The STAR Collaboration just finished some very cool experiments at the DOE’s Relativistic Heavy Ion Collider (RHIC). All of these experiments generated exciting results in the search to find a critical point in nuclear matter. Mikhail Stephanov, a nuclear theorist at the University of Illinois Chicago, helped make key predictions that have steered the observations. The critical point marked as an unusual spire in the rolling nuclear phase diagram. It represents a huge departure from the conventional understanding of how quarks and gluons traverse different phases of matter.
The STAR scientists are measuring many basic properties, in order to investigate fluctuation properties at their collision energies. Their aim is to unravel the complex potential energy landscape of nuclear phases. If confirmed, these results would indicate one half of the expected signature of a tipping point. Such an achievement would be a monumental leap forward in our own understanding of matter in extreme conditions.
Understanding the Critical Point
The critical point in nuclear matter represents an exciting transition in the nature of quark and gluon interactions. This point serves as a dividing line where the traditional behavior of nuclear matter suddenly shifts radically.
Stephanov underscores how any achievement of this inflection point, however critical, would be similar to detecting turbulence on an airplane’s flight. He states, “It’s as if a plane—whether climbing, descending, or cruising—hits turbulence. Instead of the usual steady acceleration, passengers feel sudden shifts in the direction of the acceleration.”
“Finding the critical point would put a landmark on the nuclear phase diagram,” says Xiaofeng Luo, highlighting its potential to revolutionize current understandings of nuclear physics.
Experimental Observations
Based on Stephanov’s theoretical calculations, these kurtosis values should follow certain trends in the presence of a critical point. First they are required to decrease, then increase above baseline projections, then decline again.
“These changes in direction mean that there is some particular energy at which something happens that does not happen at other points,” Stephanov adds. The results from STAR indicate that at 3 GeV, the proton kurtosis value falls slightly below that observed at 7.7 GeV.
Furthermore, as energy continues to decrease, kurtosis values seem to increase once more and fall back within baseline range at 7.7 GeV. The dip then rise in kurtosis may be the first half of the expected signature of a critical point.
The results released today in Physical Review Letters promise phenomenal statistical significance. Based on what baseline value is used, their results are between two and five sigma across the maximum.
Collaborative Efforts and Future Directions
The STAR Collaboration’s innovative analysis has been backed by a large collaborative effort involving hundreds of scientists representing the world’s institutions. Bedangadas Mohanty has been an integral part—hands, heart and soul—of the Beam Energy Scan program. This program is designed to probe the various properties of nuclear matter by scanning through various collision energies.
Stephanov cautions that a large void remains between 7.7 and 3 GeV. He acknowledges the critical need for additional data to supplant these conclusions with more robust evidence. We look forward to future experiments filling this gap, to better confirm and understand the existence and nature of the critical point.
ShinIchi Esumi notes that addressing this gap is already on STAR’s agenda. “That’s going to come up very soon in the future. It’s on the to-do list for STAR.”
As usual, Ashish Pandav had the most interesting finding. In the process, they found a distinct dip in the kurtosis data taken in one RHIC gold-ion collision at about 20 billion electron volts (GeV). This observation would give more understanding about the nuclear matter’s behavior around the critical point.