Heavy ion collisions allow us to recreate the density and temperature that existed at the very beginning of the universe, before the universe was 10-6 s old, in a laboratory environment. Studying the resulting hot dense matter, which we call a quark gluon plasma (QGP), allows us to both better understand the evolution of the universe and one of the four fundamental forces of nature, the strong force. The strong force, which is more properly called Quantum Chromodynamics (QCD), is the force that binds protons and all other hadrons together.
I will take a brief moment to remind everyone about some QCD basics before discussing the quark gluon plasma. QCD is one piece of the Standard Model, the theory that describes all subatomic particle interactions outside of gravity. QCD is carried by particles that physicists gave the tongue-in-cheek label of gluons. The only subatomic particles that can interact with gluons are quarks, of which there are 6: up, down, strange, charm, bottom and top. Each quark contains a QCD charge, which we call color. The anti-quarks have an anti-color charge, while gluons carry both color and anti-color charge. What does this mean? This means that gluons can interact with each other! This makes QCD calculations quite complicated.
There are two aspects to QCD that are important to understand with respect to the quark gluon plasma: quark confinement and asymptotic freedom. At the temperatures and densities that we observe outside of heavy ion collisions, QCD keeps quarks confined within their parent hadrons. This means we have never observed a bare quark! At extremely high energies, the QCD field strength lowers until the quarks no longer feel the force, which we call asymptotic freedom. In a very dense medium, such as what we create in heavy ion collisions it easier for quark-antiquark pairs to pop into existence than it is for these pairs to do so in a vacuum. These quark anti-quark pairs lower the QCD field strength, which lowers the energy needed for the quarks to be free.
So returning to the original question. What is a QGP? The QGP is a medium so dense and hot that the quarks and gluons within it are no longer confined to their original hadrons. But in order to discuss the properties of a medium, it needs to be in local thermalization. The concept of temperature only has meaning when thermalization has occurred because temperature is a bulk matter quantity. How hot is it? The QGP is hotter than 175 MeV, or 400,000,000 times the temperature of the surface of the sun!
What does it mean for the quarks to be deconfined? Originally, physicists tried to model the QGP as a weakly-coupled gas, but those models failed. The idea was that the deconfined quarks would behave similar to an ideal gas. Physicists have achieved reasonable success in modeling the QGP as an ideal fluid, in fact the most perfect liquid known to man . The fact that this model fits the data better means that even though the quarks are not confined within hadrons, they still interact with each other. This model of QGP is sometimes called sQGP, where the s stands for strongly coupled.
Next I will discuss some key QGP signatures and how we are looking for them at ALICE.