EMMI Workshop: Light-ion collisions 2026

Light-ion collisions at the LHC provide a unique opportunity to study parton energy loss in compact systems with controlled initial geometry and a direct $pp$ baseline at the same energy. In 2025, the ATLAS detector recorded integrated luminosities of $8~\mathrm{nb}^{-1}$ of O+O and $1~\mathrm{nb}^{-1}$ of Ne+Ne collisions, together with approximately $400~\mathrm{pb}^{-1}$ of $pp$ collisions, at $\sqrt{s_{\mathrm{NN}}}=5.36~\mathrm{TeV}$. These datasets enable a comprehensive study of medium-induced effects using both hadronic and jet observables. Using anti-$k_t$ $R=0.4$ jets, ATLAS measured the dijet transverse momentum balance via the distribution of $x_J \equiv p_{\mathrm{T},2}/p_{\mathrm{T},1}$ for back-to-back dijets with $63 < p_{\mathrm{T},1} \leq 251~\mathrm{GeV}$ as a function of event activity. A centrality-dependent modification of the $x_J$ distributions is observed in both O+O and Ne+Ne relative to $pp$, with the largest effects in the most central events, consistent with results in Pb+Pb collisions at comparable event activity. In parallel, jet azimuthal anisotropies $v_n$ are measured in O+O and Ne+Ne collisions to probe the geometric dependence of jet quenching. Charged-hadron production is also measured to determine spectra and corresponding nuclear modification factors, $R_{AA}$, providing a quantitative assessment of energy loss effects. Together, measurements of the dijet momentum balance, the jet $v_n$ and charged hadron $R_{AA}$ provide complementary constraints on the magnitude of partonic energy loss and its path-length dependence in light nuclear collision systems at LHC energies.