Role of $B_c^+ \to B_{s,d}^{(*)} \, \bar \ell \, \nu_\ell$ in the Standard Model and in the search for BSM signals

TL;DR

This paper analyzes $B_c^+$ decays to $B_{s,d}^{(*)}$ with leptons within the Standard Model and beyond, using heavy quark symmetry and lattice QCD to identify potential signals of new physics and test lepton universality.

Contribution

It provides a comprehensive study of $B_c^+$ decay modes in the SM and BSM frameworks, incorporating lattice QCD and heavy quark symmetry to identify observables sensitive to new physics.

Findings

Identification of optimized observables for new physics detection.

Analysis of correlations among observables to distinguish operator effects.

Assessment of $ ext{mu/e}$ universality tests in $B_c^+$ decays.

Abstract

The decays $B_c^+ \to B_{a} \bar \ell \nu_\ell$ and $B_c^+ \to B_{a}^{*}(\to B_a \gamma) \bar \ell \nu_\ell$, with $a=s,d$ and $\ell=e,\mu$, are studied in the Standard Model (SM) and in the extension based on the low-energy Hamiltonian comprising the full set of dimension-$6$ semileptonic $c \to s,d$ operators with left-handed neutrinos. Tests of $\mu/e$ universality are investigated using such modes. The heavy quark spin symmetry is applied to relate the relevant hadronic matrix elements and to exploit lattice QCD results on $B_c$ form factors. Optimized observables are selected, and the pattern of their correlations is studied to identify the effects of the various operators in the extended low-energy Hamiltonian.