Evidence of nuclear geometry-driven anisotropic flow in OO and Ne$-$Ne collisions at $\mathbf{\sqrt{{\textit s}_{\rm\mathbf {NN}}}}$ = 5.36 TeV

Abstract: A central question in strong-interaction physics, governed by quantum chromodynamics (QCD), is whether femto-scale droplets of quark$-$gluon plasma (QGP) form in small collision systems involving projectiles significantly smaller than heavy ions. Collisions of light ions such as $^{16}$O and $^{20}$Ne offer a unique opportunity to probe the emergence of collective behavior in QCD matter. This Letter presents the first measurements of elliptic ($v_2$) and triangular ($v_3$) flow of charged particles in $^{16}$O$-$$^{16}$O and $^{20}$Ne$-$$^{20}$Ne collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{_{\rm NN}}} = 5.36$ TeV with the ALICE detector. The hydrodynamic model predictions, explicitly incorporating the nuclear structures of $^{16}$O and $^{20}$Ne, exhibit a good agreement with the flow measurements presented. The observed increase of $v_2$ in central Ne$-$Ne collisions relative to OO collisions, driven by the nuclear geometries, highlights the importance of utilizing light nuclei with well-defined geometric shapes to constrain the initial conditions. These findings support the presence of nuclear geometry-driven hydrodynamic flow in light-ion collisions at the LHC.