Researchers from the HUN-REN Wigner Research Centre for Physics also contributed to the latest result of the CERN CMS experiment, which for the first time showed a clear sign of the plasma state of nuclear matter in collisions of low-mass nuclei.
For a long time, researchers have been studying how lead nuclei colliding at near the speed of light create an extremely hot and dense material, the quark-gluon plasma (QGP). The research evokes a state similar to the universe after the Big Bang, in which the quarks and gluons that make up protons and neutrons could move freely. Now, the CMS experiment at CERN's Large Hadron Collider (LHC) is helping to understand the conditions for the creation of QGP with new results.
| The quark-gluon plasma (QGP) is an extremely hot and dense plasma state of matter that existed for a short time after the Big Bang, in which the fundamental building blocks, the quarks and the gluons holding them together, could still move freely. From these, first protons, neutrons, and then atoms were formed. In CERN’s Large Hadron Collider, the moments after the Big Bang can also be studied because quark-gluon plasma can be created for a moment by high-energy collisions. Its creation is detected indirectly; the traces of the decaying plasma can be detected with the phenomenon of jet quenching. |
Figure 1: Expansion of the Universe; source: https://www.sentinelapologetics.org/
Figure 2: A view of an oxygen-oxygen collision in the CMS detector. The yellow curves show the trajectories of the charged particles flying out, the green columns represent the energy deposited in the detector, and the yellow cones represent the particle jets.
Chasing the quark-gluon plasma
One of the key methods for observing QGP is the phenomenon of jet quenching. During high-energy collisions, quarks and gluons form particle jets (Figure 2). When these particles pass through the hot and dense QGP, they lose energy, which inhibits the production of further particles. This phenomenon is measured with the nuclear modification factor (RAA), which shows how the number of particles created in heavy nucleus collisions related to the numbers measured in proton-proton collisions. Since QGP is not created in proton-proton collisions, they serve as a reference.
Previously, clear jet quenching was only observed in collisions with very heavy nuclei, such as lead and xenon. In smaller systems, such as proton-lead collisions, this phenomenon was absent. Physicists therefore asked the question: how large nuclei are needed for QGP to form?
Breakthrough with light nuclei
The CMS experiment now provides an answer to this question with collisions involving light nuclei, oxygen (16O) and neon (20Ne). The first analysis of the data recorded at the LHC in July 2025 confirmed the presence of jet quenching in oxygen-oxygen collisions: the measured RAA value is much smaller than 1, in some places dropping below 0.7 (Figure 3). This means that even in such a relatively small system, the hot, free state of QGP is formed. The results are consistent with theoretical models that also take into account energy loss.
Figure 3: The RAA value of charged particles in 5.36 TeV oxygen-oxygen collisions as a function of the transverse momentum (pT) of the particles. According to the CMS measurement, the measured ratio is much less than one (blue rectangles). The measurement was compared with theoretical models that did not include energy loss (left figure) and those that did (right figure): a good agreement was found with the latter.
As a next step in the research, the experiment's researchers also performed the first measurements in neon-neon collisions. Since the neon nucleus (20Ne) is slightly larger than the oxygen nucleus (16O), comparing the results with data from other larger systems (129Xe, 208Pb) provides an opportunity to study the size dependence of jet quenching in a model-independent way (Figure 4).
Figure 4: The RAA value of charged particles for various colliding nuclei (OO, NeNe, XeXe, PbPb).
The significance of the discoveries
The data from oxygen-oxygen and neon-neon collisions fill a gap between small and large collision systems. Although QGP-like phenomena have been observed in proton-lead collisions, signs of jet quenching have been missing until now. The CMS experiment has now succeeded in detecting this phenomenon for the first time in collisions of light nuclei. The study of hot, free quark matter at CERN is continuously pushing the boundaries of physics and providing new insights into the initial state of the universe.
The team of nearly twenty researchers, led by the University of Chicago and MIT (Massachusetts Institute of Technology), and the researchers and students of the HUN-REN Wigner Research Centre for Physics and the Eötvös Loránd University Faculty of Science played an important role in the data collection, analysis and evaluation, with the support of NKFI Fund K 146913 and 146914 projects. The CMS collaboration presented the new results at the Initial Stages 2025 international conference.
[1] CMS Physics Analysis Summary CMS-PAS-HIN-25-008
[2] https://indico.cern.ch/event/1479384/
[3] LHC's first oxygen colliions – CMS spots signs of small-scale quark-gluon plasma
