LHCb- Observation of $J/\psi p$ resonances consitent with pentaquark staes in $\Lambda^0_b \to J/\psi K^-p$ decays

The observation of exotic structures in the $J/\psi p$ channel, refered to as pentaquark-charmonium states, in the decay $\Lambda^0_b \rightarrow J/\psi K^- p$, are presented. An amplitude analysis is performed on the three-body final state that reproduces the two-body mass and angular distributions. To obtain a satisfactory fit of the structures seen in the $J/\psi p$ 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 $\pm$ 8 $\pm$ 29 MeV and a width of 205 $\pm$ 18 $\pm$ 86 MeV, while the second is narrower, with a mass of 4449.8 $\pm$ 1.7 $\pm$ 2.5 MeV and a width of 39 $\pm$ 5 $\pm$ 19. The preferred $J^P$ assignments are of opposite parity, with one state having spin 3/2 and the other 5/2.