Sensitivity of the photon-induced processes to the proton charge radius
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 $Λ^2=0.71$~GeV$^2$ with PDG test scenarios corresponding to $r_p=0.8751 \pm 0.0061$~fm and $r_p=0.84087 \pm 0.00039$~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 $Λ^2 = 0.465 \pm 0.056~\mathrm{GeV}^2$, corresponding to an effective radius $r_p = 1.002 \pm 0.038~\mathrm{fm}$, which indicates non-trivial 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 LHC can provide complementary model-dependent information on the proton electromagnetic forms factor.