Measurement of the charm fragmentation function in D* photoproduction at HERA

The charm fragmentation function has been measured in D* photoproduction with the ZEUS detector at HERA using an integrated luminosity of 120 pb⁻¹. The fragmentation function is measured versus z = (E + p_∥)^D*/2E^jet, where E is the energy of the D* meson and p_∥ is the longitudinal momentum of the D* meson relative to the axis of the associated jet of energy E^jet. Jets were reconstructed using the k_T clustering algorithm and required to have transverse energy larger than 9 GeV. The D* meson associated with the jet was required to have a transverse momentum larger than 2 GeV. The measured function is compared to different fragmentation models incorporated in leading-logarithm Monte Carlo simulations and in a next-to-leading-order QCD calculation. The free parameters in each fragmentation model are fitted to the data. The extracted parameters and the function itself are compared to measurements from e⁺e⁻ experiments.