First measurement of near-threshold J/$\ddot{I}ˆ$ exclusive photoproduction off the proton

Abstract

We report on the measurement of the ${\hat{I}}^{3}p\to J/\ddot{I}ˆp$ cross section from $E_{\hat{I}}^3=11.8$ 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 $d\ddot{I}ƒ/dt$ 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 $\mathcal{B}(P_c^{+}\to J/\ddot{I}ˆp)$ and cross sections $\ddot{I}ƒ({\hat{I}}^{3}p\to P_c^{+})\times \mathcal{B}(P_c^{+}\to J/\ddot{I}ˆp)$.

12 pages, 7 figures