Evidence for Parton Energy Loss in Oxygen–Oxygen Collisions with ALICE

About

The formation of a quark--gluon plasma (QGP) in collisions of heavy nuclei is firmly established through a broad range of experimental signatures, including collective flow phenomena and parton energy loss. In recent years, measurements in increasingly smaller collision systems have revealed features traditionally associated with QGP formation, raising the question of how small a collision system can be and still produce a strongly interacting medium.

The first oxygen--oxygen collisions delivered by the Large Hadron Collider in July 2025 provide a unique opportunity to address this question. Oxygen nuclei occupy an intermediate position between proton–nucleus and heavy-ion collisions, allowing the study of QGP signatures in a regime that has remained largely unexplored experimentally.

This seminar will introduce the experimental signatures used to characterize QGP formation across different collision systems and discuss the role of parton energy loss as one of the most direct probes of a hot deconfined medium. Neutral-pion production measured by ALICE's ElectroMagnetic Calorimeter (EMCal) in oxygen--oxygen and proton--oxygen collisions provides an experimental handle on disentangling cold- and hot-nuclear-matter effects. By combining both systems in a novel double-ratio observable, cold-nuclear-matter effects can be constrained experimentally and largely cancelled, enabling a direct test of parton energy loss in oxygen collisions. The results provide first evidence for parton energy loss in oxygen--oxygen collisions at the LHC and extend experimental access to the smallest nuclear collision systems in which this phenomenon has been established.

Coffee will be served at 10h30

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