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According to theoretical predictions, within a millionth of a second after the Big Bang, nucleons had not yet formed, and matter existed as a hot, dense "soup" composed of freely moving quarks and gluons. This state of matter is known as quark-gluon plasma (QGP). Finding definitive evidence for the existence of QGP is crucial for understanding cosmic evolution.

For years, number-of-constituent-quark (NCQ) scaling has been widely considered as key evidence of QGP formation. Now, a new theoretical study in Â鶹ÒùÔºics Letters B, led by researchers at the Institute of Modern Â鶹ÒùÔºics (IMP) of the Chinese Academy of Sciences (CAS), has revealed the failure of NCQ scaling in the low-energy region, casting doubt on the traditional understanding of this QGP probe.

Using an improved multiphase transport model, the researchers systematically simulated gold nucleus collisions at the center-of-mass energies between 3.0 and 7.7 gigaeV (GeV). They found that NCQ scaling fails below 3.9 GeV, even when free quarks are produced in collisions. As the energy increases above 3.9 GeV, the scaling gradually recovers.

Further analysis revealed that the cause of this anomaly lies in dual dynamic constraints. Insufficient thermalization of quarks prevents the full development of elliptic flow, while the extreme scarcity of strange quarks leads to statistical fluctuations in hadron yields.

"This finding questions the reliability of NCQ scaling as a definitive criterion for identifying the QGP phase transition," said Prof. Yong Gaochan, corresponding author of the study.

Additionally, the researchers simulated a pure hadronic scenario without free quarks. At a collision energy of 4.5 GeV, they observed a mass-ordering phenomenon: At the same kinetic velocity, lighter particles exhibit a stronger preference for collective flow than heavier particles.

This simulation shows that fluid-like expansion effects can occur even without full thermal equilibrium, challenging the conventional view that mass ordering must originate from fully hydrodynamic behavior.

"This study not only reveals complex dynamic processes in the low-energy region, but also provides new perspectives for precisely detecting quark matter and the phase transition critical point at future large-scale scientific facilities," said Yong.