How to include exclusive $J/\psi$ production data in global PDF analyses

TL;DR

This paper demonstrates that current global PDFs are compatible with exclusive $J/ heta$ photoproduction data from HERA and LHCb, and highlights the potential of LHCb data to precisely measure the gluon distribution at very low x.

Contribution

It introduces a method to include exclusive $J/\psi$ production data in global PDF analyses, improving gluon PDF constraints at low x.

Findings

Global PDFs are consistent with existing data within uncertainties.

LHCb data can significantly refine gluon PDFs at $10^{-6}<x<10^{-2}$.

Uncertainties in PDFs are large at low x, but data precision is high.

Abstract

We compare the cross section for exclusive $J/\psi$ photoproduction calculated at NLO in the collinear factorization approach with HERA and LHCb data. Using the optimum scale formalism together with the subtraction of the low $k_t<Q_0$ contribution from the NLO coefficient function to avoid double counting we show that the existing global parton distribution functions (PDFs) are consistent with the data within their uncertainties. However, at low $x$ the uncertainties of the present global PDFs are large. On the other hand, the accuracy of the LHCb data are rather good. Therefore, these data provide the possibility to directly measure the gluon PDF over the very large interval of $x$, $10^{-6}<x<10^{-2}$, at a fixed low scale.