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

Understand

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.