Researchers at Rice University have captured the temperature profile of quark-gluon plasma, the ultra-hot matter from the universe's dawn. By analyzing electron-positron emissions from atomic collisions, they determined precise temperatures at different evolutionary stages. The findings, published in Nature Communications, refine understanding of early cosmic conditions.
A team led by Rice University physicist Frank Geurts achieved a breakthrough in particle physics by measuring the temperature of quark-gluon plasma (QGP) at various stages of its evolution. This plasma, a state of matter where quarks and gluons exist freely, is believed to have filled the universe just millionths of a second after the Big Bang. The results were published on October 14 in Nature Communications.
To overcome the challenge of measuring temperatures in environments too extreme for instruments, the researchers studied thermal electron-positron pairs produced during high-speed collisions of atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. These pairs, or dileptons, pass through the plasma without distortion, serving as a reliable thermometer.
"Our measurements unlock QGP's thermal fingerprint," said Geurts, a professor of physics and astronomy and co-spokesperson of the RHIC STAR collaboration. "Tracking dilepton emissions has allowed us to determine how hot the plasma was and when it started to cool, providing a direct view of conditions just microseconds after the universe's inception."
The study revealed two distinct temperature ranges based on the mass of the dielectron pairs. In the low-mass range, the average temperature was about 2.01 trillion Kelvin, aligning with predictions for the plasma's transition to ordinary matter. Higher-mass pairs indicated an earlier, hotter phase at around 3.25 trillion Kelvin.
"Thermal lepton pairs, or electron-positron emissions produced throughout the QGP's lifetime, emerged as ideal candidates," Geurts explained. "Unlike quarks, which can interact with the plasma, these leptons pass through it largely unscathed, carrying undistorted information about their environment."
This work advances the mapping of the QCD phase diagram, which describes matter's behavior under extreme heat and density, similar to conditions in the early universe and neutron stars. Contributors include former Rice postdoctoral associate Zaochen Ye, alumnus Yiding Han, and graduate student Chenliang Jin. The research was supported by the U.S. Department of Energy Office of Science.
"This advancement signifies more than a measurement; it heralds a new era in exploring matter's most extreme frontier," Geurts concluded.