Studying the impact of the nucleon size in relativistic heavy-ion collisions

TL;DR

This study investigates how the size of nucleons influences initial conditions and observables in relativistic heavy-ion collision simulations, revealing that larger nucleon sizes dampen initial gradients and affect particle momentum distributions.

Contribution

The paper systematically analyzes the impact of nucleon width on collision simulations, clarifying its role in initial condition characteristics and experimental observable predictions.

Findings

Nucleon width significantly influences initial eccentricity and gradients.

Larger nucleon widths reduce the mean transverse momentum of particles.

Recent Bayesian analyses favor larger nucleon sizes to match experimental data.

Abstract

The hydrodynamic stage of relativistic heavy-ion collisions simulations requires an energy density profile of the system as an initial condition. In the process of converting the two colliding nuclei into such an energy distribution, some specification about the $\textit{size}$ of their constituent nucleons inevitably has to be made. Nucleons are commonly modeled as bidimensional Gaussians, and the Gaussian width (the nucleon-width) is a free parameter of the simulation. A best-fit value of the nucleon-width can be inferred by Bayesian Analyses, where the model is confronted with experimental data. Some of the most recent analyses have obtained surprisingly large values for the nucleon width parameter, exceeding in over 50 % the current value for the proton charged radius. This motivates the development of a better understanding of the role played by this parameter inside the simulation. In this work, we perform simulations of Pb-Pb collisions at $\sqrt{s_{\text{NN}}}$ = 2.76 TeV using a state-of-the-art hybrid simulation chain, using three different values of the nucleon width inside the initial condition generator T$_{\text{R}}$ENTo, and systematically investigate its effects on the initial condition characteristics and observables. The nucleon-width strongly affects the eccentricity harmonics and the gradients in the initial condition. The $\langle p_{T} \rangle$ of particles in the simulation using $w$ = 0.5 fm is much larger than experimental data. We associate this to the combination of stronger gradients in the initial condition and the coupling of a conformal pre-equilibrium dynamics to the hydrodynamic simulation. Our findings suggest that the large values of the nucleon width returned by recent Bayesian Analyses were necessary to lower the mean transverse momentum, by damping the gradients in the initial condition.