Probing the Low x Gluon with Exclusive $J/\psi$ Production
S. Jones, A. Martin, M. Ryskin, T. Teubner
We present an updated and improved extraction of the low-$x$ gluon distribution from the HERA and LHCb exclusive $J/\psi$ production data in the context of $k_T$ factorisation. Critically, we use a more precise expression for the photon flux and updated rapidity gap survival factors.
In collinear factorisation, exclusive $J/\psi$ photoproduction receives large perturbative corrections at next-to-leading order which depend sensitively on the factorisation scale. This prevents the use of the available data to extract a gluon PDF within this framework. Attempts to understand and overcome the problems caused by the large perturbative corrections are discussed.