Role of initial transverse momentum in a hybrid approach

Background: Significant theoretical uncertainties exist with respect to the initial condition of the hydrodynamic description of ultrarelativistic heavy-ion collisions. Several approaches exist, of which some contain initial momentum information. Its impact is commonly assumed to be small and final flow is seen as a linear response to the initial-state eccentricity.

Purpose:The purpose of this work is to study the effect of exchanging initial condition models in a modular hybrid approach, especially regarding changes in the event-by-event correlations of elliptic and triangular flow with initial-state quantities like eccentricity and initial-state transverse momentum.

Method:This study is performed in the hybrid approach SMASH-vHLLE, composed of the hadronic transport approach SMASH and the $(3+1)d$ viscous hydrodynamic code vHLLE. The initial condition models investigated are SMASH IC, $T_R\mathrm{ENTo}$, and IP-Glasma. Correlations are calculated on an event-by-event basis between the eccentricities and momentum anisotropies of the initial state as well as the momentum anisotropies in the final state, both for central and off-central collisions for AuAu collisions at $\sqrt{s_{NN}}=200$ GeV. The response of the final-state to initial-state properties is studied.

Results:This work demonstrates that, although averaged values for the initial eccentricities of these models are very similar, substantial differences exist both in the distributions of eccentricities, the correlations among the initial-state properties as well as in the correlations between initial-state and final-state properties. Notably, whereas initial-state momentum anisotropy is shown to not affect the final-state flow, the presence of radial flow affects the emergence of final-state momentum anisotropies.

Conclusions:Inclusion of radial flow in the linear fit improves the prediction of final-state flow from initial-state properties. The presence of transverse momentum in the initial state has an effect on the emergence of flow and is therefore a relevant part of initial-state models, challenging the common understanding of final-state momentum anisotropies being a linear response to initial-state eccentricity only.