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

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 pT the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs, log(pT /m 2 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 γg 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∗ and μ cross–sections measured by the ZEUS and H1 collaborations respectively, in a LO calculation. Talk presented by R.M. Godbole at Photon’95, Sheffield, UK, April 8-13,1995. Heisenberg Fellow Permanent address: Physics Department, University of Bombay, Vidyanagari, Bombay 400 098, India Measurements of F γ 2 in γ γ scattering at the eecolliders PEP, PETRA, TRISTAN and LEP [1] have by now yielded a lot of information on the parton content of the photon over a wide range of x and Q. However, these measurements give direct information only about the quark content of the photon. The gluon density g(x,Q) is poorly determined as it affects F γ 2 only through the QCD evolution equations. At the current values of Q 2 the charm quark contribution to F γ 2 is approximated by the quark-parton-model (QPM) matrix elements for the process γγ → cc̄ and γg → cc̄. Through the latter process, the effective charm content of the photon becomes sensitive to g(x,Q). At larger values of Q, c(x,Q) computed using the massive Altarelli-Parisi (AP) evoultion equations, is considerably different from the pure QPM predictions [2]. A study of the charm content of the photon might also help shed some light on the correct treatment of a heavy parton inside a target. The various different available parametrisations of q(x,Q) and g(x,Q) [3] treat the charm density c(x,Q) with varying amount of rigour and care. It is therefore interesting to take a phenomenological approach and think of measurements which will probe c(x,Q) directly and hence perhaps also yield information about g(x,Q). One possibility is to study production of single charm in ep collisions via the excitation processes (the subprocesses being c + q → c + q and c + g → c + g, here we neglect the contribution coming from charm in the proton) shown in fig. ??. This will give rise to a single high−pT charm particle whose transverse momentum is balanced by a light q/g jet. Of course the use of structure functions to compute this process is meaningful only for large values of the pT of the charm quark. Admittedly for lower values of pT the more reliable computation will be that of the 2 → 3 subprocesses (some of which are shown in fig. 2), but at larger values of pT the structure function language sums up the large log(p 2 T/m 2 c) terms and hence is more accurate. Another contribution to the inclusive charm signal comes from cc̄ pair production, via the ‘direct’ γg fusion subprocess as well as the ‘resolved’ processes, where the balancing high-pT jet is the c (c̄) quark jet. The excitation contribution of diagrams in fig. ?? is given, in the Weizsäcker-Williams (WW) approximation, by