Measurement of Flow Harmonics in O+O and Ne+Ne Collisions with the ATLAS Detector

Relativistic heavy-ion collisions create an exotic state of matter known as quark-gluon plasma (QGP) — a hot, dense soup of deconfined quarks and gluons that existed for the first microseconds after the Big Bang. Collider experiments have revealed that the QGP behaves like a nearly perfect liquid, exhibiting strong collective motion. The QGP fireball formed in these collisions is not perfectly spherical; its spatial anisotropies drive an azimuthally asymmetric expansion, which in turn leads to anisotropies in the final particle distributions. These azimuthal patterns are quantified using Fourier harmonics (v_n), often called "flow harmonics," which encode information about the initial geometry of the colliding nuclei and provide a stringent test of hydrodynamic models of QGP evolution. This talk presents ATLAS measurements of flow harmonics v₂–v₅ in O+O and Ne+Ne collisions at 5.36 TeV. These "light-ion" collisions are especially sensitive to fluctuations in the initial geometry, allowing us to probe whether the hydrodynamic description remains valid at the smallest QGP scales. These measurements offer new insights into how nuclear shapes and initial-state fluctuations influence the collective flow of the hottest, densest matter ever created in the laboratory.