Observable signatures of initial state momentum anisotropies in nuclear collisions

TL;DR

This paper predicts that the correlation between elliptic flow and average transverse momentum in small nuclear collisions reveals initial state momentum anisotropies, with a characteristic sign change as a function of multiplicity, serving as evidence for Color Glass Condensate effects.

Contribution

It introduces a model-based prediction of a sign change in the $\hat{\rho}(v_2^2,[p_T])$ correlator as a signature of initial state momentum anisotropies in nuclear collisions.

Findings

Predicts a sign change of the correlator in small systems at certain energies.

Shows a difference in centrality dependence between small and large systems.

Provides predictions for O+O collisions at various energies.

Abstract

We show that the correlation between the elliptic momentum anisotropy, $v_2$, and the average transverse momentum, $[p_T]$, at fixed multiplicity in small system nuclear collisions carries information on the origin of the observed momentum anisotropy. A calculation using a hybrid IP-Glasma+\textsc{Music}+UrQMD model that includes contributions from final state response to the initial geometry as well as initial state momentum anisotropies of the Color Glass Condensate, predicts a characteristic sign change of the correlator $\hat{\rho}(v_2^2,[p_T])$ as a function of charged particle multiplicity in p+Au and d+Au collisions at $\sqrt{s}=200\,{\rm GeV}$, and p+Pb collisions at $\sqrt{s}=5.02\,{\rm TeV}$. This sign change is absent in calculations without initial state momentum anisotropies. The model further predicts a qualitative difference between the centrality dependence of $\hat{\rho}(v_2^2,[p_T])$ in Au+Au collisions at $\sqrt{s}=200\,{\rm GeV}$ and Pb+Pb collisions at $\sqrt{s}=5.02\,{\rm TeV}$, with only the latter showing a sign change in peripheral events. Predictions for O+O collisions at different collision energy show a similar behavior. Experimental observation of these distinct qualitative features of $\hat{\rho}(v_2^2,[p_T])$ in small and large systems would constitute strong evidence for the presence and importance of initial state momentum anisotropies predicted by the Color Glass Condensate effective theory.