Measurement of the CKM matrix element | V c b | from B 0 → D * − ℓ + ν ℓ at Belle

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We present a new measurement of the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{cb}|$ from ${B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ decays, reconstructed with the full Belle data set of $711\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity. Two form factor parametrizations, originally conceived by the Caprini-Lellouch-Neubert (CLN) and the Boyd, Grinstein and Lebed (BGL) groups, are used to extract the product $\mathcal{F}(1){\ensuremath{\eta}}_{\mathrm{EW}}|{V}_{cb}|$ and the decay form factors, where $\mathcal{F}(1)$ is the normalization factor and ${\ensuremath{\eta}}_{\mathrm{EW}}$ is a small electroweak correction. In the CLN parametrization we find $\mathcal{F}(1){\ensuremath{\eta}}_{\mathrm{EW}}|{V}_{cb}|=(35.06\ifmmode\pm\else\textpm\fi{}0.15\ifmmode\pm\else\textpm\fi{}0.56)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, ${\ensuremath{\rho}}^{2}=1.106\ifmmode\pm\else\textpm\fi{}0.031\ifmmode\pm\else\textpm\fi{}0.007$, ${R}_{1}(1)=1.229\ifmmode\pm\else\textpm\fi{}0.028\ifmmode\pm\else\textpm\fi{}0.009$, ${R}_{2}(1)=0.852\ifmmode\pm\else\textpm\fi{}0.021\ifmmode\pm\else\textpm\fi{}0.006$. For the BGL parametrization we obtain $\mathcal{F}(1){\ensuremath{\eta}}_{\mathrm{EW}}|{V}_{cb}|=(34.93\ifmmode\pm\else\textpm\fi{}0.23\ifmmode\pm\else\textpm\fi{}0.59)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which is consistent with the world average when correcting for $\mathcal{F}(1){\ensuremath{\eta}}_{\mathrm{EW}}$. The branching fraction of ${B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ is measured to be $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}})=(4.90\ifmmode\pm\else\textpm\fi{}0.02\ifmmode\pm\else\textpm\fi{}0.16)%$. We also present a new test of lepton flavor universality violation in semileptonic $B$ decays, $\frac{\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{e}^{+}\ensuremath{\nu})}{\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\mu}}^{+}\ensuremath{\nu})}=1.01\ifmmode\pm\else\textpm\fi{}0.01\ifmmode\pm\else\textpm\fi{}0.03$. The errors quoted correspond to the statistical and systematic uncertainties, respectively. This is the most precise measurement of $\mathcal{F}(1){\ensuremath{\eta}}_{\mathrm{EW}}|{V}_{cb}|$ and form factors to date and the first experimental study of the BGL form factor parametrization in an experimental measurement.

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