Study of the sensitivity of observables to hot spot size in heavy ion collisions

TL;DR

This study investigates how initial inhomogeneities in heavy-ion collisions influence measurable observables, finding that many are insensitive to small-scale features, while some differential observables can probe short-range initial state dynamics.

Contribution

The paper introduces a method to smooth initial inhomogeneities with minimal impact on global properties, enabling assessment of their effect on various observables in heavy-ion collisions.

Findings

Most integrated and differential observables are insensitive to small-scale inhomogeneities.

Flow factorization ratio and sub-leading principal components are sensitive to granularity.

Many observables depend primarily on large-scale geometric structure.

Abstract

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, in order 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 ($v_n(p_T)$) 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 sub-leading 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.