Research: dark matter

Weakly Interacting Massive Particles (WIMPs) and axions are two of the best-motivated particle dark matter candidates. WIMPs are generically produced in the early Universe with roughly the required density and supersymmetry provides us with a concrete, well-motivated candidate in the form of the lightest neutralino. Axions arise from the Peccei-Quinn mechanism, proposed to solve the strong CP problem (namely understanding why the electric dipole moment of the neutron is so small). WIMPs and axions can both be detected directly in the lab. WIMPs can elastic scatter off nuclei in dedicated detectors, while axions can be converted into photons in a resonant magnetic cavity.

I have worked extensively on the signals (energy dependence, annual modulation, and direction dependence) expected in direct detection experiments. With Stefan Hofmann and Dominik Schwarz I studied the micro-physics of WIMPs in the early Universe and calculated the properties of the first, Earth mass, microhalos to form. With former PhD student (now at IFCA, Santander) Bradley Kavanagh, I worked on parameterising the WIMP speed distribution to obtain an unbiased measurement of the WIMP mass from future direct detection experiments. With former Leverhulme trust-funded postdoc, Mattia Fornasa, I worked on building more realistic self-consistent models of the Milky Way. Another former PhD student (currently an ARC DECRA fellow at the University of Sydney), Ciaran O’Hare, looked at how directional detection experiments can probe features in the speed distribution and also distinguish WIMPs from neutrino backgrounds. We also looked at how a future high resolution ADMX-like experiment could measure the axion velocity distribution, including streams from mini clusters.

The discovery by LIGO/Virgo of gravitational waves from mergers of multi-solar mass black holes has generated a lot of interest in Primordial Black Holes (PBHs) as a dark matter candidate. In 2020 I wrote a review on this topic with Bradley Kavanagh. I've worked extensively on the observational constraints on PBHs, especially from microlensing observations (see Publications: primordial black holes). In particular, I showed that the constraints are tighter for the expected extended mass functions than for the delta-function mass function that is traditionally assumed.

I've written some lecture notes based on the introductory lectures on 'Dark matter in Cosmology/Astrophysics' that I gave at the 2021 Les Houches Summer School on dark matter.