Astrophysical searches for New Physics

Some of our most compelling evidence for physics beyond the Standard Model comes from astrophysics – the observations which tell us that 95% of the universe’s energy budget comprises Dark Matter and Dark Energy. Astrophysics gives us access to energy and distance scales that would be impossible to reproduce in a lab, and is therefore an ideal setting to search for new physics. Possible PhD projects will use data from existing and upcoming telescopes to search for the identity of Dark Matter and for other new physics scenarios.

One well motivated Dark Matter candidate is the axion – an ultralight particle that arises in string theory compactifications and as a solution to the strong CP problem. In a background magnetic field, axions and photons can interconvert. This process would produce striking signatures in astrophysical magnetic fields, which may be observed by the next generation of telescopes. PhD projects will analyse the effect of the axion-photon interaction in stars, galaxies and galaxy clusters and aim to develop novel ways of searching for axions. As well as studying axion phenomenology, computer simulation and statistical analysis are a major part of this work. Another powerful probe of new physics is superradiance – the exponential amplification of a bosonic field around a rotating star or black hole. This work is more mathematical in nature. PhD projects will study superradiance in different settings, looking at factors that may prevent it and at how it could be observed.

Gravitational waves and particle physics

The first direct observation of gravitational waves from a binary merger was made by the LIGO/Virgo collaboration in 2015. In the next decade, gravitational wave research will enter the realm of precision science. This is a big opportunity for particle physics, but much still needs to be understood to properly interpret the data. To maximize our discovery potential, new analytic and computational tools have to be developed to translate the experimental findings to information about particles.

Possible PhD projects may include studies of early Universe sources of gravitational waves (as may be observed by the planned LISA experiment), population studies of black holes in light of the black hole mass gap, and multi-messenger studies of supernovas and neutron stars. Gravitational waves hold the potential to unravel many mysteries of our Universe, including the nature of dark matter, the strong CP-problem, and the asymmetry between matter and antimatter. The breadth of this research program means that collaboration with other members of the IPPP, the Physics Department, and members of the LISA consortium is strongly encouraged.

Precise predictions for processes involving the Higgs boson at the LHC

Particle colliders, such as the Large Hadron Collider (LHC) at CERN, provide one of the most tightly controlled environments for the exploration of the fundamental forces of nature. They have played a pivotal role in the discovery of the Standard Model (SM) of particle physics and they continue to guide and constrain current theoretical developments.

The crowning achievement of the LHC physics program to date is the discovery of a Higgs-like boson in 2012, which potentially renders the SM complete and self-consistent. However, this leaves us in a rather unsatisfying situation: on the one hand, we appear to have a complete theory, on the other hand, we are left with major unanswered questions. What is dark matter? How do neutrinos acquire their mass? Perhaps as importantly, we also have no clear explanation of why the fundamental laws of nature are what they are. In fact, we still have much to learn about the experimentally discovered Higgs-like boson: does it couple to itself and other particles precisely as predicted? Does it interact with as-yet undetected particles? Is the Higgs mechanism, through which other particles acquire their mass, realised in nature? Attempting to answer these questions is one of the most promising avenues for searching for new physics.

With no striking signals of physics beyond the Standard Model yet observed at the LHC, we can instead search for subtle discrepancies between theoretical predictions and experimental measurements. These deviations may allow us to infer the existence of new particles or interactions before we see them directly. A key part of the proposed PhD projects is the production of the precise theoretical predictions required for such studies at the LHC and future colliders. The projects will also involve contributing to the development of new analytic and numerical techniques for performing state-of-the-art perturbative calculations, as well as using high-performance computing resources.

Precise predictions for processes involving the Higgs boson at the LHC

Particle physics is entering an exciting era where new observations from neutrino and gravitational wave experiments will provide complementary windows into particles’ nature at the smallest scales and the universe at the earliest times.
However, there remains much to be done in improving the precision of our calculations and the rigour of how we compare our physics models to data.

As the Large Hadron Collider programme has been running for more than 20 years, there have been momentous theoretical physics developments to understand, simulate and analyse particle interactions. Many such calculational tools and softwares can be effectively applied to neutrino experiments and early universe physics.

Projects which harness this non-trivial synergy between collider, neutrino and early universe physics include:

1. Use ProtoDUNE data to implement a fast detector simulation of the DUNE experiment.
Apply collider tools and techniques (such as jet clustering algorithms) to construct an analysis to improve dark matter detection at the DUNE experiment.

2. Apply jet clustering algorithms and jet topologies to improve tau neutrino detection at the IceCube experiment.

3. Use soft resummation to determine the electroweak bubble wall velocity more accurately.

4. Investigate the interplay between non-standard cosmologies, such as a primordial black hole dominated early universe, and theories which explain small neutrino masses and the matter-antimatter asymmetry.

5. Exploit the complementarity between gravitational wave and neutrino data as a means of assessing the viability of Grand Unified Theories.

These projects range from more phenomenological to theoretical.