First evidence of the $B_s^0\rightarrow K^-\pi^+\gamma$ decay

The first search for the $B_s^0\rightarrow K^-\pi^+\gamma$ decay in the range $796<m(K^-\pi^+)<1800\,\text{MeV/}c^2$ is performed using data from proton-proton collisions collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb$^{-1}$. The photons are reconstructed through their conversion into an electron-positron pair, which significantly improves the mass resolution of the reconstructed decays with respect to decays with an unconverted photon. A signal excess with a significance of 3.5 standard deviations is measured, constituting the first experimental evidence for this decay. In the range $796<m(K^-\pi^+)<996\,\text{MeV/}c^2$, the ratio ${\cal R}$ between the branching fractions of the signal decay and the favoured $\kern 0.18em\overline{\kern -0.18em B}{}^0\rightarrow K^- \pi^+\gamma$ decay is measured to be ${\cal R} = (3.7\pm1.2\pm0.4)\times10^{-2}$ where the first uncertainty is statistical and the second is systematic. This measurement is consistent with the value predicted in the Standard Model. In the range $996<m(K^-\pi^+)<1800\,\text{MeV/}c^2$, the ratio ${\cal R} = (0.2\pm2.7\pm1.3)\times10^{-2}$ is measured.