Hard probes of O+O collisions at the LHC

In July 2025, the LHC collided oxygen–oxygen (O+O) nuclei for the first time, marking the only identical collision system to be studied at both RHIC and the LHC. This new system offers a unique opportunity to investigate the properties of the Quark-Gluon Plasma (QGP) in an intermediate-sized environment—larger than pp and p+Pb, but smaller than Pb+Pb. While QGP signatures such as collective behavior and parton energy loss are well established in large systems like Pb+Pb at the LHC and Au+Au at RHIC, their emergence in smaller systems remains an open question. The O+O system sits at the intersection of these regimes, providing sensitivity to both geometric effects and subnucleonic fluctuations, which are especially relevant in small systems.

This talk will present the first measurements of hard probes in proton–oxygen and oxygen–oxygen collisions at the LHC, with a focus on charged-hadron nuclear modification factors ($R_{AA}$) as a tool to probe parton energy loss and QGP-like behavior. Results from CMS, ATLAS, ALICE, and LHCb are discussed, along with comparisons to recent measurements from STAR at RHIC. These studies offer new insights into the onset of QGP phenomena and the scaling of energy loss across different system sizes. Additionally, I will briefly highlight the recent Neon–Neon collisions at the LHC as another valuable intermediate-sized system to further probe QGP-like effects.