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"
The paper is here. It also shows no lepton universality violations. The abstract of the paper is as follows:
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.
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