Session EA: Frontiers of Quark-Gluon Plasma Physics

Probing the quark-gluon plasma at the energy frontier with the LHC

After the Large Hadron Collider (LHC) turned on in 2009 at CERN, the experiments at LHC (ALICE, ATLAS, CMS, LHCb) started their own physics program by using the heavy ion beams at the highest beam energy, which give us an access to the precise determination of Quark Gluon Plasma (QGP) properties at high temperature regime. By accumulating high statistics data in the past almost 10 years, we found many interesting and new observables; for example the jet quenching and energy re-distributions in the matter, enhancement of charmonia at low transverse momenta and possible recombination, collective-flow like signal and strangeness enhancement at high multiplicity proton-lead and proton-proton events. In this talk, we summarize recent experimental results from the LHC heavy ion program, and clearify the current underfunding of the nature of QGP properties. And we also discuss the future direction of the LHC heavy ion physics program and upgrade plans.    

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.   

Determination of Quark-Gluon-Plasma properties via Bayesian analysis

A primary goal of heavy-ion physics is the measurement of the fundamental properties of the quark-gluon plasma (QGP), notably its transport coefficients and initial state properties.  Since these properties are not directly measurable, one relies on a comparison of experimental data to computational models of the time-evolution of the collision to connect measured observables to the properties of the transient QGP state.  These model-to-data comparisons are non-trivial due to the large number of model parameters and the non-factorizing sensitivity of measured observables to multiple parameters.

Over the last few years the Duke QCD group has developed techniques based on Bayesian statistics that allow for the simultaneous calibration of a large number of model parameters and the precision extraction of QGP properties including their quantified uncertainties. The computational model used is based on the Trento initial condition model, viscous relativistic hydrodynamics and a microscopic hadronic transport to describe the off-equilibrium late stage hadronic evolution. The analysis starts by selecting a set of salient model parameters - including physical properties such as temperature and/or momentum dependent transport coefficients - then evaluates the event-by-event heavy-ion collision model at a small set of points in the multidimensional parameter space, varying all parameters simultaneously.  Gaussian process emulators are used to non-parametrically interpolate the parameter space, providing fast predictions at any point in parameter space with quantitative uncertainty.  Finally, the parameter space is systematically explored using a Markov chain Monte Carlo (MCMC) to obtain rigorous constraints on all parameters simultaneously, including all correlations among the parameters.

In this talk I will review the basic components of the Bayesian analysis and discuss recent progress in the determination of QGP initial conditions and transport coefficients.

Past, present, and future of the QGP physics

In this talk, I will first give an overview of past and the past and current (mis)understanding of the QGP properties. Then, I will discuss what to do and what to be looked at in the future to understand the mystery of the QGP.