Speaker
Description
In heavy-ion collisions, directed flow ($v_1$) and elliptic flow ($v_2$) represent the first and second harmonic coefficients in the Fourier expansion of the final-state particle azimuthal distribution. These coefficients are sensitive to the equation of state (EoS) and the degrees of freedom in the produced medium. The measurement of $v_1$ and $v_2$ for identified particles provides a powerful tool for investigating the properties of nuclear matter created in heavy-ion collisions.
Using data collected by the STAR experiment during the second phase of the Beam Energy Scan (BES-II), we will present a systematic analysis of $v_1$ and $v_2$ for identified particles in Au+Au collisions at beam energies between 3.0 and 4.5 GeV. Number of Constituent Quark Scaling is observed to break down at $\sqrt{s_{NN}} \leq 3.2$ GeV, indicating a hadron-dominated equation of state. However, the scaling is gradually restored as the collision energy increases to 4.5 GeV, signaling the onset of partonic interactions. In addition, negative $v_1$ slopes for kaons are observed in the low $p_T$ region ($p_T < 0.6$ GeV/c). A comparison with JAM model calculations, with and without spectators, suggests that the observed kaon anti-flow at low $p_T$ can be explained by the shadowing effect of spectators in non-central collisions in the high baryon density region.
