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Rosi Reed | USLHC | USA

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What is the QGP?

Wednesday, November 23rd, 2011

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.

The evolution of matter in the universe from the Big Bang to the present.

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.

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Down the Rabbit Hole

Monday, November 21st, 2011

For my first blog post, I thought I would start with a very basic overview of heavy ion collisions and what we hope to learn from them. Over the next few months, I would like to fill in more details, as well as share new and breaking results.

It’s that time of year again at the LHC, when we switch from running proton beams to lead beams for heavy ion collisions. The first lead collisions for 2011 occurred early on November 6, and the beams were stabilized on November 12. An ALICE event display of this first data can be seen at this ALICE press release . Once again, the LHC is operated at its highest energy ever. While each individual nucleon (proton or neutron) does not have as much energy as the protons in earlier proton runs, each lead ion contains 208 nucleons adding a tremendous amount of energy to a very tiny volume. This run will continue until December 7, and we should start seeing interesting results even before the completion of the run!

For the ALICE experiment, this is an important time of year. ALICE stands for A Large Ion Collider Experiment, and was specifically designed to study relativistic heavy ion collisions. In future blogs, I will cover the particulars of the experiment and its design. But first, we should ask, why do we want to study heavy ion collisions as all? What do we learn from these collisions that we do not learn from proton-proton collisions?

Heavy ion collisions are the only way to increase the energy density of a system to the point where the quarks that make up the protons and neutrons within that system are no longer bound. We call this system of unbound quarks the Quark Gluon Plasma (QGP). Study of the QGP is important for several reasons. One is to increase our understanding of the early universe, where for a very brief instant, a QGP should have existed. Another is to increase our understanding of the strong force in interactions where it is no longer possible calculate the strength of the force perturbatively.

In order to study the QGP we have two classes of probes available to us. One is to study the bulk properties of the matter, such as flow, where the momentum transferred in any reaction is small. Another is to use what we call “hard probes”, where the momentum transfer is large. These include jets and heavy flavor mesons. These results are compared to proton-proton collisions by use of a variable called RAA. It is defined so that if we could treat a heavy ion collision as merely a collection of independent proton and neutron collisions, RAA would be 1. When it differs from 1, we know that something potentially interesting is happening. However, it is important to use every available probe, as studying the QGP requires us to disentangle the interesting physics due the extremely hot matter formed from all of the other effects that could cause measurements in heavy ion collisions to differ from those in proton-proton collisions.

Stay tuned for my next blog post on “What is the QGP?”

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