New results from the ALICE Collaboration suggest quark-gluon plasma may form in proton collisions, not just heavy-ion experiments.

A new analysis from the ALICE Collaboration is reshaping how physicists understand the conditions required to produce quark-gluon plasma (QGP), a state of matter thought to have existed moments after the Big Bang.

The findings, published in Nature Communications, indicate that even relatively small particle collisions can exhibit characteristics long associated only with large-scale heavy-ion experiments.

For decades, QGP has been studied by smashing heavy ions, such as lead nuclei, at extremely high energies. These collisions recreate the intense heat and density needed to free quarks and gluons from their usual confinement inside protons and neutrons.

Smaller systems, like proton–proton collisions, were generally considered incapable of reaching those conditions.

That assumption is now under increasing pressure.

Evidence emerges from proton collisions

The ALICE Collaboration analysed data from proton–proton and proton–lead collisions at the Large Hadron Collider (LHC), focusing on how particles emerge from these events.

A key observable is ‘anisotropic flow,’ a phenomenon where particles are emitted preferentially in certain directions rather than uniformly.

In heavy-ion collisions, this directional pattern is widely interpreted as evidence of collective behaviour within a quark-gluon plasma. The new study shows that a similar pattern appears in smaller systems when enough particles are produced.

More importantly, the team observed a distinct separation between two classes of particles: baryons (made of three quarks) and mesons (made of two quarks).

At intermediate momentum ranges, baryons consistently displayed stronger anisotropic flow than mesons – a hallmark signature previously linked to QGP formation.

Quark-level dynamics provide clues

This difference between particle types is typically explained by a mechanism known as quark coalescence.

In this framework, quarks flowing within the plasma combine to form composite particles. Because baryons contain an additional quark compared to mesons, they inherit a stronger collective motion.

The ALICE researchers extended this analysis across multiple particle species and a broad momentum range, isolating signals that reflect genuine collective behaviour rather than background noise.

The consistency of the pattern across different systems strengthens the argument that quark-level interactions are driving the observed effects.

These findings suggest that quarks in smaller collision systems may briefly enter a state resembling quark-gluon plasma before recombining into hadrons.

Models partially confirm the picture

To interpret the data, the collaboration compared experimental results with theoretical simulations. Models that incorporate both anisotropic flow at the quark level and subsequent hadron coalescence were able to reproduce the general trends observed in the data.

By contrast, simulations that excluded either of these processes failed to match observations, reinforcing the idea that both mechanisms are essential to understanding the results.

However, the agreement is not exact. Discrepancies remain, particularly in how the models describe the internal structure of protons and the initial geometry of collisions.

These uncertainties limit the precision with which researchers can interpret the findings and point to areas where theoretical work is still needed.

Bridging the gap between small and large systems

The implications of this study extend beyond a single set of measurements. If quark-gluon plasma can form in smaller systems, it challenges the conventional boundary between small and large collision physics and raises new questions about how QGP emerges and evolves.

Future data may help clarify the picture. Experiments involving oxygen-ion collisions, recorded in 2025, are expected to provide an intermediate system between proton and lead collisions.

These results could help determine whether the observed behaviour scales smoothly with system size or reflects distinct physical regimes.