The decays $\bar{B} \to D^{**}π$ and the Isgur-Wise functions $τ_{1/2}(w)$, $τ_{3/2}(w)$

Abstract

We perform a phenomenological analysis of the decays $B \to D^{**}π$, where $D^{**}$ is a $P$-wave excited meson with total angular momentum $j = {1 \over 2}$ or ${3 \over 2}$ for the light cloud, recently measured by the Belle Collaboration in the modes $\bar{B}^0\to D^{**+}π^-$ (Class I) and $B^- \to D^{**0}π^-$ (Class III). Making the reasonable assumption of naive factorization, that we test in $B \to D(D^*)π$ decays, Class I decays allow to extract the Isgur-Wise form factors $τ_{1/2}(w)$, $τ_{3/2}(w)$ at $w \cong w_{max}$ ($q^2 \cong 0$). We obtain $τ_{1/2}(w_{max}) < 0.20$, $τ_{3/2}(w_{max}) = 0.31 \pm 0.12$. We discuss the question of the $w$ dependence of these IW functions. We find agreement with the Bakamjian-Thomas quark model of form factors and, extrapolating at $w=1$, with Bjorken and Uraltsev sum rules. We discuss also Class III decays, where the $D^{**0}$ $(j = {1 \over 2})$ emission diagram contributes. We extract the corresponding $f_{D_{1/2}}$ decay constant, that is in agreement with theoretical estimates at finite mass. Finally, we must warn that $1/m_Q$ corrections could be large and upset the results of the present stage of this analysis. On the other hand, we confront present data on the semileptonic rate of $B$ mesons to excited states with theoretical expectations.