CGC-induced longitudinal ridge in p-Pb collisions

TL;DR

This paper addresses the origin of the ridge in high-energy proton-nucleus collisions by examining long-range rapidity correlations within the CGC framework. Using $k_T$-factorization and unintegrated gluon distributions, along with rcBK/JIMWLK evolution and MV-model averaging, the authors compute two-particle correlators and the per-trigger yield. A key finding is a rebound of the near-side correlation at large $|Δη|$, strongest near $p_T \approx 2$ GeV/$c$ and shifting with energy, with a pronounced asymmetry for unequal transverse momenta in $p$-$Pb$. These results provide a robust CGC-based mechanism for the ridge and offer a testable distinction from hydrodynamic explanations, guiding future experimental measurements.

Abstract

Within the Color Glass Condensate (CGC) effective field theory, we investigate the long-range rapidity correlations in proton-lead (p-Pb) collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. A distinctive correlation rebound is observed, where the correlation bounces after reaching a minimum at large rapidity gaps ($|Δη|>2$). The rebound means a strong correlation appears at large rapidity gap. Studying the rebound structures can thus illuminate the formation of the ridge. We find that the rebound is most obvious when the transverse momenta of two measured particles are around 2 $\mathrm{GeV/c}$, and it moves to larger rapidity gaps at higher collision energies. Beyond that, the rapidity correlations in p-Pb collisions show asymmetry when the transverse momenta of two particles differ. The asymmetry, a unique signature of the asymmetric collisions, vanishes when the transverse momenta of two particles coincide. These findings provide direct insight into gluon saturation and quantum evolution.

Figures (2)

• Figure 1: The per-trigger yield in the $\Delta y$-$\Delta\phi$ plane for 5.02 TeV p-Pb collisions with transverse momentum integrated within $1\leq p_{\rm T}(q_{\rm T})\leq 3$ GeV$/c$ and with rapidity integrated in $-2.865\leq y_{\rm p}(y_{\rm q})\leq1.935$ (the left panel). The right panel shows the near-side ridge yield projects to $\Delta y$ axis for p-Pb collisions at 5.02 and 8.16 TeV, respectively.
• Figure 2: The near-side per-trigger yield as a function of $\Delta y$ at 5.02 TeV p-Pb collisions (top row) and 7 TeV pp collisions (bottom row) within the rapidity windows $[-2.4, 2.4]$, (a) the red curve and black dotted line represent the ridge yield in transverse momentum interval $p_{\rm T}$($q_{\rm T})\in$[1, 2] and [2, 3] GeV$/c$, respectively, the ridge yield for transverse momentum interval $p_{\rm T}\in$[1, 2] GeV$/c$, $q_{\rm T}\in$[2, 4] GeV$/c$ (b), for $p_{\rm T}\in$[1, 2] GeV$/c$, $q_{\rm T}\in$[4, 8] GeV$/c$ (c), for $p_{\rm T}$($q_{\rm T})\in$[1, 2] GeV (d), for $p_{\rm T}\in$[1, 2] GeV$/c$, $q_{\rm T}\in$[2, 4] GeV$/c$ (e) and for $p_{\rm T}\in$[1, 2] GeV$/c$, $q_{\rm T}\in$[4, 8] GeV$/c$ (f), respectively.