Long range two-particle rapidity correlations in A+A collisions from high energy QCD evolution

TL;DR

This paper develops a QCD-based formalism to compute long-range two-particle rapidity correlations in heavy-ion collisions, incorporating gluon saturation effects and comparing results with experimental data from RHIC and LHC.

Contribution

It introduces a mean field approximation for nuclear wavefunction evolution using the BK equation with running coupling to predict two-gluon correlations.

Findings

Results match RHIC data on the near-side ridge.

Predictions for LHC long-range correlations are provided.

The formalism accounts for all radiative and rescattering effects at leading order.

Abstract

Long range rapidity correlations in A+A collisions are sensitive to strong color field dynamics at early times after the collision. These can be computed in a factorization formalism \cite{GelisLV5} which expresses the $n$-gluon inclusive spectrum at arbitrary rapidity separations in terms of the multi-parton correlations in the nuclear wavefunctions. This formalism includes all radiative and rescattering contributions, to leading accuracy in $\alpha_s\Delta Y$, where $\Delta Y$ is the rapidity separation between either one of the measured gluons and a projectile, or between the measured gluons themselves. In this paper, we use a mean field approximation for the evolution of the nuclear wavefunctions to obtain a compact result for inclusive two gluon correlations in terms of the unintegrated gluon distributions in the nuclear projectiles. The unintegrated gluon distributions satisfy the Balitsky-Kovchegov equation, which we solve with running coupling and with initial conditions constrained by existing data on electron-nucleus collisions. Our results are valid for arbitrary rapidity separations between measured gluons having transverse momenta $p_\perp,q_\perp\gtrsim \qs$, where $\qs$ is the saturation scale in the nuclear wavefunctions. We compare our results to data on long range rapidity correlations observed in the near-side ridge at RHIC and make predictions for similar long range rapidity correlations at the LHC.