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

TL;DR

This paper investigates the composition of excited charm states in B meson decays to address the 1/2 vs 3/2 puzzle, combining lattice QCD form factor calculations with phenomenological insights to improve understanding of V_cb determinations.

Contribution

It presents initial lattice QCD calculations of B to D** form factors at realistic quark masses and discusses new phenomenological approaches involving hadronic and semileptonic decays.

Findings

Lattice form factors for B to D** transitions are being computed at realistic quark masses.

Phenomenological studies of B to D** pi and B_s to D_s** l nu decays offer new insights into the puzzle.

The work highlights the importance of lattice inputs for resolving the 1/2 vs 3/2 puzzle.

Abstract

Understanding the composition of final states in $B \to X_c l \nu$ 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 ($D' \to D \pi$) spectrum tail could have induced an experimental overestimate of the broad states contribution to the total $B \to X_c l \nu$ 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 $B \to D^{**} l \nu$ at realistic quark masses. Cleaner processes, like hadronic decays $B \to D^{**} \pi$ and semileptonic decays $B_s \to D_s^{**} l \nu$ in the strange sector have recently been examined by phenomenologists, putting new interesting ideas on those issues with, again, the need of lattice inputs.