Theoretical Studies of Elementary Particles

Lead Research Organisation: Royal Holloway, University of London
Department Name: Physics


Our ultimate aim is to understand the nature of matter and the most basic processes that drive the universe. One of the most intriguing features of our universe is that we seem to be able to describe much of what happens very well 'theoretically' - using a a set of laws that are written down in the form of a mathematical 'model'. Isaac Newton showed this could be done with objects visible to the naked eye and modern day theorists continue the tradition for particles that have to be created by extreme events, be they cosmic or terrestrial at accelerators like the Large Hadron Collider (LHC) at CERN. The proposed research will address today's major issues in particle theory.

The first part of our proposal is to improve predictions of the effects of the strong interactions (QCD) that dominate the production of particles in collisions at the LHC. These collisions create showers of large numbers of known particles which have to be filtered out so that the rare events indicating the presence of a new particle become visible. The discovery of the Higgs boson depended on such calculations; the discovery of further new particles, and the efficient analysis of their properties, will depend on doing those calculations to even higher precision.

The Standard Model of the strong, weak and electromagnetic interactions describes most of particle physics with exquisite precision but it has defects. It does not explain dark matter, why the weak interaction scale is relatively light or why there are three generations of quarks and leptons, and it does not contain gravity. Our research will explore models that might remedy these defects and determine what experimental tests would be sensitive to the new 'beyond the Standard Model (BSM)' physics they predict. We will examine supersymmetric field theories, and models with additional space dimensions, to establish whether they contain viable candidates for dark matter without introducing other new physics incompatible with experiment. Superstring theories are a promising candidate for unifying the Standard Model with gravity. However generally they contain an abundance of particles not observed in nature; we will continue our programme to find those consistent with the Standard Model and for the first time calculate the Yukawa couplings that are needed for detailed comparison of light particle masses with experiment.

The LHC is also used to collide heavy ions producing a quark-gluon plasma. Understanding the features of this new state of matter is challenging. It reaches thermal equilibrium very quickly and affects non-trivially the behaviour of energetic particles passing through it. The so-called gauge-string duality technique can attack some of these problems but does not apply strictly to real QCD. We propose to develop a composite description that combines this method with other techniques to understand the properties of the real quark-gluon plasma.

Extensions of the Standard Model usually contain new particles that cannot be detected directly at the LHC but affect astrophysical and cosmological processes such as inflation and the generation of primordial gravitational waves. An example is the so-called ALP which arises naturally in most string theories and whose presence would be seen in astrophysical X-ray data. An important part of our proposal is to establish in detail the signals from such physics so that we can optimize strategies for detecting them in observational data. In contrast quantum gravity is not yet sufficiently well understood to tension theory against measurement so we will develop methods of computing the large scale properties of the universes in models such as causal dynamical triangulations to determine whether they are plausible candidates to describe quantum gravity.

Planned Impact

The research in this proposal, at the cutting edge of theoretical particle physics, gives opportunities for developments producing benefits to a far wider community. We anticipate wide-ranging impact beyond academia, most notably on the general public, schools and students as well as the UK economy.

The fundamental scientific understanding that our research develops has a distinctive role to play in enriching the quality of life: it contributes to a wider public understanding of our universe, what questions may be asked about it and how those questions might be answered. In engaging schoolchildren with these perspectives, it inspires the next generations of scientists, engineers and innovators, and supports their teachers in that mission. For some it will encourage them to study physics further for themselves and by our wide dissemination (e.g. through SEPnet) we aim to increase the number and diversity of students studying physics at a university level.

For public audiences our work illustrates the power of the scientific approach, for example through the ability of the LHC, coupled with analysis such as that we will develop here, to produce evidence of the origins of mass. Through social media we are increasing the capacity for the public to engage directly with research groups at the frontiers of physics.

We train PDRAs and graduate students to think independently and critically, and provide them with a sound scientific training and the ability to solve problems using high level reasoning, analytical and computational skills. Our PDRAs will, as they have done in the past, become the university faculty, leaders of industry and commerce and decision makers of the future. Our outreach activities give these developing researchers opportunities to develop their public engagement and communication skills for whichever spheres they choose to work in thereafter.

Through its necessity to solve challenging problems and analyse large and complex datasets, most particularly those now streaming from the Large Hadron Collider, our work always has the potential to bring economic benefits. For example, the development of improved methods in data analysis could have immediate benefit to those in industry using predictive analytics or working to find equally tiny signals in complex data. Through collaborations with mathematicians, astrophysicists, plasmas physicists and theoreticians in other fields the directions in which our techniques may be taken extend far beyond the realm of particle theory alone.


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March-Russell J (2020) Reproductive freeze-in of self-interacting dark matter in Physical Review D