PDF effects in high-mass Drell-Yan SMEFT analyses across flavour space

Abstract

High-mass Drell-Yan dilepton production provides one of the most sensitive probes of semileptonic four-fermion operators in the Standard Model Effective Field Theory (SMEFT), thanks to the energy growth of the corresponding contributions. At the same time, these measurements also constrain parton distribution functions (PDFs) in the large-$x$ region, where PDF uncertainties can mimic or obscure smooth new-physics effects. In this work we study how the impact of PDF profiling on SMEFT constraints depends on the quark-flavour structure of the effective operators, by performing a joint fit of SMEFT Wilson coefficients and PDF nuisance parameters. We find that PDF profiling induces a strongly flavour-dependent degradation of the SMEFT sensitivity, both with current data and in HL-LHC projections. The largest broadenings occur for operators involving first-generation quarks, whose effects are correlated with the high-$x$ valence-quark luminosities that dominate the high-mass spectrum. Operators involving heavier quark flavours are less affected, although with non-negligible operator-dependent variations. As a byproduct of our analysis, we obtain an estimate on the changes induced in the PDF sector by profiling SMEFT effects. We also show that angular information provides an important handle to reduce degeneracies among SMEFT directions and between SMEFT effects and allowed PDF deformations. These results demonstrate that the relevance of PDF uncertainties in high-energy SMEFT fits is not uniform across flavour space, and must be assessed in a flavour-dependent way.