Universal Isgur-Wise form factors from QCD sum rules in HQET.pdf

a r X i v : h e p - p h / 9 8 1 0 4 7 8 v 1 2 6 O c t 1 9 9 8 1 Universal Isgur-Wise form factors from QCD sum rules in HQET P. Colangeloa?, F. De Fazioa and N. Paverb a Istituto Nazionale di Fisica Nucleare - Sezione di Bari, via Amendola n.173, 70126 Bari, Italy b Dipartimento di Fisica Teorica dell’Universita? di Trieste and Istituto Nazionale di Fisica Nucleare - Sezione di Trieste, Strada Costiera 11, 34014 Trieste, Italy We review the role of the universal Isgur-Wise functions parameterizing the B meson semileptonic matrix elements to excited charm states in the infinite heavy quark mass limit. We also discuss the determination of such form factors by QCD sum rules in the framework of the Heavy Quark Effective Theory. 1. UNIVERSAL FORM FACTORS The heavy quark flavor and spin symmetry of QCD, exactly valid in the infinite mQ limit, has important consequences on the spectrum, the static properties and both the strong and weak decay matrix elements of the hadrons contain- ing a single heavy quark Q [1–3]. Notably, in such heavy quark effective theory (HQET), for mQ → ∞ the states can be classified in degen- erate doublets by the total angular momentum ~J and by the angular momentum of the light de- grees of freedom (quarks and gluons) ~s? = ~J?~sQ, and the semileptonic matrix elements between members of the doublets identified by s? and s?′ can be expressed in terms of “universal” form fac- tors which are functions of y = v·v′ (with v and v′ the initial and final hadron four-velocities). This is a well known result for B → D(?)?ν?: the pseu- doscalar B,D and vector B?, D? mesons have sP? = 1 2 ? , and the semileptonic matrix elements can be written in terms of a single universal form factor, the Isgur-Wise function ξ(y). The heavy flavour symmetry requires that such form factor is normalized to unity at the zero recoil point y = 1. In the case of transitions between members belonging to different HQET multiplets, addi- tional form factors are introduced. An