EMMI Workshop: Light-ion collisions 2026

The first oxygen–oxygen and proton–oxygen collisions delivered by the LHC in 2025 provide a unique opportunity to study the onset of parton energy loss in collision systems significantly smaller than those traditionally used for heavy-ion physics. While recent measurements have revealed several signatures of collective behavior in oxygen collisions, establishing the presence of parton energy loss proves challenging due to the potentially significant contribution of cold-nuclear-matter effects.

This contribution presents recent ALICE measurements of neutral-pion production in oxygen–oxygen and proton–oxygen collisions. By combining the nuclear modification factors measured in both systems in a novel double-ratio observable, cold-nuclear-matter effects can be constrained experimentally and largely cancelled. The resulting measurement provides first evidence for parton energy loss in oxygen–oxygen collisions and extends experimental access to the smallest nuclear collision systems in which this phenomenon has been established. The results are compared to theoretical calculations both with and without parton energy loss, providing a direct test of the role of final-state interactions in oxygen collisions.