Collisions of light nuclei performed at the Large Hadron Collider are providing new clues to an extreme state of matter. The ALICE, ATLAS, CMS and LHCb experiments have all observed phenomena compatible with the formation of a quark-gluon plasma.
This plasma is thought to have filled the Universe during its first microseconds. At temperatures exceeding 100,000 times that of the Sun's core, protons and neutrons lose their usual structure. Their quarks and the gluons that bind them can then move around in very hot and very dense matter.
Researchers usually produce this state by colliding heavy nuclei, such as lead. In 2025, the LHC also collided oxygen and neon nuclei. Their small size makes it possible to study the minimal conditions required for this plasma to appear.
Each experiment looked for a different signature. ALICE, ATLAS and CMS in particular measured how particles are distributed after the collision. Collective patterns appear in the data, as if the created matter briefly behaved like an expanding fluid.
CMS also observed a reduction of very energetic particles compared with expectations established from proton-proton collisions. This phenomenon can occur when quarks or gluons pass through a dense medium and lose part of their energy.
Artistic representation of the formation of the quark-gluon plasma.
Image: CERN
These light collisions occupy an intermediate area between proton collisions and heavy-ion collisions. They can help determine at what size a set of particles adopts collective behavior. This boundary remains one of the open questions in nuclear physics at very high energies.
The teams now need to compare oxygen and neon data more finely with hydrodynamic models.