Search for the radiative Ξ_b^- → Ξ − γ decay

The first search for the rare radiative decay Ξ_b^- → Ξ − γ is performed using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb − 1 . The Ξ_b^- → Ξ − J/ψ channel is used as normalization. No Ξ_b^- → Ξ − γ signal is found and an upper limit of ℬ ( Ξ_b^- → Ξ − γ ) < 1 . 3 × 10 − 4 at 95