The figure shows how a small, elongated drop of quark-gluon plasma is formed when two atomic nuclei hit each other a bit off center. The angular distribution of the emitted particles makes it possible to determine the properties of the quark-gluon plasma, including the viscosity. Credit: State University of New York
Researchers collided lead atoms with extremely high energy in the 27 km long particle accelerator. The primordial soup is a quark-gluon plasma and researchers have measured its liquid properties with great accuracy at the LHC’s top energy. A few billionths of a second after the Big Bang, the universe was made up extremely hot and dense primordial soup of quarks and gluons. By colliding lead nuclei at a record-high 5.02 TeV in the world’s most powerful particle accelerator, the 27 km long Large Hadron Collider, LHC at CERN in Geneva, it has been possible to recreate this state in the ALICE experiment’s detector and measure its properties.
“The analyses of the collisions make it possible, for the first time, to measure the precise characteristics of a quark-gluon plasma at the highest energy ever and to determine how it flows,” explains You Zhou. This state of matter behaves more like a liquid than a gas, even at the very highest energy densities. The new measurements, which uses new methods to study the correlation between many particles, make it possible to determine the viscosity accurately.
The experimental method is very advanced and is based on the fact that when 2 spherical atomic nuclei are shot at each other and hit each other a bit off center, a quark-gluon plasma is formed with a slightly elongated shape, ie the pressure difference between the centre of this extremely hot ‘droplet’ and the surface varies along the different axes. The pressure differential drives the expansion and flow and consequently one can measure a characteristic variation in the number of particles produced in the collisions as a function of the angle.
“It is remarkable that we are able to carry out such detailed measurements on a drop of ‘early universe’, that only has a radius of about one millionth of a billionth of a meter. The results are fully consistent with the physical laws of hydrodynamics, i.e. the theory of flowing liquids and it shows that the quark-gluon plasma behaves like a fluid. It is however a very special liquid, as it does not consist of molecules like water, but of the fundamental particles quarks and gluons,” explains Prof Jens Jørgen Gaardhøje, head of ALICE group at the Niels Bohr Institute, Uni of Copenhagen. They are now in the process of mapping this state with ever increasing precision – and even further back in time. http://www.eurekalert.org/pub_releases/2016-02/uoc–tup020916.php
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