This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists using the STAR detector to study particle collisions at the Relativistic Heavy Ion Collider (RHIC) have found an intriguing dip in their data in a relatively unexplored region of the nuclear phase diagram—a map of how nuclear matter behaves under various conditions of temperature and density. The dip appears in data tracking collision-by-collision variations in the momenta of particles emerging from collisions between gold nuclei at RHIC, near RHIC's lowest collision energies.
These collisions create the highest-density nuclear matter and may indicate that something interesting is happening in that dense region of the phase diagram. The findings are described in a paper just published in Physical Review Letters. RHIC, which operated as a U.S.
Department of Energy (DOE) Office of Science user facility for nuclear physics research at DOE's Brookhaven National Laboratory from 2000 to 2026, was designed to create exotic forms of matter, including the quark-gluon plasma that existed in the very early universe and matter that approaches the density of neutron stars. A dip in the momentum fluctuations—which are closely tied to the temperature of the matter—may be a sign that the way nuclei transform into these exotic substances changes character at RHIC's lower energies. Exploring whether such a change in transition behavior exists—and, if so, where a hypothesized "critical point" demarcating this change is located on the nuclear phase diagram—has been a long-sought goal of physicists conducting research at RHIC.
"The type of matter we are trying to study is a recreation, or a mini version, of the Big Bang," said Rutik Manikandhan, a STAR Collaboration member from the University of Houston and a leader on the new analysis. "The existence of a critical point would tell us about all the phases of matter that could have existed when the universe began. It would sharpen our understanding of the quark-gluon plasma, how it condensed into the protons and neutrons that make up visible matter, and how matter behaves inside neutron stars." RHIC has been central to the exploration of nuclear matter by colliding the nuclei of heavy atoms such as gold at nearly the speed of light.
The most energetic collisions completely melt the boundaries of the protons and neutrons that make up the nuclei, freeing the quarks and gluons inside to form the quark-gluon plasma. Lower-energy collisions do something different: They leave the protons and neutrons intact but squeeze them to extraordinary density, approaching the conditions inside a neutron star. By scanning across collision energies, physicists can chart different regions of the nuclear phase diagram—much as an earlier generation of scientists mapped out the gaseous, solid and liquid phases of water.
The new STAR analysis covers collision energies from 3 to 7.7 billion electron volts (GeV), exploring the lowest energies RHIC could produce and the densest matter it could create. To see the full picture, the team compared these low-energy collision data with previously published STAR data reaching up to 200 GeV. Throughout, they studied only particles emerging at right angles to the colliding beams and near the middle of the collision debris.
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