Electroweak penguins and radiative B decays at Belle II

Abstract: Decays of $B$ mesons involving the transition of a $b$ quark into an $s$ quark are good probes of physics beyond the standard model. Such decays are studied at the Belle II experiment, a detector located along the SuperKEKB electron-positron collider, and with data corresponding to an integrated luminosity of $189\,\mathrm{fb}^{-1}$ collected at the energy of the $\Upsilon(4S)$ resonance. The radiative decay of a $B$ meson into inclusive final states involving a strange hadron and a photon ($B \to X_s\gamma$) is studied by fully reconstructing the partner $B$ meson in a hadronic decay. The photon-energy spectrum and the branching fraction of the inclusive $B \to X_s\gamma$ decay are reported. In addition, the branching fraction of the decay of a $B$ meson into an excited $K$ meson and a pair of charged leptons ($B \to K^{\ast}(892)\ell^+\ell^-$, with $\ell^+\ell^-$ either an electron-positron or a muon-antimuon pair) is reported. A control channel for the study of the charmless $B \to K\ell^+\ell^-$ decay is the decay $B \to J/\psi (\ell^+\ell^-) K$, for which a lepton-flavour universality ratio is reported. All the reported measurements are in agreement with the world averages and the standard model predictions.