B → D ℓ ν ℓ semileptonic form factors from lattice QCD with Möbius domain-wall quarks

We calculate the form factors for the B → D * ℓ ν ℓ decay in 2 + 1 flavor lattice QCD. For all quark flavors, we employ the Möbius domain-wall action, which preserves chiral symmetry to a good precision. Our gauge ensembles are generated at three lattice cutoffs a − 1 ∼ 2.5 , 3.6, and 4.5 GeV with pion masses as low as M π ∼ 230 MeV . The physical lattice size L satisfies the condition M π L ≥ 4 to control finite volume effects (FVEs), while we simulate a smaller size at the smallest M π to directly examine FVEs. The bottom quark masses are chosen in a range from the physical charm quark mass to 0.7 a − 1 to control discretization effects. We extrapolate the form factors to the continuum limit and physical quark masses based on heavy meson chiral perturbation theory at next-to-leading order. Then the recoil parameter dependence is parametrized using a model independent form leading to our estimate of the decay rate ratio between the tau ( ℓ = τ ) and light lepton ( ℓ = e , μ ) channels R ( D * ) = 0.252 ( 22 ) in the Standard Model. A simultaneous fit with recent data from the Belle experiment yields | V c b | = 39.19 ( 91 ) × 10 − 3 , which is consistent with previous exclusive determinations, and shows good consistency in the kinematical distribution of the differential decay rate between the lattice and experimental data.