Onset of Quark Gluon Plasma formation: From small systems to high baryon densities

A fundamental question in nuclear physics is understanding under what conditions nuclear matter transitions into the Quark Gluon Plasma (QGP) phase - a phase of deconfined quarks and gluons. Relativistic heavy-ion collisions at top RHIC energies and LHC energies create matter in the QGP phase, and the properties of the produced QGP medium has been extensively studied. The Beam Energy Scan program at RHIC probes different regions of the QCD phase space, creating matter at lower temperatures and finite baryon densities and attempts to study the onset of partonic phase and QGP properties, and search for signatures of first order phase transition to the QGP phase and the QCD critical point. Recent results from the RHIC Beam Energy Scan Phase - II (BES-II) indicate creation of medium dominated by hadronic interactions in heavy-ion collisions at collision energies of $\sqrt{s_NN}$ = 3.2 GeV and below and partonic interactions becoming important at $\sqrt{s_NN}$ = 4.5 GeV and above. A related question is the minimum system size required for QGP formation and is being explored at both RHIC and LHC in small collision systems including O+O, p+A, p+p and $\gamma$ + A collisions.

In this talk, I will discuss the latest results from RHIC BES-II with precision measurements in Au+Au collisions at $\sqrt{s_NN}$ = 3.0 - 54.4 GeV, looking at the onset of QGP properties and search for QCD critical point and first order phase transition. I will also discuss results from both RHIC and LHC in small collision systems - O+O, p+A, p+p and $\gamma$ + A collisions - studying QGP signatures including partonic collectivity and jet quenching. The implications of these results in understanding the onset of QGP will be discussed, along with future prospects on these research directions.