Scientists at the Relativistic Heavy Ion Collider (RHIC), a user facility for nuclear physics research operated by the U.S. Department of Energy (DOE) Office of Science at DOE’s Brookhaven National Laboratory, simulated the extremely high temperatures of the early universe by colliding particles at speeds approaching that of light. The study was published in Nature Communications.
The STAR detector, which is as large as a house, specializes in tracking the thousands of particles produced by each ion collision at the Relativistic Heavy Ion Collider. Image Credit: Kevin Coughlin/Brookhaven National Laboratory
The collisions at RHIC explore the enigmas surrounding the characteristics of matter by transforming the colliding particles into a quark-gluon plasma (QGP), a mixture of fundamental particles that constitute the building blocks of protons and neutrons.
A recent analysis of data obtained from the STAR detector at RHIC has unveiled the temperature of the QGP at various stages of its evolution after gold ion collisions, the nuclei of gold atoms that have lost their electrons.
These measurements are crucial for charting how nuclear matter transforms as quarks and gluons in the heated medium cool down and merge to create more conventional nuclear particles. Investigating this phase transition at RHIC is aiding physicists in comprehending the events that transpired during the fleeting moments at the universe's inception, the last occasion the QGP was present in nature.
The QGP is the hottest matter that we can create on Earth. The temperature of the QGP is the single most important thermodynamic parameter we need to characterize the properties of this extreme matter. For the first time at RHIC, we can directly measure it using electrons and their antiparticles.
Zaochen Ye, Study Lead and Professor, South China Normal University
Zaochen Ye is a STAR Collaboration Member and a former postdoc at Rice University.
The recent STAR results obtained at various collision energies offer unique insights into the properties of the QGP during both its initial phase, as the collision fireball swiftly expands, and a subsequent phase, just prior to the cooling of the hot, dense primordial soup, which initiates the formation of subatomic particles like protons and neutrons.
The research team discovered that the temperature of the QGP peaked at approximately 3.3 trillion degrees Celsius during the earliest moments of these collisions. This temperature is roughly 220,000 times hotter than the core of the sun, as noted by the researchers. In the later phase, the temperature recorded as the QGP cooled and began to transition into conventional nuclear matter was half as high.
We were able to almost pinpoint where the phase transition occurs using this method. What we found is that all the measured late-stage temperatures at different energies are the same and coincide with the expected temperature at the transition point. That was a surprise that we were very happy to see.
Zhangbu Xu, STAR Physicist and Professor, Kent State University
Ralf Rapp, Theorist, Texas A&M University, said, “These are fantastic measurements, and it takes decades of planning and a learning curve to get to the point where you can finally extract meaningful data.”
A Rare but Ideal “Thermometer”
To investigate the temperature of the QGP system, STAR researchers focused on the emissions of pairs of electrons and their antiparticles, known as positrons. This specific combination of particles, referred to as a type of "dilepton," is rarely generated in collisions, occurring in only about one in 10,000 events, according to the researchers.
“It’s challenging to, first, find them. Then there’s a large number of background sources you have to subtract,” said Rapp.
Due to their rarity, researchers must capture and examine numerous occurrences using the dilepton method.
Statistics is the point here. We need statistics. Having the experience that we’ve built up over the many years that we’ve wanted to take these measurements, we know how to handle the data, we have the statistics, and here we are indeed extracting a temperature using this method.
Frank Geurts, Rice University
Frank Geurts is a STAR co-spokesperson.
Although challenging to measure, the distinct particle pairs monitored by the STAR detector provide an excellent "thermometer" for accurately determining the temperature of the QGP, as noted by researchers.
Previous research conducted at RHIC and the Large Hadron Collider (LHC) at CERN, which is the European Organization for Nuclear Research, utilized measurements of the energy of photons, essential particles of light, released from nuclear collisions to ascertain the temperature of the Quark-Gluon Plasma (QGP).
In 2010, the PHENIX Collaboration announced the creation of matter at approximately 4 trillion degrees Celsius by employing photons as a probe at RHIC’s maximum collision energy of 200 billion electron volts (GeV).
Due to the rapid expansion of the fireball generated by these energetic collisions, which cools during the process, the energy spectrum of the emitted photons is modified, according to researchers.
This shift in energy resembles the alteration in the sound of a train whistle as it approaches and then moves away, a phenomenon referred to as the Doppler effect. The STAR team indicated that photon measurements necessitate an additional model to separate this effect to accurately determine the true temperature at the initial stage of the QGP.
Dileptons possess a characteristic known as 'invariant mass' that renders the particle pairs unaffected by this type of 'blue' energy shift. The invariant mass of the pair accurately represents the thermal temperature, remaining unchanged by the expanding source that produces those particles.
The dileptons offer an extra dimension. I'm very excited that we can extract the temperature of the QGP and use different ranges of invariant mass to access the temperature at different stages.
Chi Yang, STAR Physicist and Professor, Shandong University
Yang specializes in direct photon and dilepton measurements.
The recent measurements obtained through dileptons were recorded at two distinct collision energies of 27 and 54.5 GeV. These measurements establish a basis for conducting the dilepton analysis again with data gathered by STAR at 200 GeV.
Researchers contrasted their measurements with supplementary baseline temperature probes collected from various nuclear physics research institutions in Europe, utilizing a different set of dileptons at reduced collision energies.
“Data obtained from those facilities provide a crucial benchmark, while our measurements at higher energies cover an important phase space unique to RHIC. With the large dataset that we’re collecting right now in RHIC’s final run, plus future data from LHC, we will be able to probe the matter under the conditions closest to the first few microseconds of the Big Bang,” said Ye.
The study received funding from the DOE Office of Science, the U.S. National Science Foundation (NSF), and various international agencies and organizations. Besides utilizing the Open Science Grid, which is directly supported by NSF, the researchers also accessed computing resources at the Scientific Data and Computing Facilities at Brookhaven Lab and the National Energy Research Scientific Computing Center (NERSC), another user facility of the DOE Office of Science located at Lawrence Berkeley National Laboratory.
Brookhaven National Laboratory is funded by the Office of Science of the U.S. Department of Energy. The Office of Science is the largest supporter of fundamental research in the physical sciences in the United States and is actively addressing some of the most urgent challenges of this era.
Journal Reference:
Aboona, E. B., et al. (2025) Temperature measurement of Quark-Gluon plasma at different stages. Nature Communications. DOI:10.1038/s41467-025-63216-5. https://www.nature.com/articles/s41467-025-63216-5