This work is advancing knowledge of the phase diagram and equation of state of strong interactions, by means of first-principle simulations. The researchers’ main goals are locating the critical endpoint and explore the strongly interacting high-density regime relevant to neutron stars and their mergers—some of the main unsolved problems in the theory of strong interactions. 

Ordinary hadronic matter undergoes a transition to a deconfined phase, quark-gluon plasma, at extremely high temperature or densities. In the universe, the reverse transition took place a few microseconds after the Big Bang: the basic building blocks of nature, the hadrons, were formed at this time. The Large Hadron Collider Heavy Ion program recreates this transition in the laboratory. Its DOE-funded detector upgrade enables them to improve the precision in the data and thus search for new phenomena, such as experimental evidence for near criticality in the QCD transition. The project continues experimental signatures to the onset of chiral critical behavior such as fluctuations of conserved charges. A hydrodynamic description of the strongly interacting plasma requires the local equation of state at nonzero light and strange density. This project computes it in the continuum limit with physical quark masses. 

Brookhaven’s Relativistic Heavy Ion Collider explores the same transition in a broad range of densities, searching for a critical point in the QCD phase diagram—a difficult task, since there are no first principles to predict its location; direct simulations at finite density have remained elusive because of a sign problem. Thanks to a recent algorithmic development, these simulations, run with physical quark masses, can overcome this barrier and scan the phase diagram for features such as the cross-over line and the critical end point.