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

TL;DR

This paper calculates the diffractive contribution to inclusive dijet photoproduction in ultraperipheral heavy-ion collisions at the LHC using NLO QCD, finding it generally small but potentially significant with looser cuts, and also explores proton-proton UPCs.

Contribution

The study provides the first detailed NLO QCD calculation of diffractive contributions in UPCs, accounting for nuclear suppression and factorization breaking effects.

Findings

Diffractive contribution is 5-10% in small-x bins with strict cuts.

Looser cuts increase the diffractive ratio to 10-20%.

In proton-proton UPCs, the ratio can reach 10-15%.

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-$x_A$ bins in the ATLAS kinematics at $\sqrt{s_{NN}}=5.02$ TeV. Its smallness is a result of the restricted kinematics ($p_{T1} > 20$ GeV and $x_A > 0.001$), 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 $p_T$ cuts, e.g., $p_{T1} > 10$ GeV and $p_{T i\neq 1} > 5-7$ GeV, we find that $(d\sigma_{\rm diff}/dx_A)/(d\sigma_{\rm inc}/dx_A)$ can reach $10-20$% at $x \approx 5 \times 10^{-4}$. Also, applying our framework to proton-proton UPCs at $\sqrt{s_{NN}}=13$ TeV, we find that the ratio of the diffractive and inclusive cross sections of dijet photoproduction $(d\sigma_{\rm diff}/dx_p)/(d\sigma_{\rm inc}/dx_p)$ is as large as $10-15$% for $x_p \sim 5 \times 10^{-4}$. An account of the contribution of Pomeron-Pomeron scattering, which is not included in our analysis, should make this ratio somewhat larger.