Possibility to study the pentaquark states Pc (4312), Pc (4440), and Pc (4457) in the reaction γp →j /ψp

Abstract

Inspired by the observation of the pentaquark states Pc(4312), Pc(4440), and Pc(4457) at LHCb, photoproduction of these three Pc states via the interaction γp→J/ψp is investigated in an effective Lagrangian approach. The t-channel Pomeron exchange diffractive process is considered as the main background for the J/ψ photoproduction. The numerical results show that the theoretical cross section, which is calculated by assuming a branching ratio Br[Pc→J/ψp]≃3%, is consistent with the existing experimental data of the γp→J/ψp process. With such a branching ratio, if experimental precision reaches 0.1 nb within a bin of 100 MeV for photon energy, two peaks are expected to be obviously observed in the J/ψ photoproduction. To observe the two-peak structure from Pc(4440) and Pc(4457), higher precision, about 0.1 nb/10 MeV, is required to distinguish two close pentaquarks. If the physical branching ratio is larger, the requirement of experimental precision will be reduced. The differential cross sections for reaction γp→J/ψp are also present. It is found that the t-channel Pomeron exchange provides a sharp increase at extreme forward angles and gives a sizable contribution at most energy points, while the contributions from the s-channel Pc exchanges play important roles at threshold energies. The experimental measurement of the γp→J/ψp process in the near-threshold energy region around Eγ≃9.4-10.5 GeV is suggested and is accessible at CEBAF@JLab and COMPASS.