Event generation at NLO for photoproduction and diffraction

Abstract

Precise predictions are key to the successful operation and data interpretation at collider experiments. While DIS is well understood and has reached very high accuracies, the precision goals of the upcoming EIC motivate revisiting lepton-hadron collisions. In this work, we implement photoproduction and diffraction which both make up significant fractions of the total cross-section at these colliders.
In the case of photoproduction, we achieve the first matched NLO simulation including beam remnants and multiple interaction modelling. Validating the calculation against data from the OPAL and ZEUS collaborations, we see satisfactory agreement. We then scrutinize the uncertainty associated with the photon PDFs by comparing eleven PDF sets and find that deviations between sets are of similar size as the Leading Order scale uncertainties. This leads to the conclusion that photon PDFs are the current bottleneck for precision photoproduction phenomenology for the EIC. We present the first fully-differential predictions for photoproduction at the EIC.
We furthermore conduct a comparative study of different implementations of photoproduction in general-purpose event generators and examine the differences between their predictions. There remain some open questions in the modelling, e.g. the transition to DIS, and future developments will benefit from past and future data from HERA and EIC, respectively.
Diffraction is described in the DIS regime by means of factorisation into the so-called Diffractive PDF and the matrix element. Implementing the corresponding terms, we again arrive at matched NLO simulations and validate our implementation against data from the H1 and ZEUS experiments and see excellent agreement. The underlying factorisation, however, breaks down in the photoproduction regime resulting in the calculation overshooting the data by approximately a factor of 2. We argue for a necessary suppression not only of the resolved but also the direct component and quantify these effects by a fit to data. We present predictions for diffractive DIS and diffractive photoproduction at the EIC where for the latter we apply the fitted suppression factors.