A new analysis of collisions of gold ions shows tantalizing signs of a “critical point,” a change in the way one form of matter changes into another. This study looked at how quarks and gluons, the building blocks of protons and neutrons in atomic nuclei, transform into the quark-and-gluon “soup” that filled the early universe. The results hint at changes in the type of transition during the shift from particles to “soup.” The transition moves from a gradual “melting” at high collision energies to a more abrupt shift at low collision energies.
Studying the phases of nuclear matter (made of quarks and gluons) is a little like learning about the solid, liquid, and gaseous forms of water. By mapping out how nuclear matter transitions from phase to phase under different temperatures, pressures, and other conditions, scientists learn about how the particles interact and what holds them together. Understanding phase changes in nuclear matter will provide insight into the evolution of the early universe. It also helps scientists better understand conditions in the cores of neutron stars.
To search for a change in the way the nuclear phase transition takes place (the “critical point”), scientists at the Relativistic Heavy Ion Collider (RHIC), a Department of Energy Office of Science user facility at Brookhaven National Laboratory, analyzed collisions at different energies from 2010 to 2017. They were looking for fluctuations in the net number of protons produced as they turned down the energy. Theorists had predicted there would be wide collision-by-collision fluctuations in such measurements as the critical point was approached. These fluctuations are somewhat like the turbulence an airplane experiences when entering a bank of clouds, but not as easy to spot as food and drinks bouncing off airplane tray tables. Physicists using RHIC’s STAR detector (the Solenoidal Tracker at RHIC) had to perform “higher order correlation function” statistical analyses. Results from some of these detailed analyses—particularly the amount of skew in the distribution of particles—showed tantalizing hints of the expected fluctuations at lower collision energies. However, the range of uncertainty in those results is still large. Analyzing additional data from a second phase of RHIC’s Beam Energy Scan (2019-2021) should narrow the uncertainty and potentially nail the discovery of a critical point.
This study was supported by the Department of Energy Office of Science, the National Science Foundation, and a wide range of international funding agencies listed in the paper. RHIC operations are funded by the DOE Office of Science.