Measurement of the semileptonic $\bar{B}^0 \to D^{*+} \ell^{-} \nu_{\ell}$ branching fraction with fully reconstructed $B$ meson decays and 34.6 fb$^{-1}$of Belle II data

TL;DR

This paper reports the first measurement of the semileptonic B meson decay branching fraction using fully reconstructed decays and Belle II data, achieving a precise result with detailed uncertainty analysis.

Contribution

It introduces a novel measurement technique employing the Full Event Interpretation algorithm with fully reconstructed B decays at Belle II.

Findings

Measured branching fraction: 4.51 ± 0.41 (stat) ± 0.27 (syst) ± 0.45 (pion)

First such measurement with Belle II data and full event reconstruction

Demonstrates the effectiveness of the Full Event Interpretation algorithm

Abstract

We present a first measurement of the $\bar{B^{0}} \rightarrow D^{*+} \ell^{-} \nu_{\ell}$ branching fraction using fully reconstructed $B$ meson decays employing the Full Event Interpretation algorithm. Collision events corresponding to an integrated luminosity of \lumi are analyzed, which were recorded by the Belle~II detector operated at the SuperKEKB accelerator complex. We measure$\cal{B}(\bar{B^{0}} \rightarrow D^{*+} \ell^{-} \nu_{\ell}) =4.51 \pm 0.41_{stat}\pm0.27_{syst} \pm0.45_{\pi_s}$, with the first and second error denoting the statistical and systematic uncertainty, respectively, and the third dominant uncertainty is from the slow pion reconstruction efficiency.