Electroweak penguins and radiative B decays at Belle II

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 $189{\mathrm{f}\mathrm{b}}^{-1}$ collected at the energy of the $\mathrm{{\rm Y}}(4S)$ 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 ($B\to K^{\ast }(892){\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$, with ${\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ either an electron-positron or a muon-antimuon pair) is reported. A control channel for the study of the charmless $B\to K{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decay is the decay $B\to J\mathrm{/}\psi ({\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-})K$, 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.