B_s → D_sℓν form factors and the fragmentation fraction ratio f_s/f_d

摘要

We present a lattice quantum chromodynamics determination of the scalar and vector form factors for the B_s → D_sℓν decay over the full physical range of momentum transfer. In conjunction with future experimental data, our results will provide a new method to extract |V_(cb)|, which may elucidate the current tension between exclusive and inclusive determinations of this parameter. Combining the form factor results at nonzero recoil with recent HPQCD results for the B → Dlν form factors, we determine the ratios f_0~(B_s→D_s)(M_π~2)/f_0~(B→D) (M_K~2) = 1.000(62) and f_0~(B_s→D_s)(M(M_π~2) = 1.006(62). These results give the fragmentation fraction ratios f_s/f_d = 0.310(30)_(stat)(21)_(syst)(6)_(theor)(38)_(latt) and f_s/f_d = 0.307(16)_(stat)(21)_(syst)(23)_(theor) (44)_(latt), respectively. The fragmentation fraction ratio is an important ingredient in experimental determinations of Bs meson branching fractions at hadron colliders, in particular for the rare decay B(B_s →μ~+μ ~-). In addition to the form factor results, we make the first prediction of the branching fraction ratio R(D_S) = B(Bs → D_sτν)/B(B_s → D_sℓv) = 0.301(6), where ℓ is an electron or muon. Current experimental measurements of the corresponding ratio for the semileptonic decays of B mesons disagree with Standard Model expectations at the level of nearly four standard deviations. Future experimental measurements of R(D_s) may help understand this discrepancy.