This work describes new simulations designed to test whether heavy-ion collisions at RHIC’s Beam Energy Scan can pass close to the hypothesized QCD critical point. The critical point is a special place on the map of hot nuclear matter where the smooth crossover between quark-gluon plasma and ordinary hadrons would end and give way to a first-order phase transition. Experiments look for signs of that point in fluctuations of emitted protons and in other observables, but interpreting the data requires realistic dynamical models of the expanding and cooling fireball created in collisions.

The authors combined a new set of equations of state (EOS) from the MUSES collaboration with a full 3+1 dimensional relativistic viscous hydrodynamics code called CCAKE, inside a simulation chain named NuclearConfectionery. The MUSES EOS tables include versions with a movable critical point and also span cross-over and first-order regions. An “EOS Inverter” converts the EOS output (temperature and baryon chemical potential) into the hydrodynamic variables the code needs (entropy and net-baryon density). The simulations were run both with a critical point placed within the reach of a chosen beam energy and with the critical point placed out of reach, to compare how the system’s path through the phase diagram changes.

Two numerical upgrades made the simulations faster and more accurate. First, the initial fluid is prepared using a WVT-“glass” relaxation that spreads Lagrangian fluid elements into an amorphous, isotropic arrangement. This resolves fine features better than a fixed grid. Second, the code uses adaptive smoothed particle hydrodynamics (SPH), so the local resolution scale changes with particle density. Together these changes improved the code’s energy conservation by about an order of magnitude and reduced run time (for example, roughly one minute less for oxygen–oxygen runs) while using fewer SPH particles.