Measurement of the photon-energy spectrum in inclusive $B\to X_s{\hat{I}}^3$ decays identified using hadronic decays of the recoil $B$ meson in 2019-2021 Belle II data

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 $\hat{I}\yen (4S)$ resonance by the Belle II experiment. The partner $B$ candidates are fully reconstructed using a large number of hadronic channels. The $B\to X_s{\hat{I}}^3$ 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.

14 pages, 3 figures