Role of Bc+→Bs,d(*)ℓ¯νℓ in the Standard Model and in the search for BSM signals

The decays ${B}_{c}^{+}\ensuremath{\rightarrow}{B}_{a}\overline{\ensuremath{\ell}}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ and ${B}_{c}^{+}\ensuremath{\rightarrow}{B}_{a}^{*}(\ensuremath{\rightarrow}{B}_{a}\ensuremath{\gamma})\overline{\ensuremath{\ell}}{\ensuremath{\nu}}_{\ensuremath{\ell}}$, with $a=s$, $d$ and $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, 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 $c\ensuremath{\rightarrow}s,d$ operators with left-handed neutrinos. Tests of $\ensuremath{\mu}/e$ 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.