Uncertainties in the Estimation of Air Shower Observables from Monte Carlo Simulation of Radio Emission

TL;DR

This study compares two Monte Carlo simulation tools, CoREAS and ZHAireS, to evaluate uncertainties in radio-based air shower measurements, finding discrepancies below 10% for electric fields and about 5% for energy estimates.

Contribution

It provides a systematic comparison of CoREAS and ZHAireS simulations, quantifying uncertainties in key air shower observables due to different simulation packages.

Findings

Discrepancies in electric field predictions are generally below 10%.

Uncertainties in primary energy estimation are below 5%.

Uncertainties in $X_{max}$ are approximately 10 g/cm^2.

Abstract

The detection of extensive air showers (EAS) induced by cosmic rays via radio signals has undergone significant advancements in the last two decades. Numerous ultra-high energy cosmic ray experiments routinely capture radio pulses in the MHz to GHz frequency range emitted by EAS. The Monte Carlo simulation of these radio pulses is crucial to enable an accurate reconstruction of the primary cosmic ray energy and to infer the composition of the primary particles. In this work, a comprehensive comparison of the predicted electric field in EAS simulated with CoREAS and ZHAireS was conducted to estimate the systematic uncertainties arising from the use of different simulation packages in the determination of two key shower observables namely, the electromagnetic energy of the EAS and the depth of maximum development ($X_{\rm max}$). For this comparison, input parameters and settings as similar as possible were used in both simulations, along with the same realistic atmospheric refractive index depending on altitude, which is crucial for the prediction of radio emission properties of EAS. In addition, simulated EAS with very similar values of depth of maximum development were selected. Good agreement was found between CoREAS and ZHAireS, with discrepancies in the dominant electric field components generally remaining below 10\% across the frequency range of a few MHz to hundreds of MHz, relevant for most radio detection experiments, translating into uncertainties in the determination of energy below $5\%$ and $\simeq 10\,\mathrm{g/cm^2}$ in $X_{\rm max}$. Our work underscores the need for further studies to clarify their origin and impact on $X_{\rm max}$ inference in composition analyses.