Quantum Physics

30.12 Quarks 993n pΣ _ Σ 0 Σ + S = _ 1Ξ _ Ξ 0S = _ 2Q = +1Q = _ 1 Q = 0(a)Λ 0 S = 0K 0 K +K _ 0K 0 S = +1S = 0S = _ 1π _ η π π +η'Q = _ 1 Q = 0(b)Q = +1Figure 30.9 (a) The hexagonaleightfold-way pattern for the eightspin 1 2 baryons. This strangeness versuscharge plot uses a horizontal axisfor the strangeness values S, but asloping axis for the charge number Q.(b) The eightfold-way pattern for thenine spin-zero mesons.are at its center. These and related symmetric patterns, called the eightfold way,were proposed independently in 1961 by Murray Gell-Mann and Yuval Ne’eman.The groups of baryons and mesons can be displayed in many other symmetricpatterns within the framework of the eightfold way. For example, the family ofspin-3/2 baryons contains ten particles arranged in a pattern like the tenpins in abowling alley. After the pattern was proposed, one of the particles was missing—ithad yet to be discovered. Gell-Mann predicted that the missing particle, which hecalled the omega minus ( ), should have a spin of 3/2, a charge of 1, a strangenessof 3, and a mass of about 1 680 MeV/c 2 . Shortly thereafter, in 1964, scientistsat the Brookhaven National Laboratory found the missing particle throughcareful analyses of bubble chamber photographs and confirmed all its predictedproperties.The patterns of the eightfold way in the field of particle physics have much incommon with the periodic table. Whenever a vacancy (a missing particle or element)occurs in the organized patterns, experimentalists have a guide for their investigations.30.12 QUARKSAs we have noted, leptons appear to be truly elementary particles because theyhave no measurable size or internal structure, are limited in number, and do notseem to break down into smaller units. Hadrons, on the other hand, are complexparticles with size and structure. Further, we know that hadrons decay into otherhadrons and are many in number. Table 30.2 lists only those hadrons that are stableagainst hadronic decay; hundreds of others have been discovered. These factsstrongly suggest that hadrons cannot be truly elementary but have some substructure.The Original Quark ModelIn 1963 Gell-Mann and George Zweig independently proposed that hadrons havean elementary substructure. According to their model, all hadrons are compositesystems of two or three fundamental constitutents called quarks, which rhymeswith “forks” (though some rhyme it with “sharks”). Gell-Mann borrowed the wordquark from the passage “Three quarks for Muster Mark” in James Joyce’s bookFinnegans Wake. In the original model there were three types of quarks designatedby the symbols u, d, and s. These were given the arbitrary names up, down, andsideways (or, now more commonly, strange).An unusual property of quarks is that they have fractional electronic charges, asshown—along with other properties—in Table 30.3 (page 994). Associated witheach quark is an antiquark of opposite charge, baryon number, and strangeness.The compositions of all hadrons known when Gell-Mann and Zweig presentedtheir models could be completely specified by three simple rules:Photo courtesy of Michael R. DresslerMURRAY GELL-MANN,American Physicist (1929–)Gell-Mann was awarded the Nobel Prizein 1969 for his theoretical studies dealingwith subatomic particles.