LONG-RANGE RAPIDITY CORRELATIONS IN HEAVY-LIGHT ION COLLISIONS

Yuri V Kovchegov,Douglas E Wertepny

doi:10.1142/s2010194514600234

Abstract

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δϕ = π with the other one centered around Δϕ = 0 (here Δϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus-nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

Highlights

This proceedings contribution is based on the paper [1]
The projectile is assumed to be smaller than the target, but still large enough a nucleus for the two gluons to be produced in collisions of different nucleons in the projectile with the target nucleus
The saturation effects in the projectile are taken at the lowest non-trivial order with α2s A11/3 1: for two-gluon correlations this corresponds to only two nucleons interacting in the projectile nucleus

Summary

General Expression for Two-Gluon Production in Heavy-Light Ion Collisions

This proceedings contribution is based on the paper [1]. Consider two-gluon production in a collision of a large heavy ion (target) with a lighter nucleus (projectile). The saturation effects in the projectile are taken at the lowest non-trivial order with α2s A11/3 1: for two-gluon correlations this corresponds to only two nucleons interacting in the projectile nucleus. The approximation used in [1] corresponds to calculating the two-gluon production in nucleus-nucleus (A + A) collisions in the McLerranVenugopalan (MV) model [2] while keeping interactions with the projectile nucleus to the lowest non-trivial order of two interacting nucleons. This setup may not directly describe the bulk of proton-nucleus (p + A) collisions ( known as the dense-dilute collisions); our setup may be relevant for high-multiplicity p + A collisions. Shortly before our work [1]

Matrix Elements of Wilson Lines in the Quasi-Classical Approximation

Nc2 in the MV model it is given by

Correlation Function

Geometric Correlations