Inclusive Charged Hadron Elliptic Flow in Au + Au Collisions at √ˢᴺᴺ = 7.7-39 GeV

A systematic study is presented for centrality, transverse momentum pT, and pseudorapidity (η) dependence of the inclusive charged hadron elliptic flow v2 at midrapidity (|η| \u3c 1.0) in Au + Au collisions at √ˢᴺᴺ = 7.7, 11.5, 19.6, 27, and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and four-particle cumulants (v2{4}), are presented to investigate nonflow correlations and v2 fluctuations. We observe that the difference between v2{2} and v2{4} is smaller at the lower collision energies. Values of v2, scaled by the initial coordinate space eccentricity, v2/ϵ, as a function of pT are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider √ˢᴺᴺ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at √ˢᴺᴺ = 2.76 TeV). The v2(pT) values for fixed pT rise with increasing collision energy within the pT range studied (\u3c2 GeV/c). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of v2(pT). We also compare the v2 results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at beam energy scan energies are discussed