Constraining the initial state granularity with bulk observables in Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV

TL;DR

This study uses a hybrid model to constrain the initial state granularity in heavy-ion collisions by analyzing bulk observables, finding specific initial condition parameters that match experimental data, but noting the need for additional observables to fully constrain granularity.

Contribution

It introduces a systematic method to constrain initial state granularity in heavy-ion collisions using a hybrid transport model and bulk observables, highlighting the importance of fluctuation-sensitive measurements.

Findings

Initial conditions are constrained to σ=1 fm and t_start=0.5 fm.

Yields and elliptic flow can be matched with these parameters.

Granularity requires additional observables for full constraint.

Abstract

In this paper we conduct a systematic study of the granularity of the initial state of hot and dense QCD matter produced in ultra-relativistic heavy-ion collisions and its influence on bulk observables like particle yields, $m_T$ spectra and elliptic flow. For our investigation we use a hybrid transport model, based on (3+1)d hydrodynamics and a microscopic Boltzmann transport approach. The initial conditions are generated by a non-equilibrium hadronic transport approach and the size of their fluctuations can be adjusted by defining a Gaussian smoothing parameter $\sigma$. The dependence of the hydrodynamic evolution on the choices of $\sigma$ and $t_{start}$ is explored by means of a Gaussian emulator. To generate particle yields and elliptic flow that are compatible with experimental data the initial state parameters are constrained to be $\sigma=1$ fm and $t_{\rm start}=0.5$ fm. In addition, the influence of changes in the equation of state is studied and the results of our event-by-event calculations are compared to a calculation with averaged initial conditions. We conclude that even though the initial state parameters can be constrained by yields and elliptic flow, the granularity needs to be constrained by other correlation and fluctuation observables.