Parton Distributions: Summary Report

TL;DR

By combining a thorough review of how parton distributions influence LHC predictions with an assessment of how well PDFs can be extracted from HERA data, the paper benchmarks current global fits, NNLO corrections, and resummation methods, and discusses strategies for integrating HERA and collider data. It provides reference results for parton evolution, examines PDF uncertainties for key LHC processes (notably W and Z production), and proposes practical approaches to reduce systematic limitations. The work highlights the state-of-the-art in PDF fitting, including Alekhin, MRST, and CTEQ benchmarks, and outlines potential improvements from future HERA data and combined analyses. Overall, the report guides precision predictions at the LHC by clarifying where PDF uncertainties dominate and how to mitigate them through higher-order theory and data synergy.

Abstract

We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of $F_L$), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques.