Determination of |Vcb| using B→ Dν_ Decays at Belle II

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element |Vcb| 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 |Vcb|=(39.2± 0.4\,(stat.) ± 0.6\,(sys.) ± 0.5\,(th.).