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  • Letter
  • Open Access

Relative difference between up and down quark structure of the proton

Mingzhe Xie1, Siqi Yang1,*, Wenhao Ma1, Minghui Liu1, Liang Han1, and C.-P. Yuan2

  • *Contact author: yangsq@ustc.edu.cn
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Phys. Rev. D 113, L011504 – Published 30 January, 2026

DOI: https://doi.org/10.1103/46f7-2s81

Abstract

We present a novel determination of the down-to-up quark composition ratio using the forward-backward asymmetry observed in the proton-proton collisions at the LHC. This method offers unique insights into the flavor-specific difference between down and up quarks, which are difficult to isolate in traditional cross-section measurements due to the inherent mixing of contributions from both flavors. In this study, we systematically measure the down-to-up quark ratio over a broad momentum fraction (x) range of 0.01 to 0.1, utilizing the sensitivity of the forward-backward asymmetry to quark-level couplings. Our findings reveal significant deviations in both the value and x dependence of this ratio compared to predictions from current parton distribution functions. These discrepancies highlight potential limitations in existing parton distribution function parametrization and emphasize the importance of flavor-separated measurements for advancing our understanding of proton structure.

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Phys. Rev. D 113, L011504– Published 30 January, 2026
  • Received 26 May 2025
  • Accepted 15 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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