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+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 $\widehat{\rho }(v_2^2,[p_T])$ as a function of charged particle multiplicity in $p+\text{Au}$ and $d+\text{Au}$ collisions at $\sqrt{s}=200\mathrm{GeV}$, and $p+\mathrm{Pb}$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. This sign change is absent in calculations without initial state momentum anisotropies. The model further predicts a qualitative difference between the centrality dependence of $\widehat{\rho }(v_2^2,[p_T])$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s}=200\mathrm{GeV}$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$, with only the latter showing a sign change in peripheral events. Predictions for $\mathrm{O}+\mathrm{O}$ collisions at different collision energy show a similar behavior. Experimental observation of these distinct qualitative features of $\widehat{\rho }(v_2^2,[p_T])$ 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.

• Received 8 July 2020
• Accepted 8 October 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.192301