Observation of Long-Range Collective Flow in O + O and Ne + Ne Collisions and Implications for Nuclear Structure Studies

The long-range collective flow of particles produced in oxygen-oxygen (O+O) and neon-neon (Ne+Ne) collisions is measured with the CMS detector at the CERN LHC. The data samples were collected at a center-of-mass energy per nucleon pair of 5.36 TeV, with integrated luminosities of 7 nb⁻¹ and 0.8 nb⁻¹ for O+O and Ne+Ne collisions, respectively. Two- and four-particle azimuthal correlations are measured over nearly five units of pseudorapidity. Significant elliptic (𝑣₂) and triangular (𝑣₃) flow harmonics are observed in both systems. The ratios of 𝑣_𝑛 coefficients between Ne+Ne and O+O collisions suggest sensitivity to the intrinsic nuclear structure of the respective Ne and O ions. Hydrodynamic models with ab initio nuclear structure inputs qualitatively reproduce the collision-overlap dependence of both the 𝑣_𝑛 values and the Ne+Ne to O+O ratios. These measurements provide new constraints when modeling the interplay of nuclear structure and collective dynamics in collisions of ¹⁶O and ²⁰Ne ions, respectively.