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Sensitivity of the photon-induced processes to the proton charge radius

Nikhil Krishna*, Mariola Kłusek-Gawenda†, and Rafał Staszewski‡

  • *Contact author: Nikhil.Krishna@ifj.edu.pl
  • †Contact author: Mariola.Klusek@ifj.edu.pl
  • ‡Contact author: Rafal.Staszewski@ifj.edu.pl
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Phys. Rev. D 114, 076003 – Published 2 October, 2026

DOI: https://doi.org/10.1103/47cq-vdfk

Abstract

We study the exclusive production of dileptons in proton-proton collisions as a probe of the proton charge-radius scale. Using a dipole form factor model, we compare the conventional choice of Λ2=0.71  GeV2 with Particle Data Group test scenarios corresponding to rp=0.8751±0.0061  fm and rp=0.84087±0.00039  fm, representative of the historical large radius from CODATA and smaller radius from muonic-hydrogen determinations, respectively. The sensitivity is greatest at large dilepton invariant masses and forward/backward rapidity. Fitting to the current ATLAS and CMS data within the adopted model gives Λ2=0.465±0.056  GeV2, corresponding to an effective radius rp=1.002±0.038  fm, which indicates nontrivial sensitivity on the proton-radius scale, but is not yet a definitive solution to the proton-radius puzzle. While this result depends on the theoretical model used in the calculation, it shows that exclusive dilepton production at the LHC can provide complementary model-dependent information on the proton electromagnetic form factor.

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Phys. Rev. D 114, 076003– Published 2 October, 2026
  • Received 30 June 2026
  • Accepted 11 September 2026
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