Inclusive Charm Production at HERA and the Charm Content of the Photon

We calculate the contribution to inclusive high tranverse momentum ($p_T$) charm production at HERA from the excitation of charm in the photon. At large values of \pt\ the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs, $\log (p_T^2\mathrm{/}{m}_c^2)$, as opposed to calculating the contribution of the $2\to 3$ subprocess in fixed order of perturbation theory. We find that this contribution is large and comparable to the contribution from $\gamma g$ fusion production of charm. Suitable cuts on the rapidity of the `away-side’ large \pt\ 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 $D^{\ast }$ and $\mu$ cross–sections measured by the ZEUS and H1 collaborations respectively, in a LO calculation.