Fragmentation contributions to $J/\psi$ photoproduction at HERA

TL;DR

This paper calculates leading-power fragmentation corrections to $J/$ photoproduction at HERA using nonrelativistic QCD, finding these corrections are small and do not resolve existing discrepancies with experimental data.

Contribution

The study provides the first comprehensive calculation of leading-power fragmentation corrections at all orders in _s for $J/$ photoproduction, showing their limited impact.

Findings

Fragmentation corrections are small compared to fixed-order contributions.

Discrepancy between theory and experiment remains unresolved.

Higher-order corrections do not significantly improve agreement with data.

Abstract

We compute leading-power fragmentation corrections to $J/\psi$ photoproduction at DESY HERA, making use of the nonrelativistic QCD factorization approach. Our calculations include parton production cross sections through order $\alpha_s^3$, fragmentation functions though order $\alpha_s^2$, and leading logarithms of the transverse momentum divided by the charm-quark mass to all orders in $\alpha_s$. We find that the leading-power fragmentation corrections, beyond those that are included through next-to-leading order in $\alpha_s$, are small relative to the fixed-order contributions through next-to-leading order in $\alpha_s$. Consequently, an important discrepancy remains between the experimental measurements of the $J/\psi$ photoproduction cross section and predictions that make use of nonrelativistic-QCD long-distance matrix elements that are extracted from the $J/\psi$ hadroproduction cross-section and polarization data.