Study of hot and dense nuclear matter in relativistic heavy ion collisions

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When temperature increases, it tends to break up matter into more elemental particles as the forces binding them get overwhelmed by thermal energy. For instance, while liquid water is made of collection of loosely bound water molecules, steam is a gas of unbound water molecules. Likewise, when atomic nucleus is heated, the nucleus undergoes a phase transition and dissolves into more elemental quarks and gluons. This occurs at around two trillion kelvin temperature. The resulting system called Quark-Gluon Plasma, or QGP, is the hottest and the densest matter ever observed in nature and it is the most "perfect fluid'' with the lowest specific viscosity ever observed.In the past two decades, high energy nuclear physics community has been focusing on the study of this primordial matter by experimentally creating it in relativistic heavy ion collisions and understanding its properties through in-depth theoretical investigations. With two relativistic heavy ion colliders in operation in the USA (the Relativistic Heavy Ion Collider) and Europe (the Large Hadron Collider), experiments on QGP has moved on well beyond the discovery phase to the precision measurement phase. Theoretical analysis of this strongly interacting QCD (Quantum Chromodynamics) many-body system is an important testing ground for our understanding of strong nuclear interaction.In the past two decades, I have made a concerted effort to theoretically understand this primordial matter. QGP created in relativistic heavy ion collisions evolves through several stages. Although QGP phase is the most important stage, understanding the whole picture is crucial in extracting the properties of QGP from experimental data. My group has been successfully conducting such a comprehensive study of QGP for many years using wide range of theoretical tools from quantum field theory to hydrodynamic simulations.In this grant proposal, I propose to add important new physics elements to our successful program of analyzing and predicting QGP phenomenology using our hydrodynamics and jet-QGP interaction simulation tools (a "jet" here refers to a high-energy quark or gluon). First, the initial state calculation will be critically improved by properly taking into account the beam-direction dynamics of gluons. Second, we will investigate the surprising phenomenon that even smaller systems created by proton-nucleus collisions exhibit evidence of QGP fluid, in particular focusing on the modification of the jet spectra fully taking into account the finite size effect. Third, we will carry out studies on the non-equilibrium corrections to the hadronic and electromagnetic signals. Lastly, we will use available Bayesian statistics tools to determine the properties of QGP such as transport coefficients in a systematic way. Through such a comprehensive and thorough study, we will be able to characterize QGP in a precise and decisive manner.