Testing the quark-gluon plasma limits with energy and species scans at RHIC

A major goal of high-energy nuclear collisions is to map the phase diagram for matter that interacts via quantum chromodynamics (QCD).  The experimentally accessible way to characterize the QCD phase diagram is in the plane of temperature and baryon chemical potential.  At high energy densities, QCD predicts a phase transition from a hadronic gas to a state of deconfined, partonic matter called the quark-gluon plasma (QGP), where the degrees of freedom are partonic rather than hadronic.  Lattice QCD calculations established that the transition from hadronic matter to the QGP is a crossover transition at a temperature around 155 MeV and zero baryon chemical potential.  Many QCD-based models predict a first-order phase transition at high baryon chemical potential, which would necessitate the existence of a critical point at a baryon chemical potential between the two regions.  The RHIC Beam Energy Scan focuses on mapping the QCD phase diagram and determining the location of a possible critical point and first order phase transition.  The first beam energy scan performed at RHIC had intriguing trends, which will be discussed.  The outlook to the second beam energy scan, with improved detector and collider capabilities will also be discussed.  In addition to changing the collision energy, which allows a scan of temperature and baryon chemical potential, the system species can also be changed.. In the RHIC species scan, multiparticle-correlation measurements of relativistic p, d or He3+Au collisions show surprising collective signatures. These measurements indicate that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles.  The results of these measurements will be discussed.