u 2 2 3 e 1 3 0 0 0 0Anti-down1d 2 1 3 e 1 3 0 0 0 0Anti-strange1s 2 1 3 e 1 3 1 0 0 0Anti-charmed1c 2 2 3 e 1 3 0 1 0 01Anti-bottom b 1 1 20 0 1 01Anti-top t 2 3 e 33 e0 0 0 123 1 31. Mesons consist of one quark and one antiquark, giving them a baryon numberof 0, as required.2. Baryons consist of three quarks.3. Antibaryons consist of three antiquarks.Table 30.4 lists the quark compositions of several mesons and baryons. Note thatjust two of the quarks, u and d, are contained in all hadrons encountered in ordinarymatter (protons and neutrons). The third quark, s, is needed only to constructstrange particles with a strangeness of either 1 or 1. Active Figure 30.10is a pictorial representation of the quark compositions of several particles.Applying Physics 30.3We have seen a law of conservation of lepton numberand a law of conservation of baryon number. Why isn’tthere a law of conservation of meson number?Explanation We can argue this from the point ofview of creating particle–antiparticle pairs from availableenergy. If energy is converted to the rest energyof a lepton–antilepton pair, then there is no netchange in lepton number, because the lepton has alepton number of 1 and the antilepton 1. Energycould also be transformed into the rest energy of aConservation of Meson Number?baryon–antibaryon pair. The baryon has baryon number1, the antibaryon 1, and there is no netchange in baryon number.But now suppose energy is transformed into therest energy of a quark–antiquark pair. By definition inquark theory, a quark–antiquark pair is a meson.There was no meson before, and now there’s a meson,so already there is violation of conservation of mesonnumber. With more energy, we can create moremesons, with no restriction from a conservation lawother than that of energy.Charm and Other Recent DevelopmentsAlthough the original quark model was highly successful in classifying particlesinto families, there were some discrepancies between predictions of the model andcertain experimental decay rates. Consequently, a fourth quark was proposed byseveral physicists in 1967. The fourth quark, designated by c, was given a propertycalled charm. A charmed quark would have the charge 2e/3, but its charmwould distinguish it from the other three quarks. The new quark would have a