# First measurement of near-threshold J/$\psi $ exclusive photoproduction off the proton

## TL;DR

This study measures the near-threshold exclusive photoproduction cross section of J/ψ off protons, revealing a less steep decline than models predict and setting limits on pentaquark production.

## Contribution

First measurement of the J/ψ photoproduction cross section near threshold, providing new data to test theoretical models and search for pentaquark states.

## Key findings

- Cross section decreases less steeply than two-gluon exchange models predict.
- Differential cross section has an exponential slope of 1.67 ± 0.39 GeV$^{-2}$ at 10.7 GeV.
- No evidence found for pentaquark production, with upper limits set on their branching fractions.

## 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 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 $\mathcal{B}(P_c^+ \rightarrow J/\psi p)$ and cross sections $\sigma(\gamma p \to P_c^+)\times\mathcal{B}(P_c^+ \to J/\psi p) $.