For 50 years, cosmic-ray air showers have been detected by their radio emission. We present the first laboratory measurements that validate electrodynamics simulations used in air shower modeling. An experiment at SLAC provides a beam test of radio-frequency (rf) radiation from charged particle cascades in the presence of a magnetic field, a model system of a cosmic-ray air shower. This experiment provides a suite of controlled laboratory measurements to compare to particle-level simulations of rf emission, which are relied upon in ultrahigh-energy cosmic-ray air shower detection. We compare simulations to data for intensity, linearity with magnetic field, angular distribution, polarization, and spectral content. In particular, we confirm modern predictions that the magnetically induced emission in a dielectric forms a cone that peaks at the Cherenkov angle and show that the simulations reproduce the data within systematic uncertainties.

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The SLAC T-510 experiment was designed to verify established microscopic models for simulation of radio emission from air-showers by reproducing the physics under controlled lab conditions. For this verification, the simulation toolkit Geant4 was expanded by the calculation of the emitted radio signal with the "endpoint" and the "ZHS" formalisms in parallel. We present and compare the results of the two simulation approaches, taking into account the details of the experimental set-up such as the beam energy, target geometry and material, and the magnetic field configuration. We put special emphasis on the discussion of the effects due to refraction and transition radiation and show a comparison of the simulation results with the measured data of the SLAC T-510 Experiment.

We report on the first observations of radio emission from transverse currents induced in a secondary cascade by a magnetic field in dense media. Only the Askaryan effect has been observed in previous experiments in ice, silica sand and in rock salt. These measurements are important because the radio detection of ultra-high energy cosmic rays (UHECRs) relies on models of radio emission from extensive air showers. In order to validate the models we performed an experiment at SLAC National Accelerator Laboratory in 2014 using a high density polyethylene (HDPE) target placed in a magnetic field, yielding results within 36% of the model predictions.

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