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

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We calculate the contribution to inclusive high tranverse momentum (pₜ) charm production at HERA from the excitation of charm in the photon. At large values of the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs, (pₜ²/mc²), 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 γg fusion production of charm. Suitable cuts on the rapidity of the `away-side' large 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.