Measurement of the photon-energy spectrum in inclusive $B\rightarrow X_{s}\gamma$ decays identified using hadronic decays of the recoil $B$ meson in 2019-2021 Belle II data

TL;DR

This paper reports a precise measurement of the photon-energy spectrum in inclusive $B ightarrow X_s\gamma$ decays using Belle II data, providing insights into rare radiative $B$ decays and testing the Standard Model.

Contribution

First measurement of the photon-energy spectrum in $B ightarrow X_s\gamma$ decays using fully reconstructed hadronic $B$ tags at Belle II.

Findings

Partial branching fractions measured above 1.8 GeV photon energy.

Integrated branching fractions consistent with world averages.

Signal yield of 343 ± 122 events in the high-energy region.

Abstract

We measure the photon-energy spectrum in radiative bottom-meson ($B$) decays into inclusive final states involving a strange hadron and a photon. We use SuperKEKB electron-positron collisions corresponding to $189~\mathrm{fb}^{-1}$ of integrated luminosity collected at the $\Upsilon(4S)$ resonance by the Belle II experiment. The partner $B$ candidates are fully reconstructed using a large number of hadronic channels. The $B \rightarrow X_s \gamma$ partial branching fractions are measured as a function of photon energy in the signal $B$ meson rest frame in eight bins above $1.8~\mathrm{GeV}$. The background-subtracted signal yield for this photon energy region is $343 \pm 122$ events. Integrated branching fractions for three photon energy thresholds of $1.8~\mathrm{GeV}$, $2.0~\mathrm{GeV}$, and $2.1~\mathrm{GeV}$ are also reported, and found to be in agreement with world averages.