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The ATLAS experiment reports that pairs of particle jets produced in collisions of oxygen and neon nuclei show a growing imbalance in momentum when the collisions are more head-on. The imbalance is measured relative to proton–proton collisions at the same energy (5.36 TeV per nucleon pair). The change with collision centrality is consistent with the idea that fast-moving quarks and gluons lose energy as they pass through a hot, dense medium formed in these collisions.
The measurement looks at dijets, pairs of jets produced nearly back-to-back in azimuthal angle. The key quantity is x_J, the ratio of the sub-leading jet transverse momentum to the leading jet transverse momentum (pT2/pT1). ATLAS used data samples of 8.0 nb⁻¹ for O+O, 1.0 nb⁻¹ for Ne+Ne collected in 2025, and 386 pb⁻¹ of pp collisions collected in 2024 as a reference. Jets were required to lie within rapidity |y|<2.1, with leading-jet transverse momentum 63 < pT1 < 251 GeV, and dijets selected to be nearly back-to-back (azimuthal separation Δφ > 7π/8). The reported x_J distributions are “self-normalized” (each distribution is scaled to unit area) and corrected for detector effects.
Centrality, a measure of how much the two nuclei overlap, was determined from the total transverse energy measured in ATLAS’s forward calorimeters. Five centrality ranges were studied: 0–10%, 10–20%, 20–40%, 40–60% and 60–80%, where smaller percentiles mean more central (larger overlap) collisions. The x_J distributions in the most central O+O and Ne+Ne collisions show larger deviations from the pp reference than in more peripheral collisions. That centrality dependence matches expectations for medium-induced partonic energy loss (often called “jet quenching”), and shows the effect persists in systems much smaller than the large lead–lead or xenon–xenon collisions previously studied.
This result matters because light-ion collisions like O+O and Ne+Ne have a substantially smaller transverse size than Pb+Pb collisions. That makes it easier to study how the energy loss depends on the distance a parton travels through the medium, a key input for understanding the properties of the medium and the onset of quark–gluon plasma (QGP) behavior. The more symmetric overlap geometry in central light-ion collisions also reduces geometric uncertainties that complicate interpretations in peripheral heavy-ion data.
There are important caveats. Determining centrality in small systems and relating it to a physical path length relies on modeling (a Monte Carlo Glauber calculation is used), and correlations between the hard process that produces jets and the soft underlying event can bias results. The ATLAS analysis reduces such “centrality-bias” effects by using self-normalized distributions rather than absolute jet rates, and by correcting for detector effects, but some model dependence and experimental uncertainty remain. The paper reports that the trends are consistent with medium-induced energy loss, but this observation alone does not by itself settle all questions about QGP formation in small collision systems. Comparisons with theoretical predictions are presented in the full paper to help further constrain interpretations.