Indirect Searches for Physics Beyond the Standard Model

Abstract

Despite the overwhelming success of the Standard Model of particle physics, there are several outstanding issues, perhaps most notably the lack of a dark matter candidate. In this thesis, we develop novel astrophysical probes to search for evidence of a few of these candidates.

In the first part of this thesis, we make use of the 20-year dataset of the XMM-Newton telescope to search for evidence of sterile neutrino dark matter decays. Sterile neutrino dark matter can decay to an active neutrino and a photon, the latter of which would appear in X-ray datasets if the sterile neutrino mass is on the keV-scale. We first show that the previously-observed 3.5 keV line does not originate from dark matter decay. We then constrain dark matter decays across the 5-16 keV mass range, setting the strongest limits to-date up to 12 keV in sterile neutrino mass.

In the second part of this thesis, we detail several searches for low-mass axion-like particles, although they do not have to be dark matter. We spend the majority of this section looking for evidence of excess hard X-ray emission in nearby magnetic neutron stars and white dwarfs. When an axion is produced inside the core of a compact object, it will free-stream out of the star and may convert into an X-ray photon in the magnetosphere of the star. This process will lead to an approximately thermal X-ray signature at the temperature of the star's core. Intriguingly, we find an excess roughly consistent with that expected from an axion in a set of nearby neutron stars known as the Magnificent Seven. However, the lack of such a signal in a nearby white dwarf somewhat disfavors the axion interpretation of that excess. We also search for axions at super star clusters, and through polarization signals at magnetic white dwarfs, and we use NS cooling observations to constrain the QCD axion mass.

Finally, we investigate macroscopic dark matter. We show that macroscopic dark matter can ignite fusion in the cores of red giant stars, leading to a distinct signal at globular clusters. We search for this signal at the globular cluster M15 to set a novel constraint on macroscopic dark matter.