Researchers at the STAR experiment, part of the Relativistic Heavy Ion Collider (RHIC), have made significant strides in recreating a primordial form of matter from the early universe through smaller atomic collisions. By colliding oxygen nuclei at nearly the speed of light, scientists found strong evidence for quark-gluon plasma (QGP), a hot and dense state of matter believed to have existed shortly after the Big Bang.
The findings, published in Physical Review Letters, indicate that high-energy particle jets lose energy as they navigate the material produced in these collisions. This phenomenon, known as jet quenching, is a critical indication of QGP formation. Previous studies at RHIC demonstrated jet quenching with larger nuclei, such as gold, as well as in experiments conducted at CERN’s Large Hadron Collider (LHC).
The latest results are noteworthy because they emerged from collisions at significantly lower energies than typically used, establishing a new lower threshold for the production of QGP in smaller systems. This advancement helps researchers refine their understanding of the conditions necessary for creating matter akin to that of the infant universe.
“The QGP is believed to be the form of matter that filled the early universe—before protons and neutrons formed,” said Li Yi, a physics professor at Shandong University who led the STAR analysis. “Understanding its properties is essential for uncovering how the early universe evolved in its first microseconds.” Yi emphasized the importance of identifying where the signatures of QGP appear and disappear in experiments.
Rongrong Ma, a physicist at Brookhaven Lab, highlighted the importance of studying smaller collisions to gain insights into the jet quenching phenomenon and the requisite conditions for generating QGP. RHIC was specifically designed to facilitate such research, typically by accelerating heavier gold nuclei and colliding them at high energies. In contrast, the choice to experiment with oxygen nuclei, consisting of eight protons and eight neutrons, allows for clearer observations due to their smaller, spherical structure.
To investigate jet quenching, the researchers employed an analysis method that identifies high-energy particle jets and measures energy levels among the particles produced in collisions. Data collected from oxygen collisions conducted at RHIC in 2021 revealed a 20% suppression of high-energy particles, indicative of jet quenching. While this level of suppression is less pronounced than what is generally observed in larger gold collisions, it still provides compelling evidence that quark-gluon plasma formed during these experiments.
As RHIC has concluded its operations, researchers are examining remaining data from its final runs, which included oxygen collisions captured by multiple detectors. This analysis aims to further understand the limits at which quark-gluon plasma can be produced, shedding light on the behavior of matter during the universe’s earliest moments. The research is supported by the U.S. Department of Energy, the National Science Foundation, and other organizations.
