In an exciting new development at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, an analysis of data from the PHENIX experiment has provided fresh evidence suggesting that even very small nuclei, when colliding with large ones, might create tiny specks of the quark-gluon plasma (QGP). This primordial substance—composed of free quarks and gluons—was present in the early universe, just moments after the Big Bang, and its creation in the lab is a major goal of modern nuclear physics research.
The QGP, which is believed to have existed shortly after the Big Bang, is an exotic state of matter where quarks and gluons, typically confined within protons and neutrons, move freely in a hot and dense “soup.” RHIC’s regular collisions of gold ions—nuclei of gold atoms stripped of their electrons—are well known for melting the nuclear building blocks and creating QGP. However, the discovery that smaller ions in collisions with larger ones may also generate tiny droplets of QGP has challenged previous assumptions about the formation of this elusive state of matter.
The Context: Small vs. Large Ion Collisions
Scientists previously thought that collisions between small nuclei (such as deuterons, which are made up of just one proton and one neutron) and large nuclei (like gold) wouldn’t create QGP. This was based on the idea that small ions wouldn’t have enough energy to “melt” the large nucleus’s protons and neutrons, which is a necessary condition for the formation of QGP. However, long before the latest data, the PHENIX experiment had observed particle flow patterns in smaller collisions that hinted at the creation of tiny regions of the QGP, even in these less energetic interactions.
The new findings, published in Physical Review Letters, provide the first direct evidence of QGP creation in small collision systems. Specifically, the research team found that energetic particles produced during small nucleus collisions sometimes lost significant amounts of energy as they traveled out of the interaction zone—a clear signal that QGP might have been formed and interacted with these particles.
Jet Quenching: A Sign of QGP
A key signature scientists look for in detecting QGP is known as jet quenching. Jets are cascades of particles produced when quarks or gluons are kicked free during a high-energy collision, such as those at RHIC. These jets are produced when a quark or gluon within a proton or neutron interacts strongly with another quark or gluon in a particle from the opposing ion. The energy from these interactions transforms the original energetic particles into a cascade of other particles, forming what are called jets.
In a collision that does not produce QGP, these energetic jets travel freely through space and can be measured by detectors. However, if QGP is present, the quarks and gluons from the jet interact with the plasma, leading to energy loss. The result is that the jets, instead of reaching the detectors with their full energy, are quenched—they lose energy as they travel through the plasma. This phenomenon has been observed in large nucleus collisions at RHIC, such as those between gold ions.
The New Results: Small Collisions and Jet Suppression
The team behind the new study, led by PHENIX Collaboration Spokesperson Yasuyuki Akiba, has successfully identified signs of jet suppression in smaller collision systems, providing strong evidence for the existence of QGP in these systems as well. “We found, for the first time in a small collision system, the suppression of energetic particles, which is one of the two main pieces of evidence for QGP,” explained Akiba, a physicist at Japan’s RIKEN Nishina Center for Accelerator-Based Science.
Jet quenching has been a primary objective at RHIC since the facility’s early days in 2000. The discovery of suppressed jets in small deuteron-gold collisions supports the hypothesis that even small nuclei can generate tiny pockets of QGP. While large, central collisions between heavy ions like gold and gold produce substantial amounts of QGP, smaller nuclei have previously been expected to be too weak to generate such conditions. However, the new findings challenge this view by showing that even these collisions can result in QGP-like behavior.
Direct Photon Measurement: A New Approach
To arrive at these conclusions, the researchers developed a more precise method for analyzing the collisions, one that offered a more direct way to measure the centrality of the collision. The previous approach used a theoretical model to estimate how many protons and neutrons were involved in the collision, but this indirect method sometimes led to unexpected results. For example, in earlier analyses, small deuteron-gold collisions sometimes appeared to produce unexpectedly high numbers of energetic jets in peripheral (off-center) collisions, a phenomenon that was difficult to explain.
To resolve this discrepancy, the team turned to a more direct measurement technique: counting “direct” photons. Photons, being uncharged, interact only very weakly with matter and are unaffected by the QGP. Thus, they serve as an ideal tracer of the energy and centrality of a collision. Since direct photons are produced in direct proportion to the number of energetic quarks or gluons involved in the collision, counting them provides a precise measure of how central or peripheral the collision is. Importantly, because photons don’t interact with QGP, their production rate in any collision system is unaffected by the formation of QGP.
By measuring the direct photons alongside the energetic particles (jets), the researchers were able to accurately assess the amount of jet suppression in the collision. This direct photon measurement technique revealed that while peripheral deuteron-gold collisions showed a small increase in energetic jets, the most central collisions—those with the highest density of interacting quarks and gluons—showed clear suppression of jets. This suppression was much more pronounced than what would have been expected in the absence of QGP.
Unexpected Results: Central Collisions Show Suppression
The key discovery from this new analysis was that, when using direct photons to better understand the centrality of collisions, the earlier observed increase in jets from peripheral collisions disappeared. The researchers also observed significant jet suppression in the most central collisions, confirming that QGP formation was taking place in these small collisions as well. “The suppression that we observed in the most central collisions was entirely unexpected,” said Niveditha Ramasubramanian, a graduate student who worked on the analysis.
This suppression of energetic jets in central small collisions is a major breakthrough in the study of QGP. It not only challenges our understanding of how small ions behave in collisions with large ones, but it also provides the first definitive evidence for the creation of tiny QGP droplets in smaller collision systems. As Akiba pointed out, “The new method relies solely on observable quantities, avoiding the use of theoretical models,” making it a more direct and accurate measure of the presence of QGP.
Next Steps and Future Research
Building on this new method, the PHENIX team plans to extend its analysis to other small collision systems, such as proton-gold and helium-3-gold collisions, to further test and refine their understanding of QGP formation. The results from these additional experiments could either confirm the current interpretation of QGP formation in small systems or suggest new avenues for exploration.
“Ongoing analyses of PHENIX’s proton-gold and helium-3-gold data with the same technique will help to further clarify the origins of this suppression to confirm our current understanding or rule it out by competing explanations,” explained Axel Drees, another leader of the analysis.
The results of this study are likely to inspire new questions and challenges for the broader nuclear physics community, as scientists work to understand the full range of conditions under which QGP can be produced and what its properties can tell us about the universe’s early moments. These findings also suggest that small-scale collisions may hold more secrets than previously thought, potentially unlocking new insights into the fundamental forces that govern the behavior of matter at its most basic level.
Conclusion
The detection of tiny droplets of quark-gluon plasma in small ion collisions at RHIC marks a significant step forward in the study of the early universe and the fundamental properties of matter. By refining measurement techniques and providing new evidence for jet suppression in these smaller systems, the PHENIX team has not only improved our understanding of QGP but has also expanded the scope of experiments that could potentially reveal more about this elusive state of matter. The work continues to be a crucial piece of the puzzle in understanding the extreme conditions that existed just moments after the Big Bang.
Reference: N. J. Abdulameer et al, Disentangling Centrality Bias and Final-State Effects in the Production of High- pT Neutral Pions Using Direct Photon in d+Au Collisions at sNN=200 GeV, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.022302. On arXiv: DOI: 10.48550/arxiv.2303.12899