Exotic States of Matter Reveal Plasma Secrets Through Particle Jets at Cern

""When the LHC accelerator at the world's largest laboratory in CERN, Geneva, collided two lead ions travelling at nearly the speed of light, for a fraction of a second ordinary matter was transformed into the most exotic state of matter known to physics: quark-gluon plasma. Analysis of the streams of particles penetrating the plasma has led to new findings about the properties of the plasma, and was recently published in the journal Physical Review Letters by the international team of physicists working at the ATLAS detector.

Immediately following the Big Bang and the formation of space-time, the Universe was filled with matter of extraordinary properties. Quarks and gluons, today only found bound within protons and neutrons, bounced about freely, comprising a homogenous 'soup'. This exceptional state of matter, appearing only at temperatures of billions of degrees, has been recreated by physicists at the LHC accelerator by colliding heavy lead ions.

Study of the quark-gluon plasma poses an enormous challenge. It appears only rarely during collisions, in extremely minute quantities, and then only for a fraction of a second. It immediately begins to expand under its own pressure, rapidly cools and transforms itself into an avalanche of ordinary particles. Modern physics has no tools at its disposal to directly observe quarks and gluons. We cannot simply proceed with the usual methods of measurement, like inserting a thermometer into the plasma and waiting a few minutes for the results. Much more refined methods are needed.

"Fortunately detectors like the ATLAS detector have suceeded in recording the decay products of particles which have interacted in the quark-gluon plasma. By carefully analysing the properties of those particles, we can come to guarded conclusions about the features of the plasma," says Prof. Barbara Wosiek of the Institute of Nuclear Physics of the Polish Academy of Sciences in Kraków, Poland, who coordinated and approved the analysis of data gathered by the ATLAS detector in 2011. The analysis was performed by a team from Columbia University.

Most of the information we have on the quark-gluon soup is provided by particles that disperse sideways as the result of a collision. As they move in this specific direction, crosswise to the initial direction of flight of the lead nuclei, it makes it relatively easy to distinguish them from thousands of other particles and assures that they resulted from the early stage of the collision. If so, immediately after the collision they had to traverse through the quark-gluon cloud, to then collapse into a concentrated narrow stream of particles, known as jets.

"These initially produced particles lose energy while going through the hot, dense plasma soup, which leads to extinguishing the high-energy jets. Through our analysis we go about reconstructing jets of an extremely high energy level, reaching 400 gigaelectronvolts," adds Prof. Wosiek.

After gathering the data on the reconstructed jets in the collision of lead nuclei, the team of physicists can correlate and compare the results with those obtained from proton-proton collisions. The idea behind such a comparison is quite simple. From a precise enough theoretical consideration it is expected that quark-gluon plasma will not arise in a proton-proton collision. In turn, theoretical models of heavy ions in collision predict the formation of dense plasma in a head-on ion-ion collision of extremely high energy. Comparison of results from the data analysis of both types of collisions enables evaluation of how the jets are disturbed by the presence of plasma.""

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