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Large-p_T Photoproduction of D^*+- Mesons in ep Collisions

B. A. Kniehl, G. Kramer, M. Spira

TL;DR

The paper develops a massless-charm factorization approach to large-p_T D^{*±} photoproduction in ep collisions, treating charm as an active parton in both the proton and photon and describing hadronization with evolved fragmentation functions. It demonstrates that full NLO evolution of fragmentation functions, especially the Peterson form, provides good agreement with H1/ZEUS data and reveals a sizable charm component in the photon’s structure via the resolved-photon contribution. The study assesses theoretical uncertainties from PDFs, fragmentation parameters, and scales, finding manageable sensitivity and emphasizing the importance of consistent scheme choices when interpreting fragmentation fits. Overall, the work offers a robust, log-resummed description for high-p_T charm production at HERA and guides future data and FF determinations.

Abstract

The cross section for the inclusive photoproduction of large-p_T D^*+- mesons is calculated at next-to-leading order, adopting different approaches to describe the fragmentation of charm quarks into D^*+- mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the photon. We present inclusive single-particle distributions in transverse momentum and rapidity, including the contributions due to both direct and resolved photons. We compare and assess the various implementations of fragmentation. We argue that, in the high-p_T regime, a particularly realistic description can be obtained by convoluting the Altarelli-Parisi-evolved fragmentation functions of Peterson et al. with the hard-scattering cross sections of massless partons where the factorization of the collinear singularities associated with final-state charm quarks is converted to the massive-charm scheme. The predictions thus obtained agree well with recent experimental data by the H1 and ZEUS Collaborations at DESY HERA.

Large-p_T Photoproduction of D^*+- Mesons in ep Collisions

TL;DR

The paper develops a massless-charm factorization approach to large-p_T D^{*±} photoproduction in ep collisions, treating charm as an active parton in both the proton and photon and describing hadronization with evolved fragmentation functions. It demonstrates that full NLO evolution of fragmentation functions, especially the Peterson form, provides good agreement with H1/ZEUS data and reveals a sizable charm component in the photon’s structure via the resolved-photon contribution. The study assesses theoretical uncertainties from PDFs, fragmentation parameters, and scales, finding manageable sensitivity and emphasizing the importance of consistent scheme choices when interpreting fragmentation fits. Overall, the work offers a robust, log-resummed description for high-p_T charm production at HERA and guides future data and FF determinations.

Abstract

The cross section for the inclusive photoproduction of large-p_T D^*+- mesons is calculated at next-to-leading order, adopting different approaches to describe the fragmentation of charm quarks into D^*+- mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the photon. We present inclusive single-particle distributions in transverse momentum and rapidity, including the contributions due to both direct and resolved photons. We compare and assess the various implementations of fragmentation. We argue that, in the high-p_T regime, a particularly realistic description can be obtained by convoluting the Altarelli-Parisi-evolved fragmentation functions of Peterson et al. with the hard-scattering cross sections of massless partons where the factorization of the collinear singularities associated with final-state charm quarks is converted to the massive-charm scheme. The predictions thus obtained agree well with recent experimental data by the H1 and ZEUS Collaborations at DESY HERA.

Paper Structure

This paper contains 9 sections, 12 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Fragmentation function approach
  3. Results
  4. Input information
  5. Refinements in fragmentation
  6. Theoretical uncertainties
  7. Comparison with H1 and ZEUS data
  8. Summary and Conclusions
  9. Mellin transform of the Peterson fragmentation function

Figures (2)

  • Figure 1: NLO $p_T$ distribution $d\sigma/dp_T$, integrated over $-1.5<y_{lab}<1$, of inclusive $D^{*\pm}$ photoproduction in $ep$ scattering with untagged positrons as in the ZEUS experiment. The branching ratio $B(c\to D^{*+})$ is taken to be unity. The fragmentation models $A$--$F$ are described in the text.
  • Figure :