Tailored PDFs for new physics searches

Abstract

Parton Distribution Functions (PDFs) at large Bjorken-$x$ are mostly constrained by high-energy measurements, and thus risk to absorb the energy-growing effects of unaccounted new physics (NP) if it were present in the high-energy tails of hadron-collider observables used in global PDF fits. If undetected, such contamination biases the resulting PDFs and can potentially hide the absorbed NP signals from subsequent searches. We illustrate this effect in a Standard Model Effective Field Theory (SMEFT) risk-assessment scenario affecting high-mass Drell-Yan production at the High-Luminosity LHC, and compare two strategies to obtain robust PDFs and SMEFT bounds: conservative fits that exclude data above an energy cut, and simultaneous fits of PDFs and Wilson coefficients performed with the {\tt SIMUnet} tool. Both approaches are shown to successfully recover the injected new physics that is otherwise absorbed and hidden by the PDFs.