Semileptonic meson decays in the quark model: An update.

We present the predictions of ISGW2, an update of the ISGW quark model for semileptonic meson decays. The updated model incorporates a number of features which should make it more reliable, including the constraints imposed by Heavy Quark Symmetry, hyperfine distortions of wave-functions, and form factors with more realistic high recoil behaviors. It has been nearly ten years since the ISGW model [1] was introduced [2,3] so it is not surprising that the heavy quark semileptonic landscape now looks very different. At that time, for both theoretical and experimental reasons, inclusive decays were the main focus of attention, and the ISGW model, which studied exclusive decays and approximated the inclusive semileptonic spectra by summing over resonant channels, was considered quite eccentric. Today, improvements in both theory and experiment have made exclusive semileptonic decays a main focus of attention. Such decays seem very likely to provide the most accurate determinations of the weak mixing angles V cb and V ub. They also provide excellent probes of hadronic structure via precision tests of Heavy Quark Symmetry (HQS) [4-7]. The ISGW model was in many respects a stepping-stone to Heavy Quark Symmetry: it is a model which respects the symmetry in the heavy quark limit near zero recoil. It also played a role in the discussion of the reliability of the free quark decay model (and its derivatives) for the endpoint region in b → u semileptonic decay. Indeed, the model had its origin in that discussion, and was designed to provide the minimum reasonable prediction for the decay rate in this region for a fixed V ub. In this paper we present an updated version of ISGW, which (with the permission of the ISGW authors) we call ISGW2 to emphasize that it is not a new model but rather an improved version of an old one [8]. The new features are described in detail in Section III, but briefly they are: 1. Heavy Quark Symmetry constraints on the relations between form factors away from zero recoil are respected, 2. Heavy Quark Symmetry constraints on the slopes of form factors near zero recoil are built in [9], 3. the naive currents of the quark model are related to the full weak currents via the matching conditions of Heavy Quark Effective Theory (HQET) [6], 3 4. Heavy-Quark-Symmetry-breaking color magnetic interactions are included, whereas ISGW only included the symmetry-breaking due to the heavy quark kinetic …