Full NNLO coefficient function for heavy-vector-meson photoproduction

Derive the full next-to-next-to-leading-order coefficient function for exclusive heavy-vector-meson photoproduction so that analyses can replace the approximate NNLO* treatment based on a constant NNLO/NLO K-factor.

Background

The paper introduces the NNLO* method because the coefficient functions needed for exclusive J/ψ and Υ photoproduction are available only at next-to-leading order. In this approximation, the missing NNLO contribution is represented by multiplying the NLO coefficient function by an effective NNLO/NLO K-factor inferred from HERA J/ψ photoproduction data.

A complete NNLO coefficient function would eliminate this approximation and enable a fully NNLO treatment of the exclusive heavy-vector-meson data within global parton-distribution analyses. The paper explicitly identifies the absence of this calculation as the reason for using NNLO* rather than full NNLO accuracy.

References

Since the coefficient function for heavy-vector meson photoproduction is not yet calculated at NNLO accuracy, we carried out the so-called NNLO$*$ method, introduced in our previous.

— Low-$x$ parton densities accounting for absorptive effects and exclusive $J/ψ$ and $Υ$ data from the LHC  (2608.25724 - Bertone et al., 26 Aug 2026) in Section 3.1, “The NNLO* method”; Section 4, Conclusions