Probing the Low x Gluon with Exclusive J/ψ Production

© 2017 Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). We present an updated and improved extraction of the low-x gluon distribution from the HERA and LHCb exclusive J/ψ production data in the context of kT factorisation. Critically, we use a more precise expression for the photon flux and updated rapidity gap survival factors. In collinear factorisation, exclusive J/ψ 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