PHENIX Experiment Finds Fresh Evidence for Tiny Drops of Quark-Gluon Plasma

Stony Brook University physicists Gabor David and Axel Drees sketch out how a signal of jet energy loss in deuteron-gold collisions at the Relativistic Heavy Ion Collider (RHIC) supports the case that these collisions create small specks of quark-gluon plasma, a form of matter that permeated the early universe. (Kevin Coughlin/Brookhaven National Laboratory)

A new analysis of data from the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory reveals fresh evidence that collisions of even very small nuclei with large ones might create tiny specks of a quark-gluon plasma (QGP).

Scientists believe such a substance of free quarks and gluons, the building blocks of protons and neutrons, permeated the universe a fraction of a second after the Big Bang. RHIC's energetic smashups of gold ions - the nuclei of gold atoms that have been stripped of their electrons - routinely create a QGP by "melting" these nuclear building blocks so scientists can study the QGP's properties.

Physicists originally thought that collisions of smaller ions with large ones wouldn't create a QGP because the small ion wouldn't deposit enough energy to melt the large ion's protons and neutrons. But evidence from PHENIX has long suggested that these small collision systems generate particle flow patterns that are consistent with the existence of tiny specks of the primordial soup, the QGP.

The new findings, recently published in Physical Review Letters, bolster the case for these tiny droplets of the QGP. The paper provides the first direct evidence that energetic particles generated in RHIC's small collision systems sometimes lose energy and slow down significantly on the way out.

Looking for the suppression of high-energy jets of particles, or jet "quenching," has been a key goal from the earliest days at RHIC, a DOE Office of Science user facility for nuclear physics research that began operating at Brookhaven Lab in 2000. Jets are created when a quark or gluon within a proton or neutron in one of RHIC's ion beams collides intensely with a quark or gluon in the nuclear particles that make up the beam traveling in the opposite direction.

These strong interactions can kick single quarks or gluons free from the colliding nuclear building blocks with tremendous amounts of energy, which quickly transforms the energetic particles into cascades, or jets, of other particles. "Those interactions lead to energy loss," explained Gabor David, a PHENIX physicist and research professor in the Department of Physics and Astronomy at Stony Brook University's College of Arts and Sciences, who was one of the leaders of the new analysis.

"You can think about it like the difference between running through air and running through water," David said. The QGP is like the water; it slows the particles down. As a result, jets reach the detector with only a fraction of their original energy.

Just as an RHIC collision can kick an energetic quark or gluon free, that interaction can also produce a high energy photon, or particle of light. These direct photons are produced in the collision right along with and in amounts proportional to the kicked-free quarks and gluons. By counting the direct photons that strike their detector, the PHENIX scientists could directly measure the centrality of the collisions and know exactly how many energetic quarks or gluons were kicked free - that is, how many jets to expect.

Niveditha Ramasubramanian (center) with her two SBU thesis advisors, Thomas Hemmick (left) and Gabor David (right). Courtesy of Niveditha Ramasubramanian.

"The more central the collision is, the more interactions there can be between the quarks and gluons of a small colliding deuteron with the quarks and gluons in the protons and neutrons of a gold ion," explained Axel Drees, Distinguished Professor of Physics and Astronomy at Stony Brook, another leader of the analysis. "So, central smashups produce more direct photons and should produce more energetic jet particles than glancing collisions do."

But unlike the quarks and gluons, the photons don't interact with the QGP. "If photons are created, they escape the QGP completely without any energy loss," Drees said.

Niveditha Ramasubramanian, who was a Stony Brook graduate student advised by David at the time, undertook the challenging task of teasing out the direct photon signals from PHENIX's deuteron-gold collision data. When her analysis was complete, the earlier, unexplained increase in jets emerging from peripheral collisions completely disappeared. But there was still a strong signal of suppression in the most central collisions.

"The initial motivation to do this complex analysis was only to better understand the strange increase in energetic jets in peripheral collisions, which we did," said Ramasubramanian, a co-author on the paper who earned her PhD - and a Thesis Award at the 2022 RHIC & AGS Users Meeting - for her contributions to this result. Now a staff scientist at the French National Centre for Scientific Research, she added, "The suppression that we observed in the most central collisions was entirely unexpected."

Read the complete story at the Brookhaven National Lab website.

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