Abstract
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 mathrm {Pb}–mathrm {Pb} collisions at CM energy 5.1,mathrm {TeV} 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}gg 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 6,mathrm {GeV}. We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.
Highlights
High energy collisions of heavy ions provide a unique opportunity to investigate the quark–gluon plasma regime of QCD
One could construct a framework in which the leading-twist shadowing gluon distribution could be obtained from the unintegrated gluon distributions (UGDs) provided they account for diffractive states and the appropriate resummation of collinear and small x logarithms would be performed in both cases
As we show later, x A can reach values of order ∼ 10−3 so that we probably venture outside the applicability domain of the saturation formalism. (ii) We focus on the kinematic region where x A < xγ, which implies that we look for a forward dijet configuration along the photon direction
Summary
In the color glass condensate (CGC) effective theory [11], which models the saturation phenomenon, the non-universality appears as a proliferation of color averages of many Wilson line operators These correlators in principle parametrize non-perturbative physics, playing a similar role to parton distributions. This, does not mean that the latter is inherently incompatible with all approaches using UGDs. In principle, one could construct a framework in which the leading-twist shadowing gluon distribution could be obtained from the UGDs provided they account for diffractive states and the appropriate resummation of collinear and small x logarithms would be performed in both cases. One of the goals of this work is to investigate whether the present LHC kinematics can give a restriction of that distribution This is done, by a direct calculation of the cross sections and nuclear modification factors for various observables taking into account existing information on WW gluon distribution.
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