The STAR Collaboration2024-05-082024-05-082023The STAR Collaboration.(2023). Elliptic flow of heavy-flavor decay electrons in Au+Au collisions at sNN<math><msqrt is="true"><mrow is="true"><msub is="true"><mrow is="true"><mi is="true">s</mi></mrow><mrow is="true"><msub is="true"><mrow is="true"></mrow><mrow is="true"><mi mathvariant="normal" is="true">NN</mi></mrow></msub></mrow></msub></mrow></msqrt></math> = 27 and 54.4 GeV at RHIC. Physics Letters B, 844, 138071. https://doi.org/10.1016/j.physletb.2023.138071https://hdl.handle.net/1911/115650We report on new measurements of elliptic flow (v2) of electrons from heavy-flavor hadron decays at mid-rapidity (|y|<0.8) in Au+Au collisions at sNN = 27 and 54.4 GeV from the STAR experiment. Heavy-flavor decay electrons (eHF) in Au+Au collisions at sNN = 54.4 GeV exhibit a non-zero v2 in the transverse momentum (pT) region of pT< 2 GeV/c with the magnitude comparable to that at sNN=200 GeV. The measured eHF v2 at 54.4 GeV is also consistent with the expectation of their parent charm hadron v2 following number-of-constituent-quark scaling as other light and strange flavor hadrons at this energy. These suggest that charm quarks gain significant collectivity through the evolution of the QCD medium and may reach local thermal equilibrium in Au+Au collisions at sNN=54.4 GeV. The measured eHF v2 in Au+Au collisions at sNN= 27 GeV is consistent with zero within large uncertainties. The energy dependence of v2 for different flavor particles (π,ϕ,D0/eHF) shows an indication of quark mass hierarchy in reaching thermalization in high-energy nuclear collisions.engExcept where otherwise noted, this work is licensed under a Creative Commons Attribution (CC BY) license. Permission to reuse, publish, or reproduce the work beyond the terms of the license or beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Journal article1-s20-S0370269323004057-mainhttps://doi.org/10.1016/j.physletb.2023.138071