A recent study of data from the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) provides new insights suggesting that even collisions between small nuclei and larger ones can create minuscule droplets of quark-gluon plasma (QGP). This form of matter, composed of free quarks and gluons—the fundamental components of protons and neutrons—is thought to have filled the universe just moments after the Big Bang. In its powerful collisions involving gold ions (the nuclei of gold atoms stripped of their electrons), RHIC typically generates QGP by “melting” these nuclear constituents, allowing scientists to explore its properties.
Initially, experts believed that smaller ions interacting with larger ones would not produce QGP since the smaller ion lacked the energy needed to disrupt the protons and neutrons of the larger ion. However, evidence gathered from the PHENIX experiment has long pointed to the fact that these smaller collision systems generate flow patterns in particles that align with the presence of tiny specks of QGP. New findings recently published in Physical Review Letters strengthen the notion of these miniature QGP droplets, providing direct evidence that high-energy particles created in RHIC’s smaller collision systems can lose energy and decelerate significantly as they exit.
“For the first time, we observed the energy suppression of high-energy particles in a smaller collision setup, which is one of the key indicators of QGP,” stated Yasuyuki Akiba, spokesperson for the PHENIX Collaboration and a physicist at the RIKEN Nishina Center for Accelerator-Based Science in Japan.
Jet suppression indicates QGP
The search for signs of high-energy jets of particles—known as jet “quenching”—has been crucial since RHIC began operations in 2000 as a user facility for nuclear physics research. Jets are produced when a quark or gluon from a proton or neutron in one of RHIC’s ion beams violently collides with a quark or gluon in the opposite beam. Such vigorous interactions can free individual quarks or gluons from their nuclear companions, generating a cascade or “jet” of particles.
In collisions that do not produce a QGP, these jets—and their subsequent decay products—can escape freely to be recorded by RHIC detectors. However, if a QGP is formed, the released quark or gluon is ensnared by interactions within the plasma, causing energy loss.
“This energy loss can be likened to running through water versus running through air,” explained Gabor David, a PHENIX physicist from Stony Brook University and a key leader of this analysis. “The QGP acts like water and slows down the particles, leading to jets arriving at the detector with diminished energy.”
To detect this suppression, researchers first predict the number of energetic particles expected from the gold-gold collisions by scaling from simpler proton-proton collisions to account for the number of nuclear particles involved. These values indicate whether two gold ions collide centrally (head-on) or peripherally (grazing one another). Central collisions are anticipated to produce more jets, but they are also more likely to yield larger QGP and thus more significant jet suppression.
This approach has been highly effective in examining gold-gold collisions. “We expected to see 1,000 times the number of energetic jets compared to proton-proton collisions in the most central gold-gold interactions,” Akiba noted. “However, we only observed about 200 times the proton-proton level, indicating a factor of five suppression.”
This suppression serves as a clear indicator that gold-gold collisions indeed generate QGP and aligns with another critical signature of QGP formation—the distinct patterns of particle flow resulting from the hydrodynamic properties of the plasma.
Upon noticing similar flow patterns in smaller collision systems that hinted at tiny blobs of QGP, PHENIX scientists aimed to investigate jet suppression in these events too. Their findings were unexpected: while central collisions among particles like deuterons (composed of one proton and one neutron) with gold ions showed potential jet suppression, peripheral collisions demonstrated an increase in energetic jets.
David remarked, “There was absolutely no explanation for this occurrence.”
Investigating direct photons
The surprising uptick in jets was found to be a result of how physicists had assessed the centrality of the collisions. They uncovered this through an alternative methodology—counting “direct” photons, which are produced in the collision along with the kicked-free quarks and gluons.
When a RHIC collision occurs, energetic quarks or gluons can generate high-energy photons. By tracking these direct photons striking the detector, PHENIX scientists could measure collision centrality more accurately, determining the exact number of energetic quarks or gluons that were freed—thereby establishing the predicted number of jets.
Axel Drees from Stony Brook University elucidated, “More central collisions lead to greater interaction opportunities among the quarks and gluons of a colliding deuteron and those in the gold ion, producing more direct photons and consequently more energetic jet particles than peripheral collisions.”
Unlike quarks and gluons, photons do not interact with the QGP. “If photons are created, they escape the QGP without losing energy,” remarked Drees.
Therefore, in the absence of QGP, direct photons and energetic particles should appear in similar amounts. However, if in central collisions fewer energetic jet particles are detected compared to the number of direct photons of similar energy, it indicates potential QGP formation that suppresses the jets.
Niveditha Ramasubramanian, who was a graduate student under David at the time, tackled the complex task of isolating direct photon signals within the deuteron-gold collision data. Upon completing her analysis, the previously noted increase in jets from peripheral collisions vanished, with clear suppression observed in the central collisions.
“Originally, we aimed to understand the puzzling increase of energetic jets in peripheral collisions, which we accomplished,” said Ramasubramanian, now a staff scientist at the French National Centre for Scientific Research. She added, “The suppression in the most central collisions was completely unanticipated.”
Akiba stated, “Using direct photons as a precise measure of collision centrality allows us to clearly see the suppression [in central collisions].”
David emphasized that “The new technique relies solely on observable factors, avoiding reliance on theoretical models.”
Next, researchers plan to apply this method to analyze additional small collision systems.
“Ongoing studies of PHENIX’s proton-gold and helium-3-gold data using the same approach will help clarify the reasons behind this suppression and whether our current theories hold true or require adjustments,” Drees explained.
This research received support from the DOE Office of Science (NP), the National Science Foundation, and various U.S. and international universities and organizations included in the scientific paper. The PHENIX experiment gathered data at RHIC from 2000 to 2016, and analysis of this data continues.
