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Fluctuations of Conserved Charges in the Quark Gluon Plasma

Scientists at the European Center for Nuclear Research (CERN) in Switzerland and at the Brookhaven National Laboratory (BNL) in the US are currently undertaking large efforts to smash atomic nuclei with high energy by letting them collide with almost the speed of light. They aim on the creation of a state of matter, the so-called quark gluon plasma (QGP), which existed in the early universe shortly after the big bang. In this state the most fundamental constituents of matter, the quarks and gluons, are not bound to protons and neutrons as in the world we see today. It is, however, not easy for the scientists at CERN and BNL to analyze the properties of QGP, as it is created in very tiny droplets in the center of the collision. After its creation the droplet expands and cools until it materializes into all kinds of small particles that can (or cannot) be measured in large detectors surrounding the center of the collision. Now, the difficulty is to conclude from measurements of the individual particles onto the properties of the QGP. This is somewhat similar to deduce from its debris the cause of an explosion. Only that the fragments measured here are all of subatomic size.

What physicists would like to know about the QGP are, e.g., its thermodynamic properties such as pressure and energy density. Also the temperature at which it breaks apart into a gas of loosely bound particles, the so-called freeze-out temperature, is an important quantity. Such kind of knowledge will allow for a deeper understanding of the evolution that went on in our early universe and that finally lead - luckily for mankind - to the evolution of live. Furthermore, there might exist places in the universe where the QGP can be found even today. Apart from the man made tiny QGP droplets in the colliders, it could be possible that the QGP does still exist in the core of some super heavy neutron stars.

Fluctuations of Conserved Charges in the Quark Gluon PlasmaSchematic phase diagram of QCD. Indicated are the positions where different experiments generate QGP droplets and the trajectories on which they cool until they freeze-out. Also shown are a possible critical point and a mixed phase region.
Copyright: © University of Bielefeld, Physics Department

The freeze-out temperature - which is a million times higher than the temperature in the core of our sun - is often estimated by simple models. A team of physicists from Bielefeld, Regensburg and Brookhaven (US) is now aiming on a model independent determination of the freeze-out temperature and density, based on the fundamental equations of the theory of strongly interacting matter, Quantum Chromodynamics (QCD). The researchers compare the fluctuations of charges measured in the detectors with a computer simulation of the QGP. The numerical calculation of a QGP droplet involves the handling of millions of variables, distributed over a large space-time lattice, and thus requires high performance computing. The team is currently performing these calculations, which were made possible through the European HPC initiative PRACE (Partnership for Advanced Computing in Europe), using supercomputers such as the petascale system JUQUEEN of GCS member centre Jülich Supercomputing Centre. The current yet preliminary results are already used in tailored analyses of the experiments at CERN and BNL.

Christian Schmidt
University of Bielefeld, Physics Department
25 University St., D-33615 Bielefeld, Germany
e-mail: schmidt@physik.uni-bielefeld.de

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