A powerful new particle detector has passed its first major test in the hunt for clues about the early universe.
The sPHENIX detector at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) proved its precision in measuring the aftermath of gold ion collisions.
Physicists say the results show the experiment is ready to investigate quark-gluon plasma, an exotic state of matter that existed moments after the Big Bang.
The sPHENIX detector is the latest experiment at RHIC. It is designed to record the products of high-speed collisions between heavy ions.
From this debris, scientists aim to reconstruct the properties of quark-gluon plasma, a short-lived soup of quarks and gluons that disappeared as the universe cooled into protons and neutrons.
In a new paper, researchers report that sPHENIX measured both the number and energy of particles produced when gold ions smashed together at nearly light speed.
Physicists call this type of measurement a “standard candle.” It provides a benchmark for judging a detector’s accuracy.
The test revealed that head-on collisions produced 10 times more charged particles, which were also far more energetic, than glancing collisions. “This indicates the detector works as it should,” said Gunther Roland, professor of physics at MIT.
He compared it to sending up a new telescope and getting back the first clear image.
“With this strong foundation, sPHENIX is well-positioned to advance the study of the quark-gluon plasma with greater precision and improved resolution,” said Hao-Ren Jheng, a graduate student in physics at MIT and a lead co-author of the paper.
Plasma gone in an instant
RHIC accelerates beams of particles close to the speed of light and then collides them. The energy released can briefly form quark-gluon plasma, thought to be the first state of matter after the Big Bang.
The plasma exists for only about a sextillionth of a second at trillions of degrees. It behaves as a “perfect fluid” before cooling into ordinary matter.
“You never see the QGP itself — you just see its ashes, so to speak,” Roland said. “With sPHENIX, we want to measure these particles to reconstruct the properties of the QGP, which is essentially gone in an instant.”
New era at RHIC
sPHENIX replaces the earlier PHENIX experiment with a faster, more powerful setup.
The detector, weighing 1,000 tons and as tall as a two-story house, sits where RHIC’s two beams meet. Its systems act as a 3D camera, tracking up to 15,000 collisions each second.
A micro-vertex subdetector, designed at MIT’s Bates Research and Engineering Center, gives it added precision.
“sPHENIX takes advantage of developments in detector technology since RHIC switched on 25 years ago, to collect data at the fastest possible rate,” said MIT postdoc Cameron Dean. “This allows us to probe incredibly rare processes for the first time.”
In late 2024, researchers gathered three weeks of data to test the detector.
They confirmed it could capture both the number and energy of particles produced, and that it responded differently depending on how direct the collisions were. “This measurement provides clear evidence that the detector is functioning as intended,” Jheng said.
The detector is now running full experiments. “The fun for sPHENIX is just beginning,” Dean said. “With all our data, we can look for the one-in-a-billion rare process that could give us insights on things like the density of QGP, the diffusion of particles through ultra-dense matter, and how much energy it takes to bind different particles together.”
The project is supported by the U.S. Department of Energy’s Office of Science and the National Science Foundation.
The study is published in the Journal of High Energy Physics.