Evidence for Factorization Breaking in Diffractive Low-$Q^2$ Dijet Production

We calculate diffractive dijet production in deep-inelastic scattering at next-to-leading order of perturbative QCD, including contributions from direct and resolved photons, and compare our predictions to preliminary data from the H1 Collaboration at DESY HERA. In contrast to recent experimental claims, evidence for factorization breaking is found only for resolved, and not direct, photon contributions. No evidence is found for large normalization uncertainties in diffractive parton densities. The results confirm theoretical expectations for the (non)cancellation of soft singularities in diffractive scattering as well as previous results for (almost) real photoproduction.

Figure 1

Diffractive scattering process $ep\to eXY$, where the hadronic systems $X$ and $Y$ are separated by the largest rapidity gap in the final state.

Figure 2

Dependence of the diffractive dijet cross section at HERA on the squared photon virtuality $Q^2$. Preliminary H1 data [19] are compared with theoretical predictions for DIS at LO (dot-dashed lines) and NLO (dotted lines) and NLO predictions including resolved virtual photon contributions with (solid lines) and without (dashed lines) a suppression factor of $R=0.34$.

Figure 3

Dependence of the diffractive dijet cross section at HERA on the parton momentum fraction in the pomeron $z_{IP}$ for different ranges of $Q^2+p_T^{*2}$. Preliminary H1 data [19] are compared with various theoretical predictions (see Fig. 2).