Cosmic ray shower rate variations detected by the ARGO-YBJ experiment during thunderstorms

The ARGO-YBJ detector, located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China), was a "full coverage" air shower array. The high altitude location and the frequent occurrence of thunderstorms, made ARGO-YBJ suitable to study the effects of atmospheric electric fields (AEF) on secondary cosmic rays. By analyzing the data of the ARGO-YBJ detector recorded during thunderstorms, significant variations of the rate of detected showers have been observed. During 20 thunderstorm episodes in 2012, the variations of the shower rates (both increases and decreases of amplitudes up to a few percent) are found to be correlated to the intensity and polarity of the AEF, and strongly dependent on the primary zenith angle. To understand the observed behavior, Monte Carlo simulations have been performed with CORSIKA and G4argo (a code based on GEANT4). We found that the data are well consistent with simulations, assuming the presence of a uniform electric field in a layer of thickness of 500 m in the atmosphere above the observation level. Due to the AEF accelerates/decelerates and deflects the secondary charged particles (mainly electrons and positrons) according to their charge, modifying the number and position of particles with energy exceeding the detector threshold. For the differences in electron and positron flux, spectrum, and lateral distribution, the AEF has an asymmetric effect on the shower particles, producing significant variations of the particle pattern on the ground, and, consequently, on the rate of detected showers, consistent with observations.