Speaker
Description
In studies of proton structure and global parton distribution function (PDF) analyses, the ratio of down to up quarks remains poorly constrained by experimental data. This is because the contributions of these two light quarks are typically entangled in physical processes and are difficult to disentangle in final-state observables. In this talk, we present a recently proposed method that enables a direct probe of the light-quark ratio through the forward–backward asymmetry (AFB) of the Drell–Yan process at hadron colliders. In this approach, the down- and up-quark contributions to AFB are factorized and defined as new structure parameters, which can be directly measured as experimental observables. We will focus on the underlying idea of this method, recent experimental results from both the Tevatron and the LHC over the past three years, and the impact of these new measurements on global PDF analyses. In particular, results for the new structure parameters obtained from Tevatron and LHC data—corresponding to pure proton–proton or proton–antiproton interactions—indicate an enhancement of the valence down–to–valence up quark ratio relative to current PDF predictions. Equivalently, these results suggest that the ratio of anti-down to anti-up quarks (the SU(2) flavor asymmetry) is reduced and approaches unity. This finding is in tension with previous results extracted from deuteron data, such as those from the NuSea and SeaQuest experiments. Beyond providing an important new insight into proton structure, these results also cast doubt on the common assumptions of small nuclear corrections in deuteron targets and exact proton–neutron isospin symmetry.
[*for the conference committee, we provide the corresponding references of these works:
Phys. Rev. D 106, 033001 (2022)
Phys. Rev. D 107, 054008 (2023)
Phys. Rev. D 110, L091101 (2024)
arXiv:2505.17608
arXiv:2510.08941]
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