Cross Sections for Charm Production in $ep$ Collisions: Massive versus Massless Scheme
B. A. Kniehl, M. Krämer, G. Kramer, M. Spira
TL;DR
The paper investigates large-p_perp charm photoproduction in ep collisions at HERA and compares two schemes: a massive charm scheme with m = 1.5 GeV and a massless scheme treating charm as an active flavor in both proton and photon structure functions. Differential cross sections d^2sigma/dy dp_perp^2 are computed for direct and resolved photon processes, including NLO corrections for the direct channel and using a delta-function fragmentation to relate charm quarks to charmed hadrons. The results indicate the massless scheme yields substantially larger cross sections, particularly in the resolved component, with most of the enhancement traced to the charm content of the photon and only a portion due to residual log(p_perp^2/m^2) terms; the massless framework effectively incorporates higher-order effects and shows better scale stability. The findings imply that measurements of large-p_perp charm production at HERA could constrain the charm content of the photon and inform the appropriate treatment of heavy quarks in photoproduction, with future work planned to use realistic fragmentation functions and explore fragmentation evolution.
Abstract
The next--to--leading order inclusive cross section for large-$p_\perp$ photoproduction of charm quarks at HERA is calculated in two different approaches. In the first approach the charm quarks are treated as massive objects which are strictly external to the proton and the photon while in the second approach the charm mass is neglected and the $c$ quark is assumed to be one of the active flavours in the proton and photon structure functions. We present single-inclusive distributions in transverse momentum and rapidity including direct and resolved photons. The cross section in the massless approach is found to be significantly larger than in the massive scheme. The deviation originates from several contributions which are disentangled. We argue that large-$p_\perp$ photoproduction of charm quarks at HERA will be sensitive to the charm content of the photon structure function.