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1.4 Quarks and Systems of Quarks

The known quarks are listed in Table 1.3. In the Standard Model, quarks, like leptons, are spin 1/2 Dirac fermions, but the electric charges they carry are 2e/3, -e/3. Quarks carry quark number, antiquarks being counted negatively. The net quark number of an isolated system has never been observed to change. However, the number of different types or flavours of quark are not separately conserved: changes are possible through the weak interaction.

Table 1.3. Properties of quarks

Quark Electric charge (e) Mass

Up u 2/3 2-8 Me V
Down d -1/3 5-15 Me V
Charmed c 2/3 1-1.6 Ge V
Strange s -1/3 100-300 Me V
Top t 2/3 168-192 Ge V
Bottom b -1/3 4.1-4.5 Ge V

A difficulty with the experimental investigation of quarks is that an isolated quark has never been observed. Quarks are always confined in compound systems which extend over distances of about 1 fm. The most elementary quark systems are baryons which have net quark number three, and mesons which have net quark number zero. In particular, the proton and neutron are baryons. Mesons are essentially a quark and an antiquark, bound transiently by the strong interaction field. The term hadron is used generically for a quark system.

The proton basically contains two up quarks and one down quark (uud), and the neutron two down quarks and one up (udd). The proton is the only stable baryon. The neutron is a little more massive than the proton, by about 1.3 MeV / c2, and in free space it decays to a proton through the weak interaction: n -> p + e- + nue, with a mean life of about 15 minutes.

All mesons are unstable. The lightest mesons are the pi-mesons or ``pions''. The electrically charged pi+ and pi- are made up of (udbar) and (ubard) pairs, respectively, and the neutral pi0 is either uubar or ddbar, with equal probabilities; it is a coherent superposition (uubar - ddbar) / sqrt2 of the two states. The pi+ and pi- have a mass of 139.57 MeV / c2 and the pi0 is a little lighter, 134.98 MeV / c2. The next lightest meson is the eta(approx 547 MeV / c2), which is the combination (uubar + ddbar) / sqrt2 of quark-antiquark pairs orthogonal to the pi0, with some ssbar component.

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