Systematic uncertainties in radio emission of extensive air showers: A comparison of CoREAS and ZHAireS

Radio detection of extensive air showers induced by ultra-high energy cosmic rays has significantly advanced in recent decades. Observables such as shower energy and depth of maximum development are largely derived by comparing experimental data with Monte Carlo simulations, making it essential to assess the systematic uncertainties associated with the simulation models. In this work, we present a comprehensive comparison between CoREAS and ZHAireS, the two most widely used simulation codes for modeling radio emission from air showers. We simulated showers initiated by different primary particles, with various arrival directions and geomagnetic conditions, ensuring similar input parameters and the same altitude-dependent atmospheric refractive index in both cases. We find a good agreement between CoREAS and ZHAireS, with differences in the electric field components, across the MHz to hundreds of MHz frequency range, that are typically below $5\%$ in the Cherenkov cone, the region where the electric field is stronger, and below $10\%$ elsewhere. These differences result in uncertainties of $\simeq 10\%$ on the energy released in the form of radio waves, and $\simeq 10 \mathrm{g/cm^2}$ in the depth of shower maximum reconstruction due to the differences between the models.