This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: High-energy nuclear collisions recreate extreme conditions similar to those in the early universe, producing quark–gluon plasma (QGP), a hot state of matter in which quarks and gluons are no longer confined inside protons and neutrons. While large systems such as lead–lead collisions can produce matter that behaves like a fluid, collective behavior has also been observed in smaller systems.
Oxygen–oxygen collisions offer an intermediate setting, but whether the produced matter reaches local thermal equilibrium—and therefore behaves like a fluid—has remained unclear. Addressing this challenge, a research team led by Professor Tetsufumi Hirano of the Faculty of Science and Technology at Sophia University in Tokyo, together with graduate student Naoya Ito, a second-year master's student, quantitatively evaluated for the first time the extent to which matter produced in high-energy oxygen–oxygen (O + O) collisions at CERN's Large Hadron Collider (LHC) reaches thermal equilibrium. They used the dynamical core–corona initialization (DCCI2) model, which separates matter that reaches local equilibrium into a "core" component from particles that remain nonequilibrated in a "corona" component.
Their findings are published in the journal Physical Review C. The researchers modeled O + O collisions at a collision energy of 5.36 TeV and examined how the balance between the two components changes with the number of charged particles produced near the center of the collision. In the model, the core is treated as an equilibrated medium whose evolution can be described using relativistic hydrodynamics, whereas the corona represents particles that do not fully equilibrate.
This approach allowed the team to quantify how much of the produced matter behaves collectively without assuming that the entire system reaches equilibrium. The analysis revealed a clear transition as collision activity increased. When the charged-particle multiplicity at midrapidity exceeded about 20, the equilibrated core contribution became larger than the corona contribution.
However, the corona did not disappear even in the most central O + O collisions; it continued to account for approximately 30% of the total hadron yield. "We were able to quantitatively clarify, for the first time, how far quark–gluon matter produced in oxygen collisions approaches thermal equilibrium," Hirano said. The result indicates that O + O collisions occupy an intermediate regime between systems dominated by nonequilibrated particles and those in which collective, fluid-like behavior is dominant.
Further analysis of particle momentum showed that the core contribution generally dominates at lower momenta, while the corona becomes increasingly important at higher momenta. This transition occurs at higher momentum for heavier particles, reflecting the stronger boost they receive from the collective expansion of the equilibrated core. The researchers also examined strange-baryon production.
Extract — continue reading at the source.
