Scientists at the Belle experiment have made progress in pinning down the value of a physical constant that represents the probability of a W boson mediated transition between bottom quarks and charm quarks. Bottom quarks can also transition to up quarks, and mass-energy conservation permitting, to top quarks.
Previously, different ways of measuring this constant were producing results that weren't very close to each other given the uncertainties of the measurements.
Previous V_cb values from inclusive & exclusive decays were only marginally compatible (3σ tension). The new Vcb exclusive obtained by Belle in this work is "in agreement with inclusive determinations"
We present a measurement of the differential shapes of exclusive B→D∗ℓν¯ℓ (B=B−,B¯0 and ℓ=e,μ) decays with hadronic tag-side reconstruction for the full Belle data set of 711fb−1 integrated luminosity.
We extract the Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor parameters and use an external input for the absolute branching fractions to determine the Cabibbo-Kobayashi-Maskawa matrix element and find |Vcb|CLN=(40.1±0.9)×10−3 and |Vcb|BGL=(40.6±0.9)×10−3 with the zero-recoil lattice QCD point (1)=0.906±0.013. We also perform a study of the impact of preliminary beyond zero-recoil lattice QCD calculations on the |Vcb| determinations.
Additionally, we present the lepton flavor universality ratio Reμ=(B→D∗eν¯e)/(B→D∗μν¯μ)=0.990±0.021±0.023, the electron and muon forward-backward asymmetry and their difference ΔAFB=0.022±0.026±0.007, and the electron and muon D∗ longitudinal polarization fraction and their difference ΔFD∗L=0.034±0.024±0.007. The uncertainties quoted correspond to the statistical and systematic uncertainties, respectively.