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

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 shortrange 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 vn, scaled vn distributions, normalized symmetric cumulants, event-plane correlations) as well as most differential observables (vn(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 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.

Measuring momentum-dependent flow fluctuations in heavy-ion collisions

“…One possibility is measuring the PCA of V n∆ , as defined in Eqs. (5), (8) and (6). This has the disadvantage of giving the same weight to all events, regardless of the number of particles, which may result in an unacceptable statistical uncertainty.…”

“…As the initial conditions fluctuate event by event, momentumdependent azimuthal correlations develop among particles in their final state. The detailed momentum dependence of two-particle correlations is, so far, the only available probe of the granularity of initial-state fluctuations, to which other more standard observables are insensitive [6][7][8]. Therefore, a thorough study of two-particle correlations and their momentum dependence is needed in order to resolve the relevant sub-nucleonic length scales present in the initial state of heavy-ion collisions.…”

“…In Ref. [4], diagonal and off-diagonal momentumdependent correlations were investigated with the introduction of the factorization breaking ratio r n (p a T , p b T ), which measures the correlation of the anisotropic flow between two different transverse momenta [4][5][6][7][9][10][11][12]. In Ref.…”

Features of the IP-Glasma

“…The more concentrated hot spots of the IP-Glasma model do not play a significant role as they are washed out quickly in the viscous hydrodynamic evolution. This point has been quantified in [29], where the effect of small scale structures, as present in the IP-Glasma initial state, was analyzed by smearing the initial energy density using cubic splines, which erase the small scale structure but keep the overall size of the system unchanged.…”

Initial conditions for nuclear collisions: theory overview

“…For large collisions systems the role of sub-nucleonic fluctuations is less evident, because viscous hydrodynamic evolution smears out the small scale structure. A recent systematic investigation up to which scale λ initial perturbation can be smeared without affecting flow observables was done in [6] using a number of initial state models (see Fig. 1(b)).…”