Inclusive and diffractive dijet photoproduction in ultraperipheral Pb-Pb collisions at the LHC

Abstract

In this contribution, we summarize NLO pQCD predictions for inclusive and diffractive dijet photoproduction in Pb-Pb UPCs at the LHC. We demonstrate that the theory describes well the preliminary ATLAS data on the inclusive cross section, which probes nuclear parton distributions (PDFs) down to $x_A \approx 0.005$ and which can reduce current uncertainties of the small-$x$ nuclear gluon distribution by approximately a factor of 2. Employing predictions of the leading twist approach to nuclear shadowing for nuclear diffractive PDFs, we calculate the cross section of diffractive dijet photoproduction and show that its $x_{\gamma}$ dependence is sensitive to the effect of nuclear shadowing and the mechanism of QCD factorization breaking in hard diffraction. We also find that due to large leading twist nuclear shadowing and restricted kinematics, the diffractive contribution to the inclusive cross section of dijet photoproduction does not exceed $5-10$%, which helps with an ambiguous interpretation of the ATLAS data.