Measurement of the azimuthal anisotropy of charged particles in √S_NN = 5.36 TeV ¹⁶O + ¹⁶O and ²⁰Ne + ²⁰NE collisions with the ATLAS detector

This paper presents the first measurements of the azimuthal anisotropy coefficients v_n, which quantify the n^th-order Fourier modulation of charged-particle azimuthal distributions, for n = 2-4 in √S_NN = 5.36 TeV ¹⁶O + ¹⁶O and ²⁰Ne + ²⁰NE collisions recorded with the ATLAS detector at the Large Hadron Collider in 2025. The v_n coefficients are measured as a function of transverse momentum p_T, collision centrality, and event multiplicity. They are extracted using two complementary methods: two-particle correlations with a template-fit subtraction of short-range non-flow contributions, and four-particle subevent cumulants, which intrinsically suppress non-flow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy v₂ > v₃ > v₄ and a non-monotonic dependence on p_T, reaching a maximum around 2 GeV, consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced v₂ in central ²⁰Ne + ²⁰NE collisions, consistent with theory expectations based on the predicted prolate deformation of neon nuclei, in contrast to the slightly tetrahedral structure predicted for oxygen. The four-particle cumulant results highlight strong event-by-event fluctuations and provide the greatest sensitivity to nuclear shape effects. These measurements can place new constraints on the initial geometry and the hydrodynamic response in light-ion collisions, offering valuable input for models of nuclear structure.