Exclusive $J/ψ$ and $Υ$ photoproduction and the low $x$ gluon

Paper

Abstract

We study exclusive vector meson photoproduction,

with

, at NLO in collinear factorisation, in order to examine what may be learnt about the gluon distribution at very low

. We examine the factorisation scale dependence of the predictions. We argue that, using knowledge of the NLO corrections, terms enhanced by a large

can be reabsorbed in the LO part by a choice of the factorisation scale. (In these exclusive processes

takes the role of Bjorken-

.) Then, the scale dependence coming from the remaining NLO contributions has no

enhancements. As a result, we find that predictions for the amplitude of

production are stable to within about

. This will allow data for the exclusive process

at the LHC, particularly from LHCb, to be included in global parton analyses to constrain the gluon PDF down to

. Moreover, the study of exclusive

photoproduction indicates that the gluon density found in the recent global PDF analyses is too small at low

and low scales.