Researchers from the Chinese Academy of Sciences’ (CAS) Institute of Modern Physics (IMP) have proposed a key indicator that could reveal the emergence of quark-gluon plasma (QGP) by examining particle “fingerprints” created in heavy-ion collisions. Their study was published in Physics Letters B on April 8th, 2025.
Schematic of cosmic evolution. Image Credit: 2013 J. Phys.: Conf. Ser. 454 012051
The finding offers a new perspective on the evolution of matter in the early universe.
The cosmos was extremely hot and dense around 13.8 billion years ago, just a millionth of a second after the Big Bang. The fundamental building blocks of matter were free quarks and gluons, a special state known as QGP, rather than protons and neutrons. The QGP progressively condensed into the modern-day atomic nuclei as the universe cooled and expanded.
Although scientists have successfully created QGP in laboratories, precisely mapping its formation process remains a significant challenge. Similar to how fingerprints identify individuals, the production ratios of different particles in collisions carry crucial information.
Gaochan Yong, Professor, Institute of Modern Physics
The scientists focused on the production patterns of four particles—protons, K+ mesons, π mesons, and Λ hyperons—by simulating violent collisions of heavy ions like calcium-40, calcium-48, and gold-197 using an enhanced multi-phase transport model. They noticed unusual variations in the yield ratios of particular particles when the collision system switched from the lighter calcium-40 to the heavier gold-197.
According to the study, a crucial marker for determining QGP creation may be the emission ratios of similar particles in heavy-light reaction systems. Multiple scatterings between hadrons are suppressed by the free flow of quarks and gluons during QGP formation, resulting in particle yields that are much lower than those predicted by the pure hadronic mode. On the other hand, ongoing hadronic collisions significantly boost particle yields in the absence of QGP.
To validate this idea, the researchers cross-checked the results with another model, confirming the link between anomalous particle yields and quark-matter production. Simulations reveal that parton rescattering has little effect on particle yields, whereas hadronic rescattering greatly increases them.
The study found that the proposed innovative probe efficiently lowers systematic errors and model uncertainties, considerably improving detection sensitivity and reliability.
“The new probe provides critical clues for mapping the complete QCD phase diagram. It not only deepens the understanding of high-density nuclear matter states, but also offers new experimental insights into the evolution of the early universe,” Yong added.
This research is supported by the National Natural Science Foundation of China and the CAS Project for Young Scientists in Basic Research.
Journal Reference:
Zhu, X. and Yong, G.-C. (2025) Exploring hadron-quark phase transition in heavy-ion collisions using particle emission ratios in heavy and light reaction systems. Physics Letters B. doi.org/10.1016/j.physletb.2025.139454