⁠Searches for Axion-Like Particles with Very High Energy Gamma Rays

We introduce the concept of axions and axion-like particles (ALPs), considering them as promising candidates for Dark Matter (DM) due to their origin in symmetry breaking. The discussion focuses on the interaction of ALPs with photons, particularly the phenomenon of photon-ALP conversion and the calculation of photon survival probability. Photon survival probability is a quantification of photon-ALP conversion, occurring due to their mixing with photons in a two-photon vertex, and resulting in the conversion of photons to an ALP and vice versa in external magnetic fields that are playing mediators for the mixing. As it is mentioned, it is calculated taking into account magnetic fields in the line of sight, as well as the interaction with the Extragalactic Background Light (EBL) in the extragalactic space, and the Cosmic Microwave Background (CMB). As an overview of the state of the art of searches for axion, experiments for ALP detection are categorized based on the type of ALPs they investigate together with the examples and corresponding limits in the ALPs parameter space. Emphasis is placed on gamma-ray astronomy, using Imaging Atmospherich Cherenkov Telescopes (IACTs) MAGIC and LST to analyze very-high-energy gamma-ray data. The production of gamma rays and their detection, together with a discussion of some principal concepts and the data analysis are explained. For the analysis of very-high-energy (VHE) data and reconstruction of the source spectrum, we used gammapy, an open-source package for gamma-ray astronomy. As mentioned before, ALPs are interacting with two photons, oscillating back and forth in the external magnetic field. In case the conversion occurs, propagation of gamma rays is affected, observable through distinctive signatures in the gamma-ray spectrum of the astrophysical sources reproduced from the IACTs data. ALPs in particular can leave alterations on the observed spectrum in different parts of it, depending on the point at which the photon-ALP conversion occurred, and the energy of the gamma-ray (photon). The central study of this thesis explores the impact of ALPs on gamma-ray spectra, providing a detailed analysis of the NGC~1275 located in the center of the Perseus galaxy cluster (GC), at the redshift z=0.0175. Our dataset includes a strong flare, post-flare and a low activity state of the NGC1275, combined in a dataset of 41 hours of observations. Methodology involves modelling magnetic fields, considering environmental factors, and using statistical methods to evaluate ALP signatures in the spectrum. To evaluate the hypothesis of the existence of axion-like particles, we modelled several magnetic fields in the line of sight: the magnetic field of the Perseus GC, the intergalactic magnetic field, the attenuation due to the interaction with the photons of the optical–infrared background light EBL, and ultimately the magnetic field of the Milky Way , in which the conversion of ALPs back to photons is expected, and hence needs to be considered. While no conclusive evidence for ALPs is found, the study establishes the most stringent limits on ALP models in the range of 40-90 neV, reaching the photon-axion coupling down to g_ay= 3.0*10^(-12) GeV^(-1). In the final chapter, preliminary results from studying blazars, particularly Markarian 421 , using LST data are discussed, along with challenges in modelling of magnetic fields combining constraints on ALPs from different sources are highlighted. Our results suggest importance of combination of data from different blazar sources, as well as careful investigation of magnetic fields in the line of sight, in particular the magnetic field of the blazar jet. We conclude by outlining prospects for future studies of ALPs with the VHE gamma-ray data.