Measurement of $b \to u \ell \nu$ at Belle II

TL;DR

This paper reports preliminary measurements of semileptonic B meson decays at Belle II, validating detector performance and providing branching fraction results consistent with world averages, crucial for CKM matrix element $V_{ub}$ determination.

Contribution

First measurements of inclusive and exclusive $B o X_u \, \ell \nu$ decays at Belle II using early data, validating detector and analysis methods.

Findings

Branching fraction of $B^0 \to \pi^- \ell^+ \nu$ measured as $(1.58 \pm 0.43_{stat} \pm 0.07_{sys})\times 10^{-4}$

Results agree with previous world average values

Validation of Belle II detector performance for semileptonic decay analysis

Abstract

The semileptonic $B$ meson decays of the type $B \to X_u \ell \nu$ are important for measuring the magnitude of the CKM matrix element $V_{ub}$. Recent measurements of inclusive $B \to X_u e \nu_e$ and hadronically tagged $B^0 \to \pi^- \ell^+ \nu$ from the Belle II experiment are presented. The results are obtained using the data collected at Belle II between March 2019 and March 2020, with a total integrated luminosity of 37.8 ${\rm fb}^{-1}$. These preliminary measurements are used to validate the detector and reconstruction software performance. The measured preliminary branching fraction of $B^0 \to \pi^- \ell^+ \nu$ decays is $(1.58 \pm 0.43_{\textrm{stat}} \pm 0.07_{\textrm{sys}})\cdot 10^{-4}$, which is in agreement with the world average, $(1.50 \pm 0.06)\cdot 10^{-4}$.