Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ^LT 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 Q ² and M ² dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ^T, as well as for σ^L, 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 = (Q ² + M ²)/s and scale K2 = (M ²c + )(1 + Q ²/M ²) where the average quark transverse momentum squared 〈k ₂ ^t ~ (M ²c . As compared to diffractive J/#x03C8; production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/#x03C8; wave function. We estimate the fraction of diffractive events that arise from cc¯ production.