Henrikas Svidras ; Arthur Bolz ; Lu Cao ; Markus Röhrken ; Kerstin Tackmann

20 October 2022

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 $189$/fb 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 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 $1.8$~GeV, $2.0$~GeV, and $2.1$~GeV are also reported, and found to be in agreement with world averages.

Keyword(s): photon ; penguin ; radiative ; ewp ; electroweak ; spectrum ; btosgamma ; bsgamma ; gamma ; photons ; xs ; hadronic tagged ; fei tagged