Inclusive charm production at HERA and the charm content of the photon

Abstract

We calculate the contribution to inclusive high tranverse momentum (_T) charm production at HERA from the excitation of charm in the photon. At large values of _T the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs, log(p² T /m²_c ), as opposed to calculating the contribution of the 2 ! 3 subprocess in fixed order of perturbation theory. We find that this contribution is large and comparable to the contribution from g fusion production of charm. Suitable cuts on the rapidity of the ‘away-side’ large p_T jet allow a very neat separation between the contributions from the excitation process and from pair-production. We further find that including this excitation contribution we can reproduce the measured inclusive D∗ and µ cross–sections measured by the ZEUS and H1 collaborations respectively, in a LO calculation.