2 - Heavy quarks
Published online by Cambridge University Press: 19 October 2009
Summary
The light u, d, and s quarks have masses mq that are small compared to the scale of nonperturbative strong dynamics. Consequently, it is a good approximation to take the mq → 0 limit of QCD. In this limit QCD has an SU(3)L × SU(3)R chiral symmetry, which can be used to predict some properties of hadrons containing these light quarks. For quarks with masses mQ that are large compared with the scale of nonperturbative strong dynamics, it is a good approximation to take the mQ → ∞ limit of QCD. In this limit QCD has spin-flavor heavy quark symmetry, which has important implications for the properties of hadrons containing a single heavy quark.
Introduction
The QCD Lagrangian in Eq. (1.82) describes the strong interactions of light quarks and gluons. As discussed in Sec. 1.4, there is a nonperturbative scale ΛQCD that is dynamically generated by QCD. A color singlet state, such as a meson made up of a quark–antiquark pair, is bound by the nonperturbative gluon dynamics. If the quarks are light, the typical size of such a system is of the order of Λ−1QCD. Consider a Q meson that contains a heavy quark with mass mQ ≫ ΛQCD, and a light quark with mass mq ≪ ΛQCD. Such a heavy-light meson also has a typical size of the order of Λ−1QCD, as for mesons containing only light quarks. The typical momentum transfer between the heavy and light quarks in the Q meson arising from nonperturbative QCD dynamics is of the order of ΛQCD.
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- Heavy Quark Physics , pp. 44 - 76Publisher: Cambridge University PressPrint publication year: 2000