Estimating nonlinear effects in forward dijet production in ultra-peripheral heavy ion collisions at the LHC

Using the framework that interpolates between the leading power limit of the color glass condensate and the high energy (or k_T) factorization we calculate the direct component of the forward dijet production in ultra-peripheral Pb –Pb collisions at CM energy 5.1TeV per nucleon pair. The formalism is applicable when the average transverse momentum of the dijet system P_T is much bigger than the saturation scale Q_s, P_T≫ Q_s, while the imbalance of the dijet system can be arbitrary. The cross section is uniquely sensitive to the Weizsäcker–Williams (WW) unintegrated gluon distribution, which is far less known from experimental data than the most common dipole gluon distribution appearing in inclusive small-x processes. We have calculated cross sections and nuclear modification ratios using WW gluon distribution obtained from the dipole gluon density through the Gaussian approximation. The dipole gluon distribution used to get WW was fitted to the inclusive HERA data with the nonlinear extension of unified BFKL + DGLAP evolution equation. The saturation effects are visible but rather weak for realistic p_T cut on the dijet system, reaching about 20% with the cut as low as 6GeV. We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.