First Measurement of Near-Threshold $J/\psi$ Exclusive Photoproduction off the Proton

Abstract

We report on the measurement of the $\gamma$p $\rightarrow$ $J/\psi$p cross section from $E_{\gamma}$ = 11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find that the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section $d\sigma$ / $dt$ has an exponential slope of 1.67 $\pm$ 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 $\mathscr{B}$($P_c+$ $\rightarrow$ $J/\psi$p) and cross sections $\sigma$($\gamma$p $\rightarrow$ $P_c+$) x $\mathscr{B}$($P_c+$ $\rightarrow$ $J/\psi$p).