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 root s(NN) = 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.

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

Carrá, S.;Cresta, T.;Gaudio, G.;Introzzi, G.;Manco, G.;Negri, A.;Pareti, A.;Rebuzzi, D. M.;Romano, E.;
2026-01-01

Abstract

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 root s(NN) = 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11571/1557200
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