Speaker
Yadav Pandit
(Kent State University)
Description
A primary goal of the RHIC Beam Energy Scan (BES) is to search for evidence of a transition
between a hadron gas and a Quark Gluon Plasma. The dependence of $v_{1}$ and higher flow
harmonics on system size and beam energy may be sensitive to the degrees of
freedom in the system, as a consequence of early pressure gradients and a potential softening in
the equation of state. In this talk, we present STAR measurements of $v_{1}$ for $\pi^\pm$, $K^\pm$,
protons and antiprotons along with $v_{n}$ for charged particles from 7.7 GeV to 200 GeV. A striking observation is that
the $v_1$ slope $F = dv_1 /dy$ for net protons, which is an estimate of the directed flow contribution from baryon number transported to the midrapidity region, changes sign twice within the BES energy range. In contrast, $F$ for all other particle types is negative at all studied energies. For charged particles, we observe a local minimum in integrated ($0.2 < p_T < 2.0$ GeV/$c$ and $|\eta| < 1.0)$ directed flow between 11.5 and 27 GeV for central 0-20\% collisions. At a similar centrality, we observe a shallow minimum in the energy dependence of $v_{3}$ for charged hadrons. We also show the ratio of the two-particle cumulant $v_{n}\{2\}$ to participant eccentricity($\varepsilon_{n, {\rm part}}$) to quantify how well the system converts initial geometry fluctuations into momentum-space correlations for different collision energies, system sizes and harmonics.
Primary author
Yadav Pandit
(Kent State University)
