New Insights into Quark-Gluon Plasma Dynamics from RHIC Collisions

Recent findings from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have revealed significant new insights into the behavior of quark-gluon plasma (QGP) during high-energy particle collisions. The study, which appears in recent issues of Physical Review Letters and Physical Review C, demonstrates how jets of particles, generated from energetic collisions, interact with the QGP, causing a phenomenon described as sideways 'splashing.' This research has crucial implications for understanding the fundamental properties of matter as it existed just after the Big Bang.

The RHIC, a prominent facility under the U.S. Department of Energy, is designed to recreate conditions similar to those just after the universe's inception by colliding gold ions at nearly the speed of light. This process generates QGP, a state of matter where quarks and gluons, the fundamental building blocks of protons and neutrons, are liberated from their confinement within atomic nuclei.

According to Dr. Peter Jacobs, a physicist at Lawrence Berkeley National Laboratory and a member of the STAR Collaboration, which conducted the research, "Measuring reconstructed jets provides a unique perspective into how the strongly interacting plasma responds to the jets as they traverse through it. This study shifts the focus from merely observing jets to understanding what the jets reveal about the QGP."

The research introduced a novel method of analyzing photon-correlated jets, which allows scientists to more accurately measure the energy dynamics within the QGP. Co-author Dr. Saskia Mioduszewski, a collaborator from Texas A&M University, noted, "By analyzing photons that do not interact with the QGP, we can establish a baseline to compare the energy of jets produced in collisions that generate QGP versus those that do not. This comparison is critical to understanding how jets lose energy in the plasma."

The data collected indicates that jets emerging from gold-gold collisions exhibit a broader distribution of energy compared to jets from proton-proton collisions, suggesting that energy is not simply lost but redistributed within the QGP. This phenomenon is likened to the way water splashes outward when a bike tires through a puddle. Dr. Jacobs elaborated, "The energy is conserved; it is just dispersed more broadly due to interactions with free quarks and gluons present in the plasma."

In their analysis, the research team employed advanced statistical techniques to isolate direct photons produced during collisions and used these to reconstruct jets with varied angular sizes. This allowed them to observe the QGP's response to jets more comprehensively. The results suggested that a jet's energy is not entirely contained within a narrow cone but is instead dispersed outward, indicating a more complex interaction with the QGP than previously understood.

The study raises further questions regarding the properties of QGP, particularly its viscosity and how it behaves under different conditions. The implications of such findings could extend beyond theoretical physics, potentially influencing our understanding of early universe conditions and the fundamental laws of nature.

As Dr. Mioduszewski mentioned, "To fully characterize energy loss and the QGP's response, we need to explore how these interactions depend on the jet's path length through the plasma and the strength of those interactions. This ongoing research will help us construct a more coherent picture of the QGP."

Overall, this groundbreaking research not only enhances our understanding of QGP dynamics but also contributes to the broader field of high-energy physics, offering new avenues for exploration into the fundamental nature of matter and the universe's origins. The STAR Collaboration's findings will continue to be crucial as they work alongside theorists to integrate these observations into a unified framework of particle physics.

### Sources: 1. B. E. Aboona et al, "Measurement of In-Medium Jet Modification Using Direct Photon+Jet and π0+Jet Correlations in p+p and Central Au+Au Collisions at sNN=200 GeV," *Physical Review Letters*, 2025, DOI: 10.1103/PhysRevLett.134.232301. 2. B. E. Aboona et al, "Semi-inclusive direct photon + jet and π0+jet correlations measured in p+p and central Au+Au collisions at sNN=200GeV," *Physical Review C*, 2025, DOI: 10.1103/8b8y-98yh. 3. Dr. Peter Jacobs, Lawrence Berkeley National Laboratory, personal communication, June 2025. 4. Dr. Saskia Mioduszewski, Texas A&M University, personal communication, June 2025. 5. RHIC STAR Collaboration reports, Brookhaven National Laboratory, June 2025.