Determination of |V₂₁| using B Dν_ Decays at Belle II

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element |V₂₁| from the decay B Dν_ using a 365~fb^-1 e^+e^-Υ (4S) B B data sample recorded by the Belle II experiment at the SuperKEKB collider. The semileptonic decay of one B meson is reconstructed in the modes B⁰ D^- (K^+π^-π^-) ^+ν_ and B^+ D⁰ (K^+π^-) ^+ν_, where denotes either an electron or a muon. Charge conjugation is implied. The second B meson in the Υ (4S) event is not reconstructed explicitly. Using an inclusive reconstruction of the unobserved neutrino momentum, we determine the recoil variable w=vB vD, where vB and vD are the 4-velocities of the B and D mesons. We measure the total decay branching fractions to be B (B⁰ D^-^+ν_) = (2. 06 0. 05\, (stat. ) 0. 10\, (sys. ) ) \% and B (B^+ D⁰^+ν_) = (2. 31 0. 04\, (stat. ) 0. 09\, (sys. ) ) \%. We probe lepton flavor universality by measuring B (B Deνₑ) /B (B Dμν_μ) =1. 020 0. 020\, (stat. ) 0. 022\, (sys. ). Fitting the partial decay branching fraction as a function of w and using the average of lattice QCD calculations of the B D form factor, we obtain |V₂₁|= (39. 2 0. 4\, (stat. ) 0. 6\, (sys. ) 0. 5\, (th. ).