Impact of Multiplicity Fluctuations on Entropy Scaling Across System Size

TL;DR

This paper examines how different models of initial state fluctuations influence entropy scaling and eccentricity calculations in heavy-ion collisions, highlighting implications for viscosity extraction.

Contribution

It compares Bayesian-constrained Trento model fluctuations with CGC-inspired models, revealing significant effects on multiplicity and eccentricity predictions.

Findings

Multiplicity distributions are significantly affected by fluctuation assumptions.

Eccentricity calculations vary notably between models.

Implications for viscosity extraction in small systems are highlighted.

Abstract

The initial state is one of the greatest uncertainties in heavy-ion collisions. A model-agnostic approach is taken in the phenomenological Trento framework which constrains parameters using Bayesian analysis. However, the color-glass condensate (CGC) effective theory predicts initial energy densities that lie outside the recent Bayesian analyses due, in part, to the assumption in Trento of event-by-event multiplicity fluctuations following a $\Gamma$ distribution. We compare the Trento-preferred $\sqrt{T_{A}T_{B}}$ scaling to CGC-like $T_{A}T_{B}$ scaling coupled with log-normal fluctuations in $AuAu$ and $dAu$ collisions and find there is a significant impact on the multiplicity distributions and on the eccentricities, which may affect the extraction of viscosity in small systems.