Beam Energy Scan Program at RHIC (BES I and BES II) – Probing QCD Phase Diagram with Heavy-Ion Collisions

Abstract

In 2010, a multi-step experimental program, the Beam Energy Scan (BES), was launched at the Relativistic Heavy Ion Collider (RHIC) to investigate the phase diagram of strongly interacting nuclear matter. The BES phase I (BES I) was completed in 2011 with Au+Au data sets in energy range from 39 GeV to 7.7 GeV. This was complemented by earlier (62.4, 130 and 200 GeV) and later (54.5 GeV) collected data sets of Au+Au collisions. Many measurements taken by the STAR (Solenoidal Tracker At RHIC) detector at $\sqrt{s}_{NN}$ below the RHIC injection energy suffer from large statistical errors, sharply increasing with decreasing energy. Nevertheless, they allowed for the first time a direct study of the QCD critical point (CP) and phase transition signatures. The results of these studies are presented. In 2015, the Beam Energy Scan program was extended to energies below $\sqrt{s}_{NN}$ = 7.7 GeV by successful implementation of the fixed-target mode of data taking (FXT) in the STAR experiment, in addition to the standard collider configuration. In the fixed target mode, ions circulating in one ring of the collider interact with a stationary target at the entrance of the STAR Time Projection Chamber. The first results from the exploratory FXT run with Au+Au collisions at $\sqrt{s}_{NN}$ = 4.5 GeV are presented. The ongoing second phase of BES (BES II), which includes both collider and FXT data taking, will provide data sets with one-order-of magnitude larger statistics in collider mode, and with two-orders-of-magnitude larger statistics in FXT mode at each of several energies down to $\sqrt{s}_{NN}$ = 3.0 GeV.