The Relativistic Correction in $J/ψ$ Photoproduction

The relativistic correction to the color singlet channel for $J/\psi$ photoproduction is calculated in the NRQCD factorisation formalism. The formalism provides a consistent framework for systematically computing production rates for quarkonia in terms of a series of non perturbative matrix elements defined within NRQCD. The correction appears at $O(v^2)$ according to the scaling rules of NRQCD. For the energy distribution, the correction is found to be negative for certain kinematic regions, leading to a decrease in the differential cross section, which is noticeable at HERA energies.