The study addresses the long-standing challenge of measuring the temperature of matter under extreme conditions where direct access is impossible. By using thermal electron-positron pairs emitted during ultrarelativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York, the researchers have decoded the thermal profile of QGP.
Temperature measurements existed previously but have been plagued by several complications such as whether they were of the QGP phase or biased by a Doppler-like effect from the large velocity fields pushing such effective temperatures.
"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."
New thermal window into nuclear matter
The properties of QGP, a deconfined state of quarks and gluons, depend heavily on its temperature. Prior methods lacked the resolution or penetrating power needed to measure QGP's inner thermal conditions without being affected by the evolution of this rapidly expanding system. With temperatures expected to exceed trillions of Kelvins, scientists needed an unobtrusive thermometer to capture real-time values.
"Thermal lepton pairs, or electron-positron emissions produced throughout the QGP's lifetime, emerged as ideal candidates," Geurts said. "Unlike quarks, which can interact with the plasma, these leptons pass through it largely unscathed, carrying undistorted information about their environment."
However, detecting these rare pairs amid a sea of particle debris required unprecedented sensitivity and data fidelity, Geurts said.
They tested the hypothesis that the energy distribution of these pairs would provide a direct measure of QGP temperature. This technique, referred to as a penetrating thermometer in theoretical discussions, integrates emission data over the plasma's lifetime, producing an average temperature profile.
The research team achieved a precise measurement despite the technological limitations of statistical data and difficulties in isolating background processes that could mimic thermal signals.
This difference indicates that thermal radiation from the low-mass range, which creates these dielectrons, is predominantly emitted later near the phase transition. In contrast, those from the higher mass range originate from the earlier, hotter stage of the QGP's evolution.
"This work reports average QGP temperatures at two distinct stages of evolution and multiple baryonic chemical potentials, marking a significant advance in mapping the QGP's thermodynamic properties," Geurts said.
By precisely measuring the temperature of the QGP at different points in its evolution, scientists gain crucial experimental data needed to complete the "QCD phase diagram," which is essential for mapping out how fundamental matter behaves under immense heat and density, akin to conditions that existed moments after the big bang and are present in cosmic phenomena like neutron stars.
"Armed with this thermal map, researchers can now refine their understanding of QGP lifetimes and its transport properties, thus improving our understanding of the early universe," Geurts said. "This advancement signifies more than a measurement; it heralds a new era in exploring matter's most extreme frontier."
Co-authors of this study include former Rice postdoctoral associate Zaochen Ye, now at South China Normal University; Rice alumnus Yiding Han, now at Baylor College of Medicine; and current Rice graduate student Chenliang Jin. Geurts' U.S. Department of Energy Office of Science Award supported the study.
Research Report:Temperature measurement of Quark-Gluon plasma at different stages
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