How large is the diffractive contribution to inclusive dijet photoproduction in UPCs at the LHC?

Abstract

Using next-to-leading order (NLO) perturbative QCD, we calculate the diffractive contribution to inclusive dijet photoproduction in Pb-Pb ultraperipheral collisions (UPCs) at the LHC and find that it does not exceed 5-10% in small-xA bins in the ATLAS kinematics at sNN=5.02 TeV. Its smallness is a result of the restricted kinematics (pT1 > 20 GeV and xA > 0.001), the large nuclear suppression of nuclear diffractive parton distribution functions predicted in the leading twist model of nuclear shadowing, and additional suppression due to QCD factorization breaking in diffraction. At the same time, using looser pT cuts, e.g., pT1 > 10 GeV and pT i≠ 1 > 5-7 GeV, we find that (dσ diff/dxA)/(dσ inc/dxA) can reach 10-20% at x ≈ 5 × 10-4. Also, applying our framework to proton-proton UPCs at sNN=13 TeV, we find that the ratio of the diffractive and inclusive cross sections of dijet photoproduction (dσ diff/dxp)/(dσ inc/dxp) is as large as 10-15% for xp 5 × 10-4. An account of the contribution of Pomeron-Pomeron scattering, which is not included in our analysis, should make this ratio somewhat larger.