In an experiment, clear evidence of the plasma state of nuclear matter was demonstrated for the first time in collisions of small atomic nuclei. Researchers and students from the ELTE Institute of Physics and Astronomy and the HUN-REN Wigner Research Center for Physics also contributed to this groundbreaking result.
Researchers have long been investigating how lead nuclei colliding at nearly the speed of light create an extremely hot and dense substance, quark-gluon plasma (QGP), which causes a state similar to the universe after the Big Bang, reads a statement from ELTE (Eötvös Loránd University, Budapest).
Researchers from the CMS experiment, including physicists from the Faculty of Science at ELTE and the HUN-REN Wigner Research Center for Physics, have now come a step closer to understanding the formation of QGP and thus the first moments of the universe with new results obtained from the collision of materials lighter than lead nuclei.
Fact
The Compact Muon Solenoid (CMS) is one of two large general-purpose particle physics detectors built on the Large Hadron Collider (LHC, the world’s largest and most powerful particle accelerator) at CERN in Switzerland and France. The goal of the CMS experiment is to investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter. The experiment is one of the largest international scientific collaborations in history, involving approximately 5,800 particle physicists, engineers, technicians, students, and support staff from 241 institutes in 54 countries.
As they wrote, one of the key methods for observing QGP is the phenomenon known as “jet quenching.” The quarks and gluons produced in high-energy collisions form particle beams called “jets.” When these particles pass through the hot and dense QGP, they lose energy, which prevents them from creating further particles. Researchers measure this phenomenon using the so-called nuclear modification factor (RAA), which indicates how the number of particles produced in heavy atomic nucleus collisions relates to the numbers measured in proton-proton collisions. Since no QGP is produced in proton-proton collisions, they serve as a reference.
Until now, clear radiation absorption has only been observed in collisions with very heavy atomic nuclei such as lead and xenon, while this phenomenon was absent in smaller systems such as proton-lead collisions.
The physicists therefore asked the question: How large must atomic nuclei be for a state similar to the universe after the Big Bang, known as QGP, to form?
The first analysis of the data collected in July 2025, at the LHC confirmed the presence of radiation absorption even in oxygen-oxygen collisions. This means that even in such a relatively small system, the hot, free state of the QGP is created. These results are consistent with theoretical models that also take energy loss into account.
CMS detector for LHC. Photo: Wikipedia
As the next step in their research, the scientists also carried out initial measurements on neon-neon collisions. Since the neon nucleus is slightly larger than the oxygen nucleus, comparing the results with data from other, larger systems allows for a model-independent investigation of the size dependence of radiation absorption. “The data from oxygen-oxygen and neon-neon collisions fill a gap by bridging the gap between small and large collision systems. Although QGP-like phenomena have already been observed in proton-lead collisions, there has been no evidence of radiation absorption until now. The CMS experiment has now succeeded for the first time in detecting this phenomenon in collisions of lighter atomic nuclei.
Research into hot, free quark matter at CERN is continuously pushing the boundaries of physics and providing new insights into the origin of the universe,”
they wrote.
Researchers and students from the ELTE Institute of Physics and Astronomy and the HUN-REN Wigner Research Center for Physics played a key role in the work of the nearly 20-member team led by researchers from the University of Chicago and the Massachusetts Institute of Technology (MIT) by collecting, analyzing, and evaluating data.
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Via elte.hu, Featured image: Pixabay
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