There is still, however, an open question of the nature of the transformation from hadronic degrees of freedom to the partonic ones, and back to the state of hadron gas. To address that, RHIC launched a multi-step experimental program to investigate the phase diagram of strongly interacting nuclear matter in 2010. The results from the exploratory phase I of the Beam Energy Scan (BES) with data sets of Au+Au collisions at energies from 39 GeV down to 7.7 GeV allowed for the first time a direct study of anticipated signatures of the critical point and first-order phase transition. The data clearly shows that observables are evolving strongly from the lowest energies accessible in the collider mode at RHIC. Some of the trends resemble the expected critical point signatures.

Figure 2. The elliptic flow of protons measured by the STAR FXT (blue stars), compared with previous data from the AGS E895 experiment (green crosses). Although the two data sets have slightly different centralities (impact parameter distributions) they are consistent with each other.

Therefore, it became crucial to extend BES program to energies below those provided by RHIC in collider mode. This was done by installing an internal stationary target (a “fixed” target, FXT), which is a thin gold foil, inside the beam pipe, where it is bombarded by one of the circulating RHIC beams. This setup lowered the collision center-of-mass energy down to 3.0 GeV and extended the coverage in baryon chemical potential in QCD phase diagram up to 720 MeV. Fig. 1 shows an FXT event in the STAR time projection chamber.

RNC physicists, together with colleagues from other STAR institutions, are leading the fixed target program, including significant contributions to data calibration and physics analyses. The first pilot run was conducted in 2015 with Au+Au collisions at √s_NN = 4.5 GeV. The LBNL group analyzed the elliptic flow, v₂, of identified particles from this run (Au+Au at 4.5 GeV). Fig. 2 shows the measured v₂ of protons, including the new 4.5 GeV point.

The FXT energies are similar to those studied by the Brookhaven Alternating Gradient Synchotron (AGS) program during the 1990s. As Fig. 2 shows, the current STAR FXT measurements are in good agreement with the prior AGS studies.

The 4.5 GeV Au+Au data from STAR FXT pilot run represents the first point at energies so low in the new precision map of the QCD phase diagram. During the 2018 and 2019 RHIC runs, a good fraction of the planned datasets over the energy range √s_NN = 3.0 – 7.7 GeV have already been collected. Analysis is in progress.