Observation of J/psip Resonances Consistent with Pentaquark States in Lambda_b^0J/psiK^-p Decays

Observations of exotic structures in the J/psip channel, which we refer to as charmonium-pentaquark states, in Lambda_b^0J/psiK^-p decays are presented. The data sample corresponds to an integrated luminosity of 3fb^-1 acquired with the LHCb detector from 7 and 8TeV pp collisions. An amplitude analysis of the three-body final state reproduces the two-body mass and angular distributions. To obtain a satisfactory fit of the structures seen in the J/psip mass spectrum, it is necessary to include two Breit-Wigner amplitudes that each describe a resonant state. The significance of each of these resonances is more than 9 standard deviations. One has a mass of 4380±8±29MeV and a width of 205±18±86MeV, while the second is narrower, with a mass of 4449.8±1.7±2.5MeV and a width of 39±5±19MeV. The preferred J^P assignments are of opposite parity, with one state having spin 3/2 and the other 5/2.