EMMI Workshop: Light-ion collisions 2026

Measurements of soft-particle production and collective flow in light-ion collisions provide powerful probes of both the initial-state geometry and the subsequent dynamical evolution of the system. This talk presents new ATLAS results from proton--oxygen (p+O), oxygen--oxygen (O+O), and neon--neon (Ne+Ne) collisions at $\sqrt{s_{NN}} = 5.36​​$ TeV collected at the LHC. Collective behavior is studied using charged-particle tracks through measurements of azimuthal anisotropy coefficients​, including two-particle and four-particle cumulants, $v_n\{2\}$ and $v_n\{4\}$, in O+O and Ne+Ne collisions. Comparisons among the different collision systems provide clear evidence of collective flow in small systems and offer stringent constraints on the geometry of highly asymmetric collisions. In particular, enhanced $v_2\{k\}$ values observed in the most central Ne+Ne collisions relative to O+O indicate the influence of the intrinsic elongated structure of the neon nucleus.

Measurements of the charged-particle pseudorapidity density, $dn/d\eta$, and average transverse momentum, $\langle p_\mathrm{T}\rangle$, are presented for O+O and Ne+Ne collisions. Charged particles are reconstructed within $|\eta|<2.5$ and $0.27 < p_{\mathrm T} < 5​$ GeV, with the measured spectra extrapolated to $p_{\mathrm T}=0$​. Results are reported as functions of pseudorapidity and collision centrality for both the fiducial and extrapolated kinematic ranges, together with ratios and differences in between Ne+Ne and O+O systems. The measurements are compared with hydrodynamic calculations employing different initial-state models. Together, these new light-ion data provide valuable constraints on nuclear structure and particle-production models, while establishing an important foundation for understanding collective dynamics and initial-state effects in small collision systems.