Saturation effects in forward-forward dijet production in p+Pb collisions

TL;DR

This study investigates saturation effects in forward-forward dijet production in p+Pb collisions at the LHC within High Energy Factorization. By comparing two nonlinear evolution schemes for the unintegrated gluon density (rcBK and KS), it provides predictions for cross sections and nuclear modification factors RpA in forward jets, showing notable suppression in p+Pb relative to p+p, especially at small x and for near back-to-back configurations. The results support stronger nuclear saturation and demonstrate the utility of forward-forward jets as a clean probe, while highlighting the need for higher-order corrections and resummations beyond the leading-order HEF framework. The work offers guidance for LHC measurements and for constraining nonlinear small-x dynamics in nuclei.

Abstract

We study saturation effects in the production of forward dijets in proton-lead collisions at the Large Hadron Collider, using the framework of High Energy Factorization. Such configurations, with both jets produced in the forward direction, probe the gluon density of the lead nucleus at small longitudinal momentum fraction, and also limit the phase space for emissions of additional jets. We find significant suppression of the forward dijet azimuthal correlations in proton-lead versus proton-proton collisions, which we attribute to stronger saturation of the gluon density in the nucleus than in the proton. In order to minimize model dependence of our predictions, we use two different extensions of the Balitsky-Kovchegov equation for evolution of the gluon density with sub-leading corrections.

Figures (4)

• Figure 1: Differential cross sections for forward-forward jet productions in p+p collisions, as functions of transverse momentum of the leading jet (left), jet rapidity (middle), and the azimuthal angle between the two hardest jets. The two bands correspond to two different unintegrated gluon distributions used for calculations. The width of the bands comes from varying the renormalization and factorizations scales by factors $\frac{1}{2}$ and 2 around the central value taken as the average $p_t$ of the two leading jets.
• Figure 2: Nuclear modification ratios, defined in Eq. (\ref{['eq:RpA']}), as functions of $p_t$s of the leading (left) and subleading (right) jets produced in the forward region. Different bands correspond to different unintegrated gluon distributions (KS and rcBK) used for calculations. To asses the uncertainty related to the nonlinear effects, the KS result was computed with two values of the $c$ parameter, defined in Eq. (\ref{['eq:radius']}), and the rcBK prediction was obtained with two values of the $d$ parameter from Eq. (\ref{['eq:QA']}). The width of the bands comes from varying the renormalization and factorizations scales by factors $\frac{1}{2}$ and 2 around the central value taken as the average $p_t$ of the two leading jets.
• Figure 3: Nuclear modification ratios, defined in Eq. (\ref{['eq:RpA']}), as functions of jet rapidity (left) and the azimuthal distance between two hardest jets produced in the forward region (right). All details as in Fig. \ref{['fig:RpA1']}.
• Figure 4: Comparisons of predictions obtained with a linear vs non-linear proton evolution, for the differential cross section in p+p collisions as a function of the azimuthal angle (left), and the nuclear modification factor as a function of $p_t$ of the subleading jet. KS gluon densities are used, with parameter $c=1$ in the nuclear case.