Tiny Particle Collisions May Unlock Secrets of Quark-Gluon Plasma Formation

New research from the ALICE Collaboration suggests that tiny particle collisions may briefly create conditions resembling quark-gluon plasma (QGP). This state, once thought exclusive to heavy-ion smashups, could now explain puzzling differences in how baryons and mesons behave in smaller systems. The findings, published in Nature Communications, challenge long-held assumptions about QGP formation.

For years, scientists believed quark-gluon plasma—a soup of free quarks and gluons—only formed in large-scale collisions, like those between heavy ions. Yet recent data from proton-proton and proton-lead collisions at the LHC show signs of anisotropic flow, a pattern previously tied to QGP. In these smaller systems, baryons exhibit stronger flow than mesons at intermediate momentum, hinting at collective quark behaviour before particles recombine.

Theoretical models now suggest that when local energy densities exceed ~5–10 GeV/fm³ and temperatures climb above ~150–170 MeV, even tiny collision zones—lasting less than 2 femtometres per light-speed unit—can mimic QGP. Unlike heavy-ion collisions, these fleeting droplets expand hydrodynamically for just ~0.5–2 fm/c, far shorter than the ~10 fm/c lifetimes seen in larger systems. Simulations incorporating quark-level flow and subsequent hadron coalescence successfully replicate the observed trends. Planned oxygen-ion collisions in 2025 could clarify whether this behaviour scales smoothly with system size or marks a distinct physical regime. The results may redefine the boundaries of QGP formation, pushing beyond traditional heavy-ion experiments.

The study implies that quarks in small collisions may briefly enter a plasma-like state before forming hadrons. This mechanism, called quark coalescence, could resolve why baryons and mesons behave differently in such events. Future experiments will test whether these findings hold across varying collision sizes, potentially reshaping our understanding of QGP creation.