The LHCb Collaboration announced the discovery of two J/ψ–p resonances consistent with pentaquark states in 2015. They found these charmed-pentaquark states, Pc(4380) and Pc(4450), to have a combined significance of more than 15σ. This discovery has spurred a new excitement in the high-energy and nuclear physics communities, generating over 600 publications in the two years since the original press release.
The true nature of these Pc states is still unclear: even the exact spin-parity assignments cannot be fully constrained by the existing data. Are the Pc(4380) and Pc(4450) truly new states, in the form of a true five-quark state, or some form of molecular binding between the J/ψ and the nucleon? Or is this observation a remarkable consequence of kinematic enhancements through the anomalous triangle singularity (ATS), or other final-state interactions? These are the central questions that need to be answered to obtain a much deeper understanding of the signal measured by the LHCb Collaboration.
Real photo-production is the ideal tool to distinguish between both explanations: a real Pcstate can be created in photo-production through the s-channel, while kinematic enhancement through the ATS is not possible in this process. The Pc(4450) translates to a sharp peak at a photon energy of 10 GeV, close to the threshold for elastic J/ψ production. The precise behavior of J/ψ production in this near-threshold region is still a mystery due to a lack of existing measurements. JLab is the ideal laboratory to study the Pc and the near-threshold region, due to luminosity, resolution, and energy reach at threshold.
The J/ψ-007 experiment (E12-16-007) in Hall C was approved by the PAC44 with the highest possible rating: A/high-impact. It leverages the difference in the angular dependence of the (s-channel) resonant Pc production, compared to the (t-channel) elastic J/ψ production to maximize the discovery potential of the possible charmed-pentaquark states. The experiment will use the HMS and SHMS in a standard configuration, with a standard 10cm liquid hydrogen target. It will use a 50μA electron beam incident on a 9% copper radiator for a total 10% radiation length. The J/ψ decay leptons will be detected in coincidence, with the SHMS running in positive and the HMS in negative polarity. The experiment will spend the first 2 days in a symmetric configuration to precisely calibrate against the elastic J/ψ cross section in the near-threshold region, and 9 days in an asymmetric configuration that is optimized to maximize the (Pc) signal over (t-channel J/ψ) background ratio, for a total of 11 days. The significance of this experiment will far exceed the 5σ level necessary for discovery down to a branching ratio (Pc to J/ψ–p) of 1.3%. In only 11 days, this experiment will provide an answer to the question of the true nature of what was observed by the LHCb collaboration, either in the form of a confirmation, or a strong exclusion limit on the branching ratio. Furthermore, the calibration measurement, in itself, will be a valuable first high-precision measurement of the absolute elastic J/ψ cross section near threshold, useful for future experimental endeavors in Halls A, B and D.