A search for dark matter in dwarf spheroidal galaxies with VERITAS

Understanding the nature and identity of dark matter (DM) is a key goal in the physics community. As weakly interacting massive particles (WIMPs) interact only through gravity and the weak nuclear force they are popular candidates for the identity of DM. WIMPs are a particularly attractive candidate due to the so called ”WIMP Miracle” which states that the predicted relic abundance of a WIMP DM is the same as the measured abundance of DM measured in the Universe today. Certain DM theories suggest that WIMPs may decay or annihilate into standard model particles with electromagnetic radiation up to very-high-energy (VHE) gamma-rays (greater than 100 GeV) produced in the process. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four imaging atmospheric Cherenkov telescopes (IACTs) that can indirectly detect VHE gamma rays in an energy range of 100 GeV to > 30 TeV, making it an ideal instrument to detect signatures of DM decay or annihilation. Dwarf spheroidal galaxies (dSphs) are chosen as the targets of this search for WIMP DM due to their high dark matter content and their low gamma-ray fluxes from other processes. The goal of this thesis is to use the full VERITAS dSph data set to derive the improved upper limits on the WIMP annihilation cross section to date. The newly implemented VERITAS image template method (ITM) is used to improve gamma-ray sensitivity during the analysis of each dSph. The DM mass range on which the upper limits are placed is extended beyond the unitary limit of a point-like DM particle by assuming above this mass limit DM particles are no longer point-like but instead have a geometrical cross
section. This allows upper limits to be placed on the annihilation cross section of DM masses up to 10s of PeV as well as on the geometrical cross section of composite DM particles above ∼ 100 TeV. This thesis also investigates the benefits of treating dSphs as largely extended sources in order to contain a larger fraction of their DM content. This required an investigation into maximum angular cut possible with the current VERITAS analysis pipeline. In order to complete these analyses a method of deriving the VERITAS point-spread function was required. This was initially performed in the absence of gamma-ray simulations using observations of the Crab Nebula and later with said gamma-ray simulations. The upper limits derived in this thesis are the first results presented using the most state-of-the-art, physically motivated J-Factors. Despite these J-Factors predicting lower DM densities in most dSphs, the limits derived are the most constraining limits from an indirect search in dSphs with the largest DM mass range. The extended-source analysis in this thesis uses the largest angular cut to date on dSphs and shows the benefits of such an analysis.