Long-Range Rapidity Correlations in Heavy-Light Ion Collisions

TL;DR

This paper investigates long-range rapidity correlations in heavy-light ion collisions within the Color Glass Condensate framework, identifying various correlation types and their potential to mimic flow signals in experiments.

Contribution

It provides a detailed calculation of two-gluon production including all-order saturation effects, revealing new insights into azimuthal correlations and their impact on flow measurements.

Findings

Identifies four types of correlations: geometric, HBT, away-side, and near-side azimuthal.

Shows long-range correlations can mimic flow signals, affecting experimental flow measurements.

Demonstrates that early-time CGC dynamics produce azimuthal non-flow correlations with similar amplitudes and shapes.

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 \Delta \phi =\pi\ with the other one centered around \Delta \phi =0 (here \Delta \phi\ 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.