Speaker
Description
Directed flow of particles is an important feature seen in heavy-ion collisions and is a
sensitive probe of the equation of state (EoS) of the matter produced in the collisions.
Model calculations have also predicted that directed flow could be a sensitive probe of the
softening of the EoS associated with a first order phase transition. Directed flow of protons
and anti-protons are also of interest as they offer sensitivity to both the contributions from
the transported quarks and also the medium generated component from the produced
quarks. Measurements of proton and net proton directed flow from BES-I have shown that
there is a non-monotonous dependence on collision energy. We will present
measurements of the directed flow of protons and anti-protons from the collision energies
of 7.7, 9.2, 11.5, 14.5, 17.3, 19.6, and 27 GeV Au+Au collisions, using high statistics BES-II
data from STAR. We will also present a decomposition of proton directed flow into a
medium generated component and a component ($v_{1}$ excess) attributed to transported
protons. The v1 excess component is found to show a simple scaling from a center of mass
energy of 200 GeV to ~10 GeV, but to break scaling below 10 GeV. The new results have
significantly reduced uncertainties compared to those from BES-I and also allow
differential measurements in centrality and transverse momentum. Measurements will be
compared to different model calculations and implications to the understanding of the
QCD phase structure and EoS of the medium will be discussed.
