Observation of J /ψ p Resonances Consistent with Pentaquark States in Λb0→J /ψ K-p Decays

Observations of exotic structures in the J /ψ p channel, which we refer to as charmonium-pentaquark states, in Λ_b⁰→J /ψ K^-p decays are presented. The data sample corresponds to an integrated luminosity of 3 fb^-1 acquired with the LHCb detector from 7 and 8 TeV p p 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 /ψ p 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 ±29 MeV and a width of 205 ±18 ±86 MeV , while the second is narrower, with a mass of 4449.8 ±1.7 ±2.5 MeV and a width of 39 ±5 ±19 MeV . The preferred J^P assignments are of opposite parity, with one state having spin 3 /2 and the other 5 /2 .