Exclusive production of heavy quarkonia as a probe of the low x and low scale gluon PDF

Understand

We discuss the exclusive $J/\psi$ photoproduction process, as measured recently at LHCb, as a means of constraining and ultimately determining the low $x$ and low $Q$ gluon PDF.

• The scale dependence of the theoretical prediction for this process is shown to be systematically improved via a taming of the known $\bar{\text{MS}}$ result, this amounts to resumming logarithmically enhanced small-$x$ terms and implementing a small-$Q$ power correction.
• The cross section level predictions allow the behaviour of the gluon PDF in the low $(x, Q)$ domain to be determined.