Searching for axionlike particles with gamma-ray observations of blazars

Thesis

Many theories beyond the Standard Model of particle physics predict the existence of axionlike particles (ALPs) that mix with photons in the presence of a magnetic field. Searching for the effects of ALP-photon mixing in astrophysical gamma-ray observations of blazars—jetted active galactic nuclei with their jets pointed towards us—has provided some of the strongest constraints on ALP parameter space so far. The focus of this thesis is a type of search where the magnetic field of the blazar jet itself, as opposed to the various other mixing environments usually used, is the dominant region for ALP-photon mixing. I show that mixing in jets can be important for an un-probed region of ALP parameter space at high masses (close to the ALP dark matter space), making them promising mixing environments for an actual search. In order to perform such a search, however, the modelling of the jet field structure, and the effects of photon-photon dispersion within the jet, must be understood. I develop a framework to investigate and address these challenges. This framework is then used to perform an ALP search. In previous searches, only individual sources have been analysed. I perform a search with a combined analysis of Fermi Large Area Telescope data of three bright, flaring flat-spectrum radio quasars. For the first time, I include a full treatment of photon-photon dispersion within the jet, and account for the uncertainty in the B-field model by leaving the field strength free in the fitting. Overall, I find no evidence for ALPs, but am able to exclude ALP masses, m ≲ 200 neV, and ALP-photon couplings, g ≳ 5 × 10^−12 GeV^−1, with 95% confidence. This is an improvement on previous gamma-ray constraints in this mass range. Finally, I develop a proof-of-concept for an ALP search based on population-level observations of blazars, as opposed to individual source spectra. From simulated number counts with and without ALPs, I show that this method could potentially allow couplings down to g ∼ 1 × 10^−12 GeV^−1 to be probed for masses m ∼ 10 neV.

Authors: Davies, J

• DOI: 10.5287/ora-de6bjyzjw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Axions; Blazars; Gamma rays; Axionlike particles; Beyond the Standard Model
• Subjects: Gamma-ray astronomy; Astroparticle physics; Astrophysics; High-energy astrophysics