Impact of Geometric Inflation on Nucleon Size Sensitivity in Relativistic Heavy-Ion Collisions

TL;DR

This paper shows that correcting for geometric inflation in initial-state models of heavy-ion collisions alters the sensitivity of key observables to nucleon size, impacting the extraction of nuclear structure and QGP properties.

Contribution

The study introduces a self-consistent density correction to remove geometric inflation, revealing its significant effect on observable sensitivities in heavy-ion collision simulations.

Findings

Removing geometric inflation reduces sensitivity of elliptic flow and mean pT to nucleon size.

Enhanced sensitivity of flow fluctuations and correlations to nucleon position fluctuations.

Bias in nucleon structure and QGP property extraction if geometric inflation is uncorrected.

Abstract

The intrinsic transverse size of nucleons, parameterized by a Gaussian width $w$, is a critical yet uncertain input in the initial-state modeling of relativistic heavy-ion collisions. Using a finite $w$ in standard initial geometry models introduces an unintentional ``geometric inflation'' that alters the initial nuclear density profile. In this study, we implement a self-consistent density correction to eliminate this artifact and investigate its impact on final-state observables. Through hybrid (viscous hydrodynamics + hadronic transport) simulations of $^{208}$Pb+$^{208}$Pb collisions at the LHC, we demonstrate that removing geometric inflation significantly modifies the sensitivity of observables to the nucleon width $w$. While elliptic flow and mean transverse momentum ($\langle [p_{\rm T}]\rangle$) become less sensitive to variations in $w$, the Pearson correlation coefficient $\rho(v_{n}^{2}, \delta p_{\rm T})$, $[p_{\rm T}]$ fluctuations, and triangular flow exhibit enhanced sensitivity to fluctuations in nucleon positions. Our results indicate that uncorrected geometric inflation can bias the extraction of nucleon structure and quark-gluon plasma properties. This underscores the necessity of a self-consistent initial-state geometry for reliable Bayesian inference in heavy-ion collisions.