A New Phase of Matter: Quark-Gluon Plasma Beyond the Hagedorn Critical Temperature

TL;DR

This paper reviews the development of the concept of quark-gluon plasma, highlighting experimental discoveries from SPS to LHC and discussing future research directions like locating the QCD critical point.

Contribution

It provides a personal historical overview of the experimental and theoretical progress in understanding quark-gluon plasma beyond the Hagedorn temperature.

Findings

Discovery of quark-gluon plasma at RHIC as a nearly perfect liquid

Experimental signatures of deconfinement through strangeness production

Future prospects include searching for the QCD critical point

Abstract

I retrace the developments from Hagedorn's concept of a limiting temperature for hadronic matter to the discovery and characterization of the quark-gluon plasma as a new state of matter. My recollections begin with the transformation more than 30 years ago of Hagedorn's original concept into its modern interpretation as the "critical" temperature separating the hadron gas and quark-gluon plasma phases of strongly interacting matter. This was followed by the realization that the QCD phase transformation could be studied experimentally in high-energy nuclear collisions. I describe here my personal effort to help develop the strangeness experimental signatures of quark and gluon deconfinement and recall how the experimental program proceeded soon to investigate this idea, at first at the SPS, then at RHIC, and finally at LHC. As it is often the case, the experiment finds more than theory predicts, and I highlight the discovery of the "perfectly" liquid quark-gluon plasma at RHIC. I conclude with an outline of future opportunities, especially the search for a critical point in the QCD phase diagram.