Factorisation issues in diffraction

The high centre-of-mass energies of the HERA ep collider and of the Tevatron collider allow us to study the diffractive interactions in the presence of a hard scale and to describe them in terms of perturbative QCD. In QCD, the diffractive exchange is described in terms of partons, and the factorisation theorem states that the cross section for hard interactions can be expressed as convolution of universal diffractive parton distribution functions and process-dependent coefficients, perturbatively calculable. The recent experimental data from the H1 and ZEUS Collaborations at HERA are presented for diffractive dijet and D∗ meson production, in both deep-inelastic scattering (DIS) and the photoproduction regimes and are compared to next-to-leading order QCD predictions using diffractive parton distributions. While good agreement is found for dijets in DIS and for D∗ production in both DIS and photoproduction, the dijet photoproduction data are overestimated by NLO predictions at lower transverse momentum of the jets, indicating the breaking of QCD factorisation.