RHIC experiment records decline in particle correlations in gold collisions — Live Science
In the United States, the STAR experiment at the Relativistic Heavy Ion Collider RHIC recorded an unexpected pattern in particles produced after collisions of gold nuclei at speeds close to the speed of light. Researchers found a decline in correlations of particles’ transverse momentum, which could potentially point to a critical point of nuclear matter, Live Science reports.
Unexpected decline
In each collision, charged particles fly outward with different transverse momentum. Scientists analyzed whether pairs of particles in a single collision simultaneously received a stronger or weaker push than average. Such correlations reflect temperature fluctuations and the expansion of matter that occurs after the impact.
Physicists expected the magnitude of these fluctuations to change smoothly as collision energy changed. Instead, the indicator first decreased and then increased. The signal was strong enough to make a statistical origin unlikely, but researchers stress that this is only an indication, not proof of the existence of a critical point.
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Searching for the properties of nuclear matter
Protons and neutrons consist of quarks held together by the strong interaction, whose carriers are gluons. At extremely high temperature or density, they can transition into a state of quark-gluon plasma — a hot mixture of free quarks and gluons. Such matter is believed to have filled the Universe in the first microseconds after the Big Bang.
A critical point would separate two regimes of nuclear matter’s transition to quark-gluon plasma: a smooth one at very high temperatures and an abrupt one at higher density. Near such a point, the heat capacity of matter is expected by physicists to increase sharply, so temperature fluctuations and particle correlations should weaken.
The team analyzed about one billion collisions at energies from 3 to 7.7 GeV per pair of protons or neutrons — at the lower end of RHIC’s operating range, which reaches 200 GeV. During the lowest-energy runs, a beam of gold nuclei was directed at thin gold foil inside the detector to obtain the densest matter available at the collider. The study’s results were published in the journal Physical Review Letters.
