Beam-energy dependence of the longitudinal broadening of two-particle transverse momentum correlations from STAR

Two-particle transverse momentum correlator is 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 entropy density $\eta/s$ of the quark-gluon plasma formed in the collisions [1,2]. We used the data of Au--Au collisions at $\sqrt{s_{\rm NN}}$ = 200-11.5 GeV to investigate: (i) the longitudinal broadening of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ correlator, (ii) the long range azimuthal dependence of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ [3]. We will present the centrality and event shape dependence of the longitudinal width $\sigma\left( \Delta\eta \right)$ and the azimuthal harmonics $a^{p_T}_{n}$ of the $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ correlator for different beam energies. Our measurements are compared to similar LHC measurements [4]. The $G_{2}\left(\Delta\eta,\Delta\varphi\right)$ beam-energy dependence is expected to reflect the temperature ($T$) and chemical potential ($\mu_{B}$) dependence of $\eta/s$.

[1] S. Gavin and M. Abdel-Aziz, Phys. Rev. Lett. 97, 162302 (2006)

[2] V. Gonzalez, et al., Eur.Phys.J.C 81 (2021) 5, 465

[3] N. Magdy, et al., arXiv:2101.01555

[4] ALICE Collaboration Phys.Lett.B 804 (2020) 135375