Study of Exclusive $B \to \pi e^+ \nu_e$ Decays with Hadronic Full-event-interpretation Tagging in 189.3 fb$^{-1}$ of Belle II Data

TL;DR

This paper reports on the measurement of exclusive semileptonic B meson decays to pions using Belle II data, providing new partial branching fractions and an initial determination of the CKM matrix element |V_ub|.

Contribution

First Belle II measurement of B to pi e nu decays with hadronic full-event-interpretation tagging, including partial branching fractions and |V_ub| extraction.

Findings

Partial branching fractions determined for B0 and B+ decays.

First Belle II measurement of |V_ub| from exclusive decays.

Results consistent with previous measurements within uncertainties.

Abstract

We present a reconstruction of the semileptonic decays $B^0 \to \pi^- e^+ \nu_e$ and $B^+ \to \pi^0 e^+ \nu_e$ in a sample corresponding to 189.3 fb$^{-1}$ of Belle II data, using events where the partner $B$-meson is reconstructed from a large variety of hadronic channels via a tagging algorithm known as the full-event-interpretation. We determine the partial branching fractions in three bins of the squared momentum transfer to the leptonic system using fits to the distribution of the square of the missing mass. The partial branching fractions are summed to determine $\mathcal{B}(B^0 \to \pi^- e^+ \nu_e)$ = (1.43 $\pm$ 0.27(stat) $\pm$ 0.07(syst)) $\times 10^{-4}$ and $\mathcal{B}(B^+ \to \pi^0 e^+ \nu_e)$ = (8.33 $\pm$ 1.67(stat) $\pm$ 0.55(syst)) $\times 10^{-5}$. We extract a first Belle II measurement of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\mathrm{ub}}|$, with $|V_{\mathrm{ub}}|$ = (3.88 $\pm$ 0.45) $\times 10^{-3}$.