Light hadron production measurements with Au+Au Collisions from $\sqrt{s_{NN}} = 3.2$--$4.5$ GeV with STAR

Abstract

One of the main physics goals of the Beam Energy Scan program at RHIC is to study the QCD phase diagram, specifically around the phase transition between the quark-gluon plasma and hadronic matter. Beam Energy Scan Phase-I studied Au+Au collisions from center-of-mass energy ($\sqrt{s_{NN}}$) of 7.7 to 62.4 GeV. Beam Energy Scan Phase-II extended these measurements in several important ways, one of which was the addition of a fixed-target program that pushed the collision energy down to 3.0 GeV (or baryon chemical potential, $μ_B$, up to 720 MeV). Fixed-target collisions at STAR allow for a more extensive scanning of the QCD phase diagram to an important region where the QCD critical point may lie, and to a region dominated by dense baryonic matter. One key measurement in the fixed-target program is the spectrum of the lightest hadrons ($π^{\pm}$, $K^{\pm}$, p) as a function of transverse momentum, rapidity, and collision centrality. From the $p_T$ spectra, a blast-wave model is used to study the temperature at kinetic freeze-out and the surface velocity of the expanding matter. These results provide important input to models of heavy ion collisions at these energies, and can help constrain the equation of state of QCD matter. Here, results are shown for four collision energies in the fixed-target range: $\sqrt{s_{NN}} = 3.2$, 3.5, 3.9, and 4.5 GeV.