The p_T Spectrum in Heavy-Flavour Photoproduction

TL;DR

The paper develops a fixed-order plus next-to-leading-log (FONLL) framework for heavy-flavour photoproduction, merging NLO fixed-order results with a resummed massless scheme while preserving exact mass terms up to alpha_em alpha_s^2. It carefully implements a scheme change to include the heavy flavour as an active parton, derives the massless limit of the fixed-order contribution, and establishes a robust matching between resummed and fixed-order components for the full cross section, including photon splitting contributions. The authors analyze power-suppressed mass effects, introduce a smearing mechanism to control low-pT behavior, and demonstrate the approach with charm production at HERA, highlighting the distinct roles of direct and resolved photon contributions. The work provides a predictive, cross-validated method that describes the pT spectrum across the full range and informs interpretation of photon-hadron collision data.

Abstract

We consider the transverse-momentum distribution of heavy flavours in photon-hadron collisions. We present a formalism in which large transverse-momentum logarithms are resummed to the next-to-leading level, and mass effects are included exactly up to order alpha_em alpha_s^2, so as to retain predictivity at both small and large transverse momenta. Phenomenological applications relevant to charm photoproduction at HERA are given.

Figures (13)

• Figure 1: Comparison of the $\rm FO_{pnt}$ and $\rm FOM0_{pnt}$ differential cross sections as a function of the logarithm of the mass, at $p_{\rm T}=20$ GeV and $y=1$.
• Figure 2: As in figure \ref{['fig:lim_all']}, for $\gamma g$ (left) and $\gamma q$ (right) components alone.
• Figure 3: Photon splitting into a heavy-quark pair.
• Figure 4: $\rm FOM0_{pnt}$ compared to the sum of $\rm RS_{pnt}$ and PQ. The parameters are as in fig. \ref{['fig:lim_all']}. The charm quark mass was set equal to 1.5 GeV.
• Figure 5: $\rm FO_{pnt}$ and $\rm FOM0_{pnt}$ at Born and full ${\cal O}(\alpha_{\rm em}\alpha_{\rm s}^2)$ level, plotted as a function of the mass and at fixed transverse momentum (left figure) or fixed transverse mass (right figure).