Semi-leptonic B s → K ν and B s → D s ν decays provide an alternative b-decay channel to determine the CKM matrix elements |V ub | and |V cb | or to obtain R-ratios to investigate lepton flavor universality violations. In addition, these decays may shed further light on the discrepancies seen in the analysis of inclusive vs. exclusive decays. Using the nonperturbative methods of lattice QCD, theoretical results are obtained with good precision and full control over systematic uncertainties. This talk will highlight ongoing efforts of the B-physics program by the RBC-UKQCD collaboration.

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We present updates for our nonperturbative lattice QCD calculations to determine semileptonic form factors for exclusive B → π ν, B → D ν, B s → K ν, and B s → D s ν decays. Our calculation is based on RBC-UKQCD's set of 2 + 1-dynamical-flavor gauge field ensembles. In the valence sector we use domain wall fermions for up/down, strange and charm quarks, whereas bottom quarks are simulated with the relativistic heavy quark action. The continuum limit is based on three lattice spacings. Using kinematical z expansions we aim to obtain form factors over the full q 2 range. These form factors are the basis for predicting ratios addressing lepton flavor universality or, when combined with experimental results, to obtain CKM matrix elements |V ub | and |V cb |. * Speaker. † Combined contribution of both speakers.

Precise theoretical predictions derived from the Standard Model are a key ingredient in searches for new physics in the flavor sector. The large mass and long lifetime of the b quark make processes involving b quarks of particular interest. We use lattice simulations to perform nonperturbative QCD calculations for semileptonic B (s) decays. We present results from our determinations of B s → D s ν and B s → K ν semileptonic form factors and provide an outlook for our B → π ν calculation. In addition we discuss the determination of R-ratios testing lepton-flavor universality and suggest use of an improved ratio. Our calculations are based on the set of 2+1 flavor domain wall Iwasaki gauge field configurations generated by the RBC-UKQCD collaboration featuring three lattice spacings of 1/a = 1.78, 2.38, and 2.79 GeV. Heavy b-quarks are simulated using the relativistic heavy quark action.

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