Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ for the diffractive production of open charm (cc) from longitudinally and transversely polarised photons (of virtuality Q2) incident at high energy ( √ s) on a proton target. We study both the Q2 and M2 dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ , as well as for σ, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the square of the gluon density g(x,K2), and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for x = (Q2+M2)/s and scale K2 = (mc+〈k T 〉)(1+Q2/M2) where the average quark transverse momentum squared 〈k2 T 〉 ∼ mc . As compared to diffractive J/ψ production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/ψ wave function. We estimate the fraction of diffractive events that arise from cc production.