Quark-Gluon Plasma Found in Tiny Particle Collisions, Defying Expectations

A new study by the ALICE Collaboration, published in Nature Communications, is reshaping our understanding of quark-gluon plasma (QGP). Previously, scientists believed this extreme state of matter only formed in high-energy collisions of heavy ions. Now, evidence suggests even small particle crashes—like proton-proton or proton-lead collisions—can briefly create QGP-like conditions under the right circumstances.

The research challenges long-held assumptions about the conditions needed for QGP formation. Current models state that QGP requires energy densities above ~5-10 GeV/fm³ and temperatures exceeding ~150-170 MeV. These thresholds were thought to apply mainly to large systems, such as gold or lead ion collisions at facilities like RHIC and the LHC. However, the new analysis shows that smaller systems can also reach these local conditions, despite producing fewer particles overall.

One key observation is a clear separation between baryons and mesons in intermediate momentum ranges. Baryons exhibit stronger anisotropic flow—a pattern usually linked to QGP—than mesons. The study proposes that this difference arises from quark coalescence, where quarks briefly enter a plasma-like state before recombining into hadrons. The findings align best with theoretical simulations that combine quark-level anisotropic flow and hadron formation through coalescence. This consistency across different collision systems reinforces the idea that quark interactions are driving the observed effects. The discovery blurs the traditional boundary between small and large collision physics, raising fresh questions about how and when QGP emerges. Looking ahead, planned oxygen-ion collisions in 2025 could provide further clarity. These experiments will test how the observed behaviour scales with system size, potentially confirming whether QGP-like states are more common than previously thought.

The study reshapes the understanding of QGP by showing its possible formation in smaller collision systems. Anisotropic flow and particle separation patterns, once considered hallmarks of heavy-ion experiments, now appear in proton-proton and proton-lead crashes. Future experiments will determine whether these effects scale predictably, offering deeper insights into the fundamental nature of quark-gluon plasma.