- Production in Diffractive DIS

We calculate the cross section for q ¯ qg-production in diffractive DIS with finite fermion masses and zero momentum transfer. The calculation is done in the leading log(1/x P) approximation and is valid for the region of high diffractive masses (small β). We apply our cross section formula to diffractive charm production at HERA: in a Monte-Carlo-Simulation of diffractive D * ± meson production we include both massive q ¯ q-and massive q ¯ qg-production, and we compare with preliminary H1 results. After adjusting an infrared cutoff parameter of our model which affects the overall normalization of the cross section we find reasonable agreement with the data. In particular, the slope of the β-distribution can be reproduced. 1. In the process of diffractive deep inelastic scattering, γ * +p → p+X, one can separate perturbative and non-perturbative contributions by filtering out particular diffractive final states. Examples of diffractive states which are perturbatively calculable are longitudinal vector particles or final states which consist of hard jets (and no soft remnant). In the latter case the hard scale which allows the use of pQCD is provided by the large transverse momenta of the jets, and the Pomeron exchange is modelled by the unintegrated gluon density. Another particularly interesting example is diffractive charm production, since the charm quark mass justifies pQCD, even for not so large transverse momenta of the outgoing quarks and gluons. Calculations for the diffractive production of massless open q ¯ q states and of massless q ¯ qg states have been reported in [1, 2, 3] and in [4], resp., and a comparison of diffractive two-jet and three-jet events observed at HERA with these calculations has been presented in [5]. Final states with finite quark masses have been calculated, so far, only for q ¯ q production [6] which is expected to be the dominant final state in the region of small diffractive masses (large β). However, as there are recent results from measurements of H1 and ZEUS at HERA on diffractive production of (charmed) D * ± mesons, which extend into the small-β-region, gluon radiation can certainly not be neglected, and a comparison of the perturbative two gluon model with HERA data has not been possible yet. In this letter we report on a calculation of massive q ¯ qg-production in DIS diffraction, and we present a comparison of our cross section formula with preliminary H1 data. 2. …