Constraining Dark Matter Annihilation through High-Energy Gamma-Ray Line Searches in the Galactic Center

The nature and origin of dark matter (DM) remain among the most compelling open questions in modern physics. This thesis presents a search for mono-energetic gamma-ray signatures from dark matter annihilation in the Galactic Center (GC), using data from the Large-Sized Telescope (LST-1) located at La Palma, Spain. Approximately 27% of the Universe’s mass-energy content is believed to be DM. Indirect detection via gamma-ray observations offers a complementary approach to direct and collider searches, with GC offering an optimal setting for investigating DM. This work analyzes five years of LST-1 observations at large zenith angles, maximizing sensitivity to high-mass DM using the enhanced effective area for photons above 500 GeV. This occurs because, at larger zenith angles, gamma-ray-induced air showers travel through a longer atmospheric path, resulting in a larger light pool on the ground that effectively increases the telescope’s collection area. Special attention is given to systematic uncertainties arising from energy threshold and reconstruction effects, as well as the analysis approach. The developed pipeline reconstructs physical parameters from the telescope data, enabling a robust search for sharp, monochromatic spectral lines characteristic of WIMP annihilation. The analysis with LST-1 data from the GC region shows no significant excess above the background. Consequently, 95% confidence level (C:L:) upper limits on the annihilation cross-section for dark matter masses between 1 and 100 TeV are derived assuming annihilation exclusively into photon pairs. The results demonstrate the capability of LST-1 for future DM searches, showing great potential and significant improvement in the current best limits when projected for similar exposure. The thesis discusses the implications of these findings within the broader context of indirect detection efforts and outlines the prospects for the upcoming CTAO observatory, CTA+ project, and continued research at TeV scales. It outlines the possibility of multi-instrument studies using MAGIC data in the near future. During my PhD, along with the LST-1 analysis, I also contributed to the development of high-level processed MAGIC data observations from the GC region, to bring the fles into a more commonly used fle format, to be analyzed using the official pipeline Gammapy. This work gives the possibility to perform a joint analysis of LST-1 and MAGIC data, and extend the possibility of multi-wavelength analysis at other energy ranges, combining with observations of different instruments. The development of this algorithm provides the opportunity to include MAGIC data in the wider framework of multi-wavelength and multi-messenger studies and builds the basis for the next generation studies by CTAO. The thesis is organized as follows: Chapter 1 gives a brief introduction to the thesis. Chapter 2 introduces the quest for the DM by discussing some of the astrophysical evidence of its existence, while listing out various detection techniques and possible candidates. Chapter 3 describes the Cherenkov telescopes studying gamma-ray astronomy, with a focus on LST-1 and its observations of GC. Chapter 4 explains the data observations and processing of LST-1. Chapter 5 demonstrates the DM line-search, presenting the first WIMP DM search results from the LST-1 collaboration, and highlights our contribution to the study of systematic uncertainties. The thesis concludes with the discussion of results in Chapter 6 and conclusions in Chapter 7.