Extensive Air Showers at Large Zenith Angles

Abstract

The recent discovery by the LHAASO experiment of gamma-ray emission in the PeV energy range indicates the existence of galactic sources capable of accelerating particles to multi-PeV energies, motivating further exploration of gamma-ray sources above 100 TeV with Imaging Air Cherenkov Telescopes (IACTs). With increasing gamma-ray energy, the sensitivity of IACTs mainly depends on the collection area Aeff . Despite achieving large collection areas, even up to 1 km² , the expected photon rate above 100 TeV from the Crab Nebula is less than one photon per 100 hours, highlighting the challenges in detecting such high energy phenomena. This thesis explores the possibilities of observing extensive air showers (EAS) at large zenith angles (> 70 deg ) with IACTs, in order to significantly increase the collection area compared to moderate zenith angle observations. In particular, the performance of H.E.S.S. CT5 is investigated. The study focuses on observations at a zenith angle of 80 deg , simulating gamma-ray, proton, and helium induced air showers using CORSIKA and sim telarray. The derived image parameters serve as inputs for evaluating the gamma-hadron separation power, angular resolution, and energy resolution at a zenith angle of 80 deg using dedicated Random Forest Classifier and Random Forest Regressor techniques. The investigation showcases the impressive performance of H.E.S.S. CT5, revealing an collection area of Aeff ≥ 5 - 6 km² at 1 PeV after gamma-hadron separation and direction cuts when operating as a stand-alone telescope at a zenith angle of 80 deg. It was found that H.E.S.S. CT5 achieves a quality factor Q on the order of Q ≥ 5 at a zenith angle of 80 deg. The angular resolution is estimated at theta_68% = 0.12 deg for energies > 10 TeV, improving to theta_68% < 0.1 deg for energies > 100 TeV. Moreover, the energy resolution proves to be better than 18% at energies > 10 TeV. Monte Carlo simulations, such as CORSIKA, present a notable drawback in terms of increased computation time and storage size as the primary particle energy increases.This thesis introduces a 3.5-dimensional simulation of extended air showers and their subsequent emission of fluorescence and Cherenkov light. Utilizing parametrizations for electron-positron distributions, the simulation tool, termed EASpy, adopts a novel geometrical approach to determine the number of detected photons by an IACT. This approach significantly reduces computation time compared to traditional ray-tracing methods. Furthermore, EASpy can simulate the detector response, including the imaging of the simulated air shower.