Speaker
Description
We present a new measurement of the lepton-flavor-universality ratios $R(D^{(*)})$ utilizing the entire Belle data set, which corresponds to an integrated luminosity of 711 fb${}^{-1}$. The analysis employs hadronic tag-side reconstruction, leveraging the capabilities of the full-event-interpretation algorithm developed for Belle II. This results in a significant efficiency improvement of approximately a factor of two with respect to the previously utilized tag-side reconstruction method. The analysis utilizes the reconstructing the mass squared of missing neutrinos originating from leptonic tau decays ($\tau \to \ell \bar \nu_\ell \, \nu_\tau$) and unassigned neutral energies registered in the Belle calorimeter. By exploiting these two observables, we separate $B \to D^{(*)} \tau \bar \nu_\tau$ decays from background processes and the normalization mode $B \to D^{(*)} \ell \bar \nu_\ell$ with $\ell = e, \mu$ using a two-dimensional fit. One of the key backgrounds of this analysis are decays involving higher charm resonances ($D^{**}$). Their branching fractions and decay dynamics are poorly known. The talk also covers related measurements from Belle, such as the first observation of the $B \to D_1 \ell \bar \nu_\ell$ decay with $D_1 \to D \pi \pi$ and new measurements of $B \to D^{(*)} \pi \ell \bar \nu_\ell$ and $B \to D^{(*)} \pi \pi \ell \bar \nu_\ell$ branching fractions. These results provide further insights into the difference between the inclusive $B \to X_c \ell \bar \nu_\ell$ branching fraction and the sum over exclusive contributions from $D, D^*$ and $D^{**}$ contributions.
| Collaboration / Activity | Belle Collaboration |
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