OBSERVATION OF ULTRA-HIGH-ENERGY GAMMA RAYS AND SEARCH FOR DARK MATTER SIGNATURES IN THE GALACTIC CENTER WITH THE HAWC OBSERVATORY

Abstract

The center of our Galaxy is an intriguing region in astrophysics, providing a unique opportunity to study various astrophysical processes. However, our line of sight is obscured by dense layers of dust and gas, making optical emission observation impossible. Fortunately, we can observe other portions of the electromagnetic spectrum, such as radio, X-rays, and gamma rays. The latter serve as probes of cosmic-ray acceleration to extremely high energies and, in theory, for indirect \ac{DM} detection. This dissertation analyzes gamma-ray data from the \ac{GC}, obtained with the \ac{HAWC} Gamma-Ray Observatory. The \ac{HAWC} Observatory, situated on the Sierra Negra volcano in Mexico at an altitude of 4,100 m, detects gamma rays with energies from 0.1 to greater than 100 TeV. It has a wide field of view of about 2 sr, and with an operational duty cycle of over 95%, it observes two-thirds of the sky daily. The observatory uses the water-Cherenkov detection technique to detect Cherenkov emission from secondary air-shower particles. This study presents improvements to the reconstruction algorithms that retrieve the properties of the primary gamma rays. In particular, the angular resolution and background rejection have improved by a factor of four for gamma-ray events coming from high-zenith angles and with energies above 40 TeV. Thanks to these improvements, \ac{HAWC} can detect the \ac{GC}. Although multiple sources of cosmic-ray acceleration to PeV energies, known as PeVatrons, have been proposed within the Galaxy, they remain insufficient to fully account for the observed flux. In this context, the \ac{GC} was proposed as one of the main Galactic PeVatrons with a power-law spectrum that extends up to 50 TeV without a cutoff. Here, we present, for the first time, observations of gamma rays with energies above 100 TeV, which we suggest originated from PeV protons---accelerated in the \ac{GC}---that interact with the dense ambient gas. While the angular resolution of \ac{HAWC} at this high zenith angle is not enough to distinguish the specific PeVatron source, we provide the first confirmation of its existence. The \ac{GC} is also one of the most promising candidates for indirectly detecting \ac{DM} via gamma rays. While the nature of \ac{DM} remains a fundamental question in modern physics, \acp{WIMP} are considered potential candidates for \ac{DM} and naturally emerge in several extensions of the \ac{SM} of particle physics. The relic density of thermally produced \acp{WIMP} in the early Universe can account for all the \ac{DM} observed in the Universe, as measured from cosmological observations. In theory, \acp{WIMP} self-annihilate in dense astrophysical environments, like the \ac{GC}, producing gamma rays in the final state from processes such as hadronization, radiation, and decay of \ac{SM} particles. We conduct a follow-up study of the above analysis and compare the spatial and spectral morphology of the residual gamma-ray emission to the one theorized for \ac{DM} annihilation. Finding no significant emission, we place for the first time to date \acp{UL} at the 95% \ac{CL} on the velocity-weighted cross section, for \ac{DM} particles with masses well above 70 TeV using \ac{GC} gamma-ray data.