Observation of $J/\psi$ p resonances consistent with pentaquark states in$\wedge_ b^0$ → $J/\psi K^−p$ decays

The observation of structures consistent with charmonium-pentaquark states decaying to $J/\psi p$ in $\wedge_b^0$ → $J/\psi K^−p$ decays is presented. The data sample analyzed corresponds to an integrated luminosity of $3 fb^−1$ acquired with the LHCb detector from 7 and 8 TeV $pp$ collisions. An amplitude analysis was performed which utilized all six kinematic degrees of freedom in the decay. It was shown that adequate descriptions of the data are unattainable with only $K^−p$ resonances in the amplitude model. For satisfactory fits of the data, it was found to be necessary to include two $J/\psi p$ resonances, with each having significances of over 9 standard deviations. One has a mass of $4449.8 \pm1.7\pm2.2 MeV$ and a width of $39\pm5\pm16 MeV$, while the second is broader, with a mass of $4380 \pm8\pm29 MeV$ and a width of $205\pm18\pm87 MeV$. The $J^P$ assignments could not be uniquely determined, though there is a preference for one to have spin 3/2 and the other spin 5/2 with an opposite parity to the first. The data sample was also inspected in a model-independent way for the presence of $J/\psi p$ or$ J/\psi K^−$ contributions. It was demonstrated at more than 9 standard deviations that the data cannot be described with $K^−p$ contributions alone, and that $J/\psi p$ contributions play a dominant role in this incompatibility. These model-independent results support the model-dependent evidence for $P_c^+ \rightarrow J/\psi p$ charmonium-pentaquark states provided by the amplitude analysis.