First measurement of near-threshold J/ψ exclusive photoproduction off the proton

From GlueXWiki
Revision as of 13:28, 12 June 2019 by Sdobbs (Talk | contribs)

Jump to: navigation, search


Abstract

We report on the measurement of the γp → J/ψp cross section from Eγ=11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67 ± 0.39 GeV−2 at 10.7 GeV average energy. The LHCb pentaquark candidates P+c can be produced in the s-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P+c → J/ψp.

Publication Information

Summary

In summary, we have made the first measurement of the J/ψ exclusive photoproduction cross section from Eγ = 11.8 GeV down to the threshold, which provides important inputs to models of the gluonic structure of the proton at high x. The data were collected by the GlueX experiment located in Hall D at Jefferson Lab during 2016 and 2017, representing about 25% of the total data accumulated by the experiment to date. We measure the exclusive reaction γp → J/ψp, and identify the J/ψ by its decay into an electron-positron pair. We normalize the J/ψ total cross section to that of the non-resonant Bethe-Heitler process in the invariant mass range 1.20−2.50 GeV, thus canceling uncertainties from factors like luminosity and common detector efficiencies. The measured cross section is also used to set model-dependent upper limits on the branching fraction of the LHCb P+c states 90% confidence level of <4.6%, <2.3%, and <3.8% for P+c (4312), P+c(4440), and P+c(4457), respectively., which allow to discriminate between different pentaquark models.

Figures and captions

Caption Figures
(a) Photon beam intensity versus energy as measured by the pair spectrometer (not corrected for instrumental acceptance). (b) Photon beam polarization as a function of beam energy, as measured by the triplet polarimeter, with data points offset horizontally by &pm;0.015 GeV for clarity. pdf