Reconstruction of very inclined air showers with radio data and application to the first GRANDProto300 cosmic-ray candidates
Radio detection of very inclined extensive air showers (EAS) offers a promising way to study ultra-high-energy cosmic rays (UHECRs) and neutrinos above $10^{17}$ eV. We present a method to reconstruct the arrival direction and energy of EAS with zenith angles above $60^\circ$, applicable in principle to upward-going neutrino-induced showers. The approach combines a point-source-like description of the radio wavefront with the Angular Distribution Function (ADF), a phenomenological model describing the angular pattern of electric field amplitudes in the 50-200 MHz band. On realistic cosmic-ray simulations, we achieve better than $0.07^\circ$ resolution on arrival direction and ~15\% on primary energy. Applied directly to voltage traces, this method allows efficient cosmic-ray selection with accurate direction reconstruction and first-order energy estimation. We illustrate the method on the first cosmic-ray candidates detected by the GRANDProto300 prototype of GRAND, validating its detection principle and paving the way for high-precision neutrino searches.