Comparison of exclusive decay rates for b-->u and b-->c transitions.

Abstract

We present a variety of estimates for nonleptonic and semileptonic few-body decays of the B meson which indicate that b\ensuremath{\rightarrow}u transitions tend to produce multibody final states. This implies that channels such as B\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi} or B\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\rho} offer little chance to improve the current upper bound on the b\ensuremath{\rightarrow}u Kobayashi-Maskawa matrix element. Also the leptons in b\ensuremath{\rightarrow}u semileptonic transitions tend to be emitted together with massive multibody states rather than a single \ensuremath{\pi} or \ensuremath{\rho}. This makes the b\ensuremath{\rightarrow}u spectrum somewhat softer and the task of distinguishing such leptons from B\ensuremath{\rightarrow}Dl\ensuremath{\nu}, say, more difficult.