B→D* and Bs→Ds* vector, axial-vector and tensor form factors for the full q2 range from lattice QCD

Abstract

We compute the complete set of Standard Model (SM) and tensor B→D*ℓν¯ and Bs→Ds*ℓν¯ semileptonic form factors across the full kinematic range of the decays using second generation MILC nf=2+1+1 highly improved staggered quark (HISQ) gluon field configurations and HISQ valence quarks, with the method. Lattice spacings range from 0.09 to 0.044 fm with pion masses from ≈300 MeV down to the physical value and heavy quark masses ranging between ≈1.5mc and 4.1mc≈0.9mb; currents are normalized nonperturbatively. Using the recent untagged B→D*ℓν¯ℓ data from Belle and Bs→Ds*μν¯μ from LHCb together with our form factors, we determine a model independent value of Vcb=39.03(56)exp(67)latt×10−3, in agreement with previous exclusive determinations and in tension with the most recent inclusive result at the level of 3.6σ. We also observe a ≈1σ tension between the shape of the differential decay rates computed using our form factors and those measured by Belle. We compute a purely theoretical Standard Model value for the ratio of semitauonic and semimuonic decay rates, R(D*)=0.273(15), which we find to be closer to the recent Belle measurement and heavy flavor averaging group average than theory predictions using fits to experimental differential rate data for B→D*ℓν¯ℓ. Determining Vcb from our form factors and the experimental total rate for B→D*ℓν also gives a value in agreement with inclusive results. We also compute the longitudinal polarization fraction for the semitauonic mode, FLD*=0.395(24), which is in tension at the level of 2.2σ with the recent Belle measurement. Our calculation combines B→D* and Bs→Ds* lattice results in a simultaneous chiral continuum extrapolation, maintaining correlations between both modes. We then give results for both B→D* and Bs→Ds*, with the Bs→Ds* results superseding our previous lattice computation. We also give the chiral perturbation theory needed to analyze the tensor form factors. Published by the American Physical Society 2024