Measurement of the azimuthal anisotropy of charged particles in sqrt(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 vn, which quantify the nth-order Fourier modulation of charged-particle azimuthal distributions, for n = 2-4 in sqrt(sNN) = 5.36 TeV 16O+16O and 20Ne+20Ne collisions recorded with the ATLAS detector at the CERN Large Hadron Collider in 2025. The vn coefficients are measured as a function of transverse momentum (pT), collision centrality, and event multiplicity. They are extracted using two complementary methods: two-particle correlations with a template-fit subtraction of short-range nonflow contributions, and four-particle subevent cumulants, which intrinsically suppress nonflow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy v2 > v3 > v4 and a nonmonotonic dependence on pT, 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.