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Rosi Reed | USLHC | USA

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Down the Rabbit Hole

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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