Experimental studies of the quantum chromodynamics phase diagram at the STAR experiment

We review the STAR experiment's results to date from the Beam Energy Scan (BES) at Brookhaven's Relativistic Heavy Ion Collider, and outline future plans and prospects in this area. BES Phase-I is based on Au + Au data taken in 2010 and 2011 at √sNN = 7.7, 11.5, 19.6, 27 and 39 GeV, and when interpreted in conjunction with the large datasets available at 62.4 and 200 GeV, permits an initial exploration of the phase diagram of quantum chromodynamics (QCD) matter. The three goals of BES Phase-I are as follows: (1) a search for turn-off of the promising signatures of quark gluon plasma (QGP) already reported at the top RHIC energies; (2) a search for evidence of a possible first-order phase transition such as a signature of softening of the QCD equation of state (EoS); (3) a search for a critical end point as expected in a scenario where there is a cross-over transition from hadronic matter to QGP at the highest RHIC energies, but a first-order phase transition at lower energies with finite net-baryon density. We summarize several analyses of BES data from 2010 and 2011 that are either published or submitted, as well as several more that have been reported at meetings in preliminary forms. The physics interpretation of BES Phase-I measurements is frequently limited by the increasing statistical error bars as the beam energy decreases, and the planned BES Phase-II will have much improved capabilities in this regard. (author)