Evidence of a J/ψΛ structure and observation of excited Ξ^- states in the Ξ_b ^-→J/ψΛK^- decay

First evidence of a structure in the J/ψΛ invariant mass distribution is obtained from an amplitude analysis of Ξ_b ^-→J/ψΛK^- decays. The observed structure is consistent with being due to a charmonium pentaquark with strangeness with a significance of 3.1σ including systematic uncertainties and look-elsewhere effect. Its mass and width are determined to be 4458.8±2.9_-1.1 ^+4.7MeV and 17.3±6.5_-5.7 ^+8.0MeV, respectively, where the quoted uncertainties are statistical and systematic. The structure is also consistent with being due to two resonances. In addition, the narrow excited Ξ^- states, Ξ1690^- and Ξ1820^-, are seen for the first time in a Ξ_b ^- decay, and their masses and widths are measured with improved precision. The analysis is performed using pp collision data corresponding to a total integrated luminosity of 9 fb^-1, collected with the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV.