G. Aad, E. Aakvaag, B. Abbott, S. Abdelhameed, K. Abeling, N. J. Abicht, S. H. Abidi, M. Aboelela, A. Aboulhorma, H. Abramowicz, Y. Abulaiti, B. S. Acharya, A. Ackermann, C. Adam Bourdarios, L. Adamczyk, S. V. Addepalli, M. J. Addison, J. Adelman, A. Adiguzel, T. Adye, A. A. Affolder, Y. Afik, M. N. Agaras, A. Aggarwal, C. Agheorghiesei, F. Ahmadov, S. Ahuja, S. Ahuja, X. Ai, G. Aielli, A. Aikot, M. Ait Tamlihat, B. Aitbenchikh, T. P. A. Åkesson, A. V. Akimov, D. Akiyama, N. N. Akolkar, S. Aktas, G. L. Alberghi, J. Albert, U. Alberti, P. Albicocco, G. L. Albouy, S. Alderweireldt, Z. L. Alegria, M. Aleksa, I. N. Aleksandrov, C. Alexa, T. Alexopoulos, F. Alfonsi, M. Algren, M. Alhroob, B. Ali, H. M. J. Ali, S. Ali, S. W. Alibocus, M. Aliev, G. Alimonti, W. Alkakhi, C. Allaire, B. M. M. Allbrooke, D. R. Allen, J. S. Allen, J. F. Allen, P. P. Allport, A. Aloisio, F. Alonso, C. Alpigiani, Z. M. K. Alsolami, A. Alvarez Fernandez, M. Alves Cardoso, M. G. Alviggi, M. Aly, Y. Amaral Coutinho, A. Ambler, C. Amelung, M. Amerl, C. G. Ames, T. Amezza, D. Amidei, B. Amini, K. Amirie, A. Amirkhanov, S. P. Amor Dos Santos, K. R. Amos, D. Amperiadou, S. An, C. Anastopoulos, T. Andeen, J. K. Anders, A. C. Anderson, A. Andreazza, S. Angelidakis, A. Angerami, A. V. Anisenkov, A. Annovi, C. Antel, E. Antipov, M. Antonelli, F. Anulli
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 N N = 5.36 TeV O 16 + O 16 and Ne 20 + 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 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 v 2 in central Ne 20 + 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.