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Quarks, Gluons and Particle Jets
May 7, 2015   
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Shortly after the Big Bang, the universe was filled with a chaotic primordial “soup” of quarks and gluons, particles that are now trapped inside protons and neutrons. The study of this quark-gluon plasma requires the use of the most advanced theoretical and experimental tools.

A group of physicists working at the Large Hadron Collider (LHC) facility in Geneva and using the ATLAS detector have now gained a closer insight into the quark-gluon plasma and its properties.

The Large Hadron Collider researchers collided two lead ions traveling at nearly the speed of light and, 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 prestigious journal Physical Review Letters by the international team of physicists working at the ATLAS detector.

Study of the quark-gluon plasma poses an enormous challenge. The plasma appears rarely, only during collisions and 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.

“Fortunately, detectors like ATLAS have succeeded in recording the decay products of particles that have interacted with the quark-gluon plasma. By carefully analyzing the properties of those particles, we can come to conclusions about the features of the plasma,” says Prof. Barbara Wosiek from the Polish Academy of Sciences’ Institute of Nuclear Physics in Cracow, southern Poland, who coordinated and approved the analysis of data gathered by the ATLAS detector at the European Organization for Nuclear Research (CERN) laboratory near Geneva, Switzerland, in 2011. The analysis was performed by a team from Columbia University in New York.

Most of the information on the quark-gluon plasma is provided by particles that disperse sideways as a result of a collision. The specific direction of their movement, crosswise to the initial trajectory of lead nuclei, makes it relatively easy to distinguish them from thousands of other particles and at the same time guarantees that they are a result of an early stage of the collision. Immediately after the collision they have to traverse through the quark-gluon cloud only to subsequently collapse into a concentrated narrow stream of particles known as jets.

“These initially produced particles lose energy while going through the hot, dense plasma, which leads to extinguishing the high-energy jets,” says Wosiek. “In the course of our analysis we reconstructed jets with extremely high energies reaching 400 gigaelectronvolts.”

After reconstructing the jets detected in lead nuclei collisions, the physicists compared the results with those obtained from proton-proton collisions. The comparison showed that the quark-gluon plasma cannot emerge as a result of a proton-proton collision. In turn, theoretical models of heavy ion collision predict the formation of dense plasma in a head-on ion-ion collision with an extremely high energy. The comparison of the results of the analysis of both types of collisions makes it possible to evaluate how the jets are affected by the presence of plasma.

“In the collisions of the lead nuclei we recorded half the number of jets occurring in proton-proton collisions,” says Wosiek. “This indicates that particles ensuing from the initial collision lose energy as they interact with the plasma, and the high-energy jets are thus extinguished. This is an important result because it allows us to discard some of the theoretical models of quark-gluon plasma which do not provide for such a high rate of suppression.”

The ATLAS detector, built with the help of Polish research institutions, including the Institute of Nuclear Physics in Cracow, is an extraordinarily sophisticated instrument the size of a multi-story building.

Studies of lead nuclei collisions are only one element of the research undertaken by international groups of scientists conducting experiments at the Large Hadron Collider accelerator. The research focuses on proton-proton collisions with a view to putting the current theory of particle physics, the so-called Standard Model, to the test, and to exploring phenomena extending beyond the Standard Model. The most spectacular success of the physicists working with the ATLAS and CMS detectors at the Large Hadron Collider was the discovery of the so-called Higgs boson in 2012, after a half-century search. Also known as the God Particle, the Higgs boson was the only particle in the standard model of particle physics that defied observation for decades.
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