Measurement of the azimuthal anisotropy of charged particles in $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV $^{16}$O+O$^{16}$ and $^{20}$Ne + Ne$^{20}$ 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 $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV $^{16}$O+O$^{16}$ and $^{20}$Ne + Ne$^{20}$ collisions recorded with the ATLAS detector at the CERN 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 nonflow contributions, and four-particle subevent cumulants, which intrinsically suppress nonflow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy $v_2>v_3>v_4$ and a nonmonotonic dependence on pT, reaching a maximum around 2GeV, consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced $v_n$ in central $^{20}$Ne + Ne$^{20}$ 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.