Improved nonflow control for multiparticle correlators and system-size-dependent evolution from OO to Pb--Pb collisions at the LHC

Multiparticle correlations of event-plane angles, known as symmetry plane correlators (SPCs), and correlations of anisotropic flow amplitudes of different harmonics, known as symmetric cumulants (SCs), help constrain the initial conditions and the transport properties of the system formed in heavy-ion collisions. Such multiparticle correlations are robust against nonflow biases in heavy-ion collisions; however, in small systems the nonflow contribution is sizeable and can distort SPCs and SCs at low multiplicity, mimicking a flow-like signal. Evidence of nuclear-geometry-induced collectivity in light-ion collisions, such as OO and Ne--Ne, has been established at the LHC. Measurements of SPCs and SCs in these systems, and their comparison to heavy-ion collisions, can therefore clarify the system-size dependence of these correlations and the strength of nonlinear mode coupling. In this work, an improved nonflow subtraction framework based on multiplicity reweighting ($\czero$) and $v_1$-estimator-based ($\cone$) scaling is validated for selected SPCs and SCs using a controlled toy model and the \textsc{Pythia~8~Angantyr} model, where collective flow is absent by construction. We find that $\cone$ provides the more reliable nonflow control and apply it to predict the SPCs and normalized SCs in OO collisions at $\sqrt{s_{\rm NN}}=5.36$~TeV with the \textsc{ampt} string-melting model. The results are compared with published Pb--Pb measurements from ALICE and from \textsc{ampt} as a function of midrapidity charged-particle multiplicity $\langle N_{\rm ch}\rangle(|η|<0.5)$, placing light- and heavy-ion systems on a common system-size axis.