Sensitivity of observables to coarse-graining size in heavy-ion collisions

An open question in the field of heavy-ion collisions is to what extent the size of initial inhomogeneities in the system affects measured observables. Here we present a method to smooth out these inhomogeneities with minimal effect on global properties, to quantify the effect of short-range features of the initial state. We show a comparison of hydrodynamic predictions with original and smoothened initial conditions for four models of initial conditions and various observables. Integrated observables (integrated v(n), scaled v(n) distributions, normalized symmetric cumulants, event-plane correlations) as well as most differential observables {[}v(n)(pT)] show little dependence on the inhomogeneity sizes and instead are sensitive only to the largest-scale geometric structure. However, other differential observables such as the flow factorization ratio and subleading principal components are more sensitive to the granularity and could be a good tool to probe the short-scale dynamics of the initial stages of a heavy-ion collision, which are not currently well understood. (AU)