Measurement of charm fragmentation ratios and fractions in photoproduction at HERA

TL;DR

This paper measures charm hadron production in ep scattering at HERA, determining fragmentation ratios and fractions, and compares them with results from other experiments to test the universality of charm fragmentation.

Contribution

It provides new measurements of charm fragmentation ratios and fractions in photoproduction at HERA, extending the understanding of charm hadronization in ep collisions.

Findings

Measured R_u/d and gamma_s agree with deep inelastic scattering and e+e- results.

P_v^d value is smaller but consistent with previous measurements.

Fragmentation fractions are consistent with prior results, with some differences in specific channels.

Abstract

The production of D^*+, D^0, D^+, D_s^+ and Lambda_c^+ charm hadrons and their antiparticles in ep scattering at HERA was measured with the ZEUS detector using an integrated luminosity of 79 pb^-1. The measurement has been performed in the photoproduction regime with the exchanged-photon virtuality Q^2 < 1 GeV^2 and for photon-proton centre-of-mass energies in the range 130 < W < 300 GeV. The charm hadrons were reconstructed in the range of transverse momentum p_T(D, Lambda_c) > 3.8 GeV and pseudorapidity |eta(D, Lambda_c)| < 1.6. The production cross sections were used to determine the ratio of neutral and charged D-meson production rates, R_u/d, the strangeness-suppression factor, gamma_s, and the fraction of charged D mesons produced in a vector state, P_v^d. The measured R_u/d and gamma_s values agree with those obtained in deep inelastic scattering and in e^+e^- annihilations. The measured P_v^d value is smaller than, but consistent with, the previous measurements. The fractions of c quarks hadronising as a particular charm hadron, f(c -> D, Lambda_c), were derived in the given kinematic range. The measured open-charm fragmentation fractions are consistent with previous results, although the measured f(c -> D^*+) is smaller and f(c -> Lambda_c^+) is larger than those obtained in e^+e^- annihilations. These results generally support the hypothesis that fragmentation proceeds independently of the hard sub-process.