Research

My papers can be found on Inspires.

I am interested in a range of topics all broadly in the area of Beyond the Standard Model physics, and particularly Dark Matter. There are many promising candidates for Dark Matter that have not been fully explored, but could soon be discovered! Some examples of things I've worked on so far are discussed below, and the University of Liverpool has a collaboration between theorists and experimentalists on Dark Matter and Dark Energy. 

Axions

Both the QCD axion, and other light axion-like-particles are excellent candidates to be dark matter (DM). Not only are they well motivated from string theory, but their dynamics in the early universe automatically produces a significant relic abundance, which behaves like cold dark matter and can easily match the observed density without having to add any extra symmetries by hand to prevent them decaying. The case of the QCD axion is even more interesting, since this is introduced for a totally unrelated purpose, solving the Standard Model strong CP problem.

The complex dynamics of axions make many aspects of their phenomenology challenging to study, but also extremely theoretically interesting. Research in this areas includes aspects of cosmology and astrophysics, strongly coupled gauge theories, string phenomenology, and big numerical simulations, as well as having crucial links to experimental and observational work!

Papers I've written on this topic include an analysis of the properties and cosmology of the QCD axion, in which we used chiral perturbation theory, data from lattice simulations, and finite temperature QFT to obtain precise and reliable results that are important for interpreting experimental data. I'm also interested miniclusters formed of axion-like-particles. These are dense astrophysical objects made out of axion-like-particles, which could provide a way of detecting or constraining large classes of models using astronomical observations.

Left: An example snapshot from a numerical simulation of axion strings. Right: The minimum QCD axion coupling to photons as a function of the axion mass for different models (due to a contribution to this coupling from mixing with the pion, it is extremely rare for models to have a smaller coupling than that shown for E/N=2).

Glueballs

Another possible candidate to be dark matter is a glueball of a hidden sector gauge group that runs into strong coupling. Such hidden sectors are very common in string theory, and in large classes of string compactifications seem to be generic.

The glueballs that naturally appear in these hidden sectors are good candidates for DM, but there's a problem. Unless the temperature of the hidden sector is initially much lower than that of the visible sector, the glueball relic abundance is much too large (although very asymmetric initial temperatures might be possible in some models of reheating after inflation). Luckily there's a solution: string theory models also often have light moduli, and these can decay at relatively late times during the Universe's cosmological history. In a recent paper, I showed that this greatly expands the viable parameter space of glueball DM models.

Constraints on the glueball relic abundance in models with a thermal cosmology. Lambda is the hidden sector confinement scale, and B is the inflaton's branching fraction to the hidden sector. To get the observed relic abundance B must be extremely small, corresponding to a much lower temperature in the hidden sector compared to the visible sector, which seems unusual from a UV perspective. Modifying the cosmological history allows for a hidden sector temperature much closer to that of the visible sector.

Finite temperature QFT and astrophysical constraints on new light particles

Important constraints on new light particles come from observations of stars and other astrophysical objects, such as supernova. The new particles can be produced in the hot cores of these objects, changing the way that energy is transported. In some cases this results in the system cooling down, whereas in others as energy is removed a star contracts and heats up. Either way, if the new particles are produced at a high enough rate, this will lead to significant differences between models and observations.

However, since the new light particles are produced in an environment with high temperature and density it is important to consistently include effects from these in the analysis. For example, Lorentz symmetry is broken by the medium's rest frame, and as a result the photon gets a longitudinal degree of freedom. In a recent paper I showed that this can be resonantly converted to a new light vector boson, or scalar, dramatically increasing the production rate, and as a result strengthening the constraints on the couplings of new particles to the visible sector by more than an order of magnitude.

Left: Examples of the Feynman diagrams that need to be included when calculating the production rate of new particles inside a star. Mixing between the medium and the new particles can lead to much more efficient production, compared to if these effects were neglected. Right: Constraints on the mixing sin theta of a new light scalar with the Standard Model Higgs from observations of red giant (RG) and horizontal branch (HB) stars. By including resonance effects, we find that a mixings as small as 10^-9 are ruled out.

New Approaches to the hierarchy problem

I am also interested in new solutions to the hierarchy problem, especially given the lack of a hint of new physics from the LHC. Some of these include new axion-like-particles, and often rely on dynamics in the early universe and finite temperature effects (for example, a model I proposed in this paper)

Heterotic String theory and its phenomenology

On a more formal side, I'm interested in heterotic string theory and in particular the moduli that appear when the extra spatial dimensions are compactified. These are new light scalars, which must obtain masses in order to be consistent with observations from 5th force experiments, and successful predictions from big bang nucleosynthesis. The number and properties of the moduli from a compactification are mathematically interesting, and phenomenologically important.

Computer code and algorithms

I've developed a range of small and not so small numerical codes, which at some point I will make-available in open source format.