$B \to D^{**}$ -- puzzle 1/2 vs 3/2

Abstract

Understanding the composition of final states in $B \to X_c l ν$ could help to get a feedback on the persisting disagreement between exclusive and inclusive determinations of $V_{cb}$. In particular the series of orbital excitations $D^{**}$ and radial excitations ($D'$, $D^*\,'$) has received a lot of attention; a misinterpretation as a scalar state of the ($D' \to D π$) spectrum tail could have induced an experimental overestimate of the broad states contribution to the total $B \to X_c l ν$ width with respect to theoretical expectations, all of them made however in the infinite mass limit: it is the so-called 1/2 vs 3/2 puzzle. We describe first attempts to measure on the lattice form factors of $B \to D^{**} l ν$ at realistic quark masses. Cleaner processes, like hadronic decays $B \to D^{**} π$ and semileptonic decays $B_s \to D_s^{**} l ν$ in the strange sector have recently been examined by phenomenologists, putting new interesting ideas on those issues with, again, the need of lattice inputs.

Proceedings of the 8th International Workshop on the CKM Unitarity Triangle (CKM 2014), Vienna, Austria, September 8-12, 2014