Measurement of the CKM Matrix Element $|V_{cb}|$ from $B^{0} \to D^{*-} \ell^+ ν_\ell$ at Belle

Abstract

We present a new measurement of the CKM matrix element $|V_{cb}|$ from $B^{0} \to D^{*-} \ell^+ ν_\ell$ decays, reconstructed with the full Belle data set of $711 \, \rm fb^{-1}$ integrated luminosity. Two form factor parameterizations, originally conceived by the Caprini-Lellouch-Neubert (CLN) and the Boyd, Grinstein and Lebed (BGL) groups, are used to extract the product $\mathcal{F}(1)η_{\rm EW}|V_{cb}|$ and the decay form factors, where $\mathcal{F}(1)$ is the normalization factor and $η_{\rm EW}$ is a small electroweak correction. In the CLN parameterization we find $\mathcal{F}(1)η_{\rm EW}|V_{cb}| = (35.06 \pm 0.15 \pm 0.56) \times 10^{-3}$, $ρ^{2}=1.106 \pm 0.031 \pm 0.007$, $R_{1}(1)=1.229 \pm 0.028 \pm 0.009$, $R_{2}(1)=0.852 \pm 0.021 \pm 0.006$. For the BGL parameterization we obtain $\mathcal{F}(1)η_{\rm EW}|V_{cb}|= (34.93 \pm 0.23 \pm 0.59)\times 10^{-3}$, which is consistent with the World Average when correcting for $\mathcal{F}(1)η_{\rm EW}$. The branching fraction of $B^{0} \to D^{*-} \ell^+ ν_\ell$ is measured to be $\mathcal{B}(B^{0}\rightarrow D^{*-}\ell^{+}ν_{\ell}) = (4.90 \pm 0.02 \pm 0.16)\%$. We also present a new test of lepton flavor universality violation in semileptonic $B$ decays, $\frac{{\cal B }(B^0 \to D^{*-} e^+ ν)}{{\cal B }(B^0 \to D^{*-} μ^+ ν)} = 1.01 \pm 0.01 \pm 0.03~$. The errors correspond to the statistical and systematic uncertainties respectively. This is the most precise measurement of $\mathcal{F}(1)η_{\rm EW}|V_{cb}|$ and form factors to date and the first experimental study of the BGL form factor parameterization in an experimental measurement.