Light-Ion Collisions at the LHC

The Large Hadron Collider is not only a machine for discovering new particles. It is also a laboratory for creating and studying a new phase of QCD matter. When lead nuclei collide at ultrarelativistic energies, the resulting energy densities far exceed the QCD confinement scale and melt hadrons into a Quark-Gluon Plasma (QGP), a strongly coupled, nearly perfect fluid that is well described by relativistic viscous hydrodynamics. Yet a fundamental question remains. How does a handful of elementary particles thermalize and form a collective medium in such a vanishingly short window of time?

Light-ion collisions probe the critical regime where nuclear structure, perturbative QCD, and quark–gluon plasma physics intersect. The first oxygen–oxygen and neon–neon runs at the LHC in July 2025 opened a new chapter in the LHC ion program, bridging the gap between proton–proton and heavy-ion collisions in a controlled and theoretically tractable way. For a decade, experiments had found QGP-like collective flow even in proton–proton collisions, while jet quenching, the canonical signature of the QGP, remained elusive in small systems. The 2025 runs have changed this picture. First results show evidence of jet quenching in systems with as few as ten participating nucleons, and collective flow measurements in oxygen–oxygen and neon–neon collisions agree well with hydrodynamic predictions with input from ab initio nuclear structure calculations.

This talk will discuss what these results reveal about thermalization, collectivity, and the boundaries of the QGP phase, and what might come next for the ion program at the LHC.