Azimuthal dependence of two-particle transverse momentum current correlations

TL;DR

This study investigates how two-particle transverse momentum correlations depend on azimuthal angle in heavy-ion collisions, aiming to constrain the quark-gluon plasma's shear viscosity to entropy density ratio using model comparisons and potential experimental measurements.

Contribution

It demonstrates the sensitivity of azimuthal transverse momentum correlations to the shear viscosity over entropy density ratio and compares model predictions to guide experimental constraints.

Findings

Different models produce distinct Fourier coefficients of $G_2$.

These coefficients show significant dependence on $ ext{η/s}$ in the AMPT model.

Measurements of $G_2$ can help distinguish theoretical models and constrain $ ext{η/s}$.

Abstract

Two-particle transverse momentum correlation functions are a powerful technique for understanding the dynamics of relativistic heavy-ion collisions. Among these, the transverse momentum correlator $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ is of particular interest for its potential sensitivity to the shear viscosity per unit of entropy density $\eta/s$ of the quark-gluon plasma formed in heavy-ion collisions. We use the UrQMD, AMPT, and EPOS models for Au--Au at $\sqrt{s_{\rm NN}}$ = 200~GeV and Pb--Pb at $\sqrt{s_{\rm NN}}$ = 2760~GeV to investigate the long range azimuthal dependence of $G_{2}\left(\Delta\eta,\Delta\varphi\right)$, and explore its utility to constrain $\eta/s$ based on charged particle correlations. We find that the three models yield quantitatively distinct transverse momentum Fourier harmonics coefficients $a^{p_{\rm T}}_{\rm n}$. We also observe these coefficients exhibit a significant dependence on $\eta/s$ in the context of the AMPT model. These observations suggest that exhaustive measurements of the dependence of $G_{2}\left(\Delta\varphi \right)$ with collision energy, system size, collision centrality, in particular, offer the potential to distinguish between different theoretical models and their underlying assumptions. Exhaustive analyses of $G_{2}\left(\Delta\varphi \right)$ obtained in large and small systems should also be instrumental in establishing new constraints for precise extraction of $\eta/s$.