Universe’s Beginning was Fluid, Physicists Say

Physicists from the ALICE (A Large Ion Collider Experiment) Collaboration at CERN have gained new insights into the properties of the quark-gluon plasma, a state of matter thought to have existed just after the Big Bang. The findings are published in the journal Physics Letters B.

An event from the first xenon-xenon collision at the Large Hadron Collider at 5.44 TeV registered by ALICE; a colored track (blue) corresponds to the trajectory of a charged particle produced in a single collision. Image credit: ALICE Collaboration.

An event from the first xenon-xenon collision at the Large Hadron Collider at 5.44 TeV registered by ALICE; a colored track (blue) corresponds to the trajectory of a charged particle produced in a single collision. Image credit: ALICE Collaboration.

The quark-gluon plasma, as the name suggests, is a special state consisting of the fundamental particles, the quarks, and the particles that bind the quarks together, the gluons.

The ALICE team obtained new results by replacing lead — usually used for collisions — with xenon.

“Xenon is a ‘smaller’ atom with fewer nucleons in its nucleus,” they explained.

“When colliding ions, we create a fireball that recreates the initial conditions of the Universe at temperatures in excess of several thousand billion degrees.”

“In contrast to the Universe, the lifetime of the droplets of the quark-gluon plasma produced in the laboratory is ultra short, a fraction of a second.”

“Under these conditions the density of quarks and gluons is very high and a special state of matter is formed in which quarks and gluons are quasi-free, dubbed the strongly interacting quark-gluon plasma.”

The experiments revealed that the primordial matter, the instant before atoms formed, behaved like a liquid that can be described in terms of hydrodynamics.

“One of the challenges we are facing is that, in heavy ion collisions, only the information of the final state of the many particles which are detected by the experiments are directly available — but we want to know what happened in the beginning of the collision and first few moments afterwards,” said team member Dr. You Zhou, a physicist at the Niels Bohr Institute at the University of Copenhagen, Denmark.

“We have developed new and powerful tools to investigate the properties of the small droplet of the quark-gluon plasma (early Universe) that we create in the experiments.”

“They rely on studying the spatial distribution of the many thousands of particles that emerge from the collisions when the quarks and gluons have been trapped into the particles that the Universe consists of today.”

“This reflects not only the initial geometry of the collision, but is sensitive to the properties of the quark-gluon plasma. It can be viewed as a hydrodynamical flow,” he added.

“The transport properties of the quark-gluon plasma will determine the final shape of the cloud of produced particles, after the collision, so this is our way of approaching the moment of quark-gluon plasma creation itself.”

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S. Acharya et al (ALICE Collaboration). 2018. Anisotropic flow in Xe–Xe collisions at √sNN = 5.44 TeV. Physics Letters B 784: 82-95; doi: 10.1016/j.physletb.2018.06.059

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