In the first few microseconds after the birth of the universe, the protons and neutrons that make up today's atomic nuclei had not yet formed. Matter existed as a hot "soup" of quarks and gluons, known as quark-gluon plasma (QGP). As the universe cooled, quarks became bound inside hadrons, such as protons and neutrons. Studying how quark matter forms and evolves at high temperatures and densities is an important way to understand the evolution of matter in the early universe and explore the strong interaction.

In a recent theoretical study published in Physics Letters B, researchers from the State Key Laboratory of Heavy Ion Science and Technology at the Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS), have found that particles with different numbers of strange quarks respond differently to QGP formation in intermediate-energy gold-gold collisions: the more strange quarks a particle contains, the stronger its response to quark matter.

The researchers simulated gold-gold collisions at center-of-mass energies of 3.0-7.2 GeV using the Parton-Hadron-String Dynamics transport model, which tracks the collision process and particle evolution. They compared two modes: one with a quark phase and one without it. The former allows quarks and gluons to evolve as independent particles, while the latter includes only hadrons and their production processes. The researchers found that even when the central collision region met the assumed conditions for hadron-quark conversion, the final collision products showed no clear signs of a phase transition.

The study indicated that differences were more obvious in strange-particle yields. Starting at a center-of-mass energy of 6.2 GeV, the response to quark matter began to show a trend toward proportionality with the number of strange quarks.

At 7.2 GeV, the model with a quark phase predicted an approximately linear increase in the response with strange-quark number. Compared with the mode without a quark phase, the yields of Lambda hyperon (contains one strange quark) increased by about 20%-30%, those of cascade hyperon (contains two strange quarks) rose by about 65%, and those of Omega hyperon (contains three strange quarks) rose by about 93%.

These results show that the response of particle yields to a quark phase grows with the number of strange quarks the particles contain. "Multi-strange particles could be a promising probe of the hadron-quark phase transition in heavy-ion collisions," said Prof. YONG Gaochan at IMP, the corresponding author of this study. "The study offers a new approach to experimentally searching for quark matter in baryon-rich environments and exploring the phase diagram of nuclear matter.”

Moreover, the researchers proposed probing the hadron–quark transition by jointly measuring energy-dependent yield ratios of multi-strange to singly strange particles and proton elliptic flow at higher transverse kinetic energies.

The study was supported by the National Natural Science Foundation of China.

Figure. The phase diagram of nuclear matter and the evolution of the universe. The vertical axis shows temperature, while the horizontal axis shows baryon chemical potential, which characterizes how baryon-rich the matter is. The sequence above starts with the Big Bang and highlights QGP, hadrons, atoms, stars and galaxies, and the Solar System and Earth. The phase boundaries and possible critical point are qualitative illustrations. (Image by YONG Gaochan)

DOI: https://doi.org/10.1016/j.physletb.2026.141031