# Bulk quantities in nuclear collisions from running coupling $k_{T}$-factorization and hybrid simulations

## TL;DR

This paper uses a CGC-based $k_T$-factorization approach combined with hybrid hydrodynamic simulations to analyze bulk observables in heavy-ion collisions, assessing initial conditions versus evolved states and comparing to experimental data.

## Contribution

It introduces a hybrid simulation framework that combines initial CGC calculations with hydrodynamic evolution to better understand bulk observables in nuclear collisions.

## Key findings

- Hydrodynamic evolution can increase multiplicity by up to 50%.
- The dependence of multiplicity on system size and energy matches experimental data within 5%.
- Hydrodynamics significantly alters momentum distributions, aligning them closer to measurements.

## Abstract

Starting from a Color Glass Condensate (CGC) framework, based on a running-coupling improved $k_T$-factorized formula, we calculate bulk observables in several heavy-ion collision systems. This is done in two ways: first we calculate the particle distribution directly implied from the CGC model, and we compare this to the case where it is instead used as initial conditions for a hybrid hydrodynamic simulation. In this way, we can assess the effects of hydrodynamic and hadronic evolution by quantifying how much they change the results from a pure initial state approach and, therefore, to what extent initial condition models can be directly compared to experimental data. We find that entropy production in subsequent hydrodynamic evolution can increase multiplicity by as much as 50\%. However, disregarding a single overall normalization factor, the centrality, energy, and system size dependence of charged hadron multiplicity is only affected at the $\sim$5\% level. Because of this, the parameter-free prediction for these dependencies gives reasonable agreement with experimental data whether or not hydrodynamic evolution is included. On the other hand, our model results are not compatible with the hypothesis that hydrodynamic evolution is present in large systems, but not small systems like p-Pb, in which case the dependence of multiplicity on system size would be stronger than seen experimentally. Moreover, we find that hydrodynamic evolution significantly changes the distribution of momentum, so that observables such as mean transverse momentum are very different from the initial particle production, and much closer to measured data. Finally, we find that a good agreement to anisotropic flow data cannot be achieved due to the large eccentricity generated by this model.