A Search for the LHCb Charmed 'Pentaquark' using Photo-Production of $J/ψ$ at Threshold in Hall C at Jefferson Lab

Abstract

We propose † to measure the photo-production cross section of J/ψ near threshold, in search of the recently observed LHCb hidden-charm resonances P c (4380) and P c (4450) consistent with 'pentaquarks'. The observation of these resonances in photo-production will provide strong evidence of the true resonance nature of the LHCb states, distinguishing them from kinematic enhancements. A bremsstrahlung photon beam produced with an 11 GeV electron beam at CEBAF covers the energy range of J/ψ production from the threshold photo-production energy of 8.2 GeV, to an energy beyond the presumed P c (4450) resonance. The experiment will be carried out in Hall C at Jefferson Lab, using a 50 µA electron beam incident on a 9% copper radiator. The resulting photon beam passes through a 15 cm liquid hydrogen target, producing J/ψ mesons through a diffractive process in the t-channel, or through a resonant process in the s-and u-channel. The decay e + e − pair of the J/ψ will be detected in coincidence using the two highmomentum spectrometers of Hall C. The spectrometer settings have been optimized to distinguish the resonant s-and u-channel production from the diffractive t-channel J/ψ production. The s-and u-channel production of the charmed 5-quark resonance dominates the t-distribution at large t. The momentum and angular resolution of the spectrometers is sufficient to observe a clear resonance enhancement in the total cross section and t-distribution. We request a total of 11 days of beam time with 9 days to carry the main experiment and 2 days to acquire the needed t-channel elastic J/ψ production data for a calibration measurement. This calibration measurement in itself will greatly enhance our knowledge of t-channel elastic J/ψ production near threshold. † This document is an updated version of the original proposal PR12-16-007, which was approved with an 'A' rating and a 'high-impact' label by the Jefferson Lab PAC 44 in July 2016. The experiment was awarded 11 days of beam time.

Looking for the hidden-charm pentaquark resonances in $J/ψp$ scattering

“…All these narrow pentaquarks are worth searching for or being confirmed in future experiments. The Jefferson Lab has proposed to look for the hidden-charm pentaquarks by using photoproduction of J/ψ at threshold in Hall C [46]. Moreover, the pentaquarks with charm quarks can also be observed by the PANDA/FAIR [47].…”

Pion-induced production of hidden-charm pentaquarks P(4312),P(4440), and P(4457)

“…Then, the productions of these predicted pentaquark state via pion-or kaon-induced reaction were calculated [36,37]. After the states P c (4450) and P c (4380) were experimentally observed, many theoretical calculations about these two pentaquarks produced in different processes appeared [39][40][41][42][43][44]. After new LHCb results was released, we also updated the predictions about the photoproductions of three P c states [45], which is accessible at JLab.…”

Holographic charm and bottom pentaquarks III: Excitations through photo-production of heavy mesons

“…A more detailed way to understand the structure of the holographic pentaquarks, is through their strong two-body decay channels [10], or even better by directly producing them using photo-production of charmonium (bottomonium) on the proton in the threshold region [30][31][32] as currently pursued at JLab [33]. Precision data are required to disentangle the s-channel production from the dominant t-channel contribution, in the nearly diffractive regime.…”