Sensitivities to PDFs in parton shower MC generator reweighting and tuning

Abstract

Evaluating parton density systematic uncertainties in Monte~Carlo event generator predictions has long been achieved by reweighting between the original and systematic PDFs for the initial state configurations of the individual simulated events. This weighting is now pre-emptively performed in many generators, providing convenient weight factors for PDF and scale systematics -- including for NLO calculations where counterterms make the weight calculation complex. This note attempts a pedagogical discussion and empirical study of the consequences of neglecting the effects of PDF variations on the beyond-fixed-order components of MC models, and the implications for parton shower & MPI tuning strategies. We confirm that the effects are usually small, for well-understood reasons, and discuss the connected issue of consistent treatment of the strong coupling between PDFs and parton showers, where motivations from physical principles and the need for good data-description are not always well-aligned.