First Evidence of the Bs0 →k-π+γ Decay

The first search for the Bs0→K-π+γ decay in the range 796<m(K-π+)<1800 MeV/c2 is performed using data from proton-proton collisions collected by the LHCb experiment at center-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-π+)<996 MeV/c2, the ratio R between the branching fractions of the signal decay and the favored B¯0→K-π+γ decay is measured to be R=(3.7±1.2±0.4)×10-2 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-π+)<1800 MeV/c2, the ratio R=(0.2±2.7±1.3)×10-2 is measured.