Collider reveals sharp change from 'quark soup' to atoms

Their study is published in Physics Review Letters.

The Brookhaven’s Relativistic Heavy Ion Collider acts as a time machine, said Yadav Pandit, UIC postdoctoral fellow in physics.

“We can’t go back in time, but we can reproduce the conditions present just after the Big Bang and try to understand the evolution of the early universe."

A familiar phase transition is when water is heated to boiling. The water’s temperature rises until it begins to turn into steam. The temperature stops rising as the heat’s energy goes into the change of state from liquid to gas.

Pandit was looking for the signature of that moment—when the energy going in stops producing one effect and goes into producing the phase change instead.

For more than 30 years physicists have been looking for this sharp phase transition, a so called first order phase transition, between the quantum plasma of the early universe, and the normal matter of atoms and molecules that make up the stars and planets of the universe we know, said Pandit.

“This is a very significant observation,” said Olga Evdokimov, UIC associate professor of physics and a principal investigator in UIC’s heavy ion research group. “It addresses one of the central questions of our field.”

Pandit analyzed data on heavy gold nuclei collisions at various energies from Brookhaven’s Relativistic Heavy Ion Collider STAR collaboration, one of two groups of physicists conducting experiments at Brookhaven using huge particle detectors.

He found the signature – a temporary disappearance of a particular kind of particle flow observed coming out of the collisions.

“When the collision takes place at an energy close to a first-order phase transition, the expansion and the resulting deflection of the emitted particles is ‘softened,’” said Pandit. “Energy that would normally expand the system is instead going into changing the state of matter—melting the hadrons to free the quarks and gluons.”

“These results have certainly brought us several steps closer to a complete understanding of the big picture of the phase structure of the nuclear matter,” said David Hofman, UIC professor and head of physics and a principal investigator in UIC’s heavy ion research group.

All members of the RHIC’s STAR collaboration are co-authors of the paper. Zhenyu Ye, assistant professor of physics; Yadav Pandit, Yaping Wang, postdoctoral research associates; and graduate students K. Kauder and Z. H. Khan are also members of the UIC heavy ion group involved in the STAR collaboration. The work was done at Brookhaven National Laboratory and funded by the Department of Energy’s Office of Science.