Theoretical Studies of Particles & Strings

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Our ultimate aim is to understand the nature of matter & forces, and the content & evolution of the universe as a whole. One of the most intriguing features of reality is what the physicist Eugene Wigner called "the unreasonable effectiveness of mathematics in the natural sciences" in being able to provide an accurate & predictive description of observed phenomena. As theoretical physicists we exploit this by motivating and constructing mathematical models of physical systems, for which we also devise experimental tests whenever possible - as these are ultimately the arbiter of truth.

Our focus is on the fundamental constituents of matter - particles and the hypothetical 'strings' that may underlie them. To create and study elementary particles requires accelerators like the Large Hadron Collider at CERN. We also make use of natural particle beams like the high energy neutrinos generated in extreme astrophysical environments that are studied by the IceCube experiment at the South Pole. Our research addresses most of the current issues in particle & string theory. A new direction in this proposal is devising novel laboratory experiments using quantum sensors to look for subtle effects of new physical phenomena.

The first part of our proposal is to improve predictions of the effects of the strong interactions 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 very 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. We will employ sophisticated mathematical techniques to calculate the 'amplitudes' which yield rates for complex processes.

The Standard Model (SM) of the strong, weak & electromagnetic interactions describes experimenal data with exquisite precision but it is incomplete. It does not explain why the weak interaction scale is relatively low or why there are 3 generations of quarks & leptons, and it does not contain gravity. Moreover it does not explain why there is matter but no antimatter in the universe and what is the dark matter. Our research explores models that might remedy these defects and determine what experimental tests would be sensitive to the new 'beyond the Standard Model' physics they predict.

The LHC also collides heavy ions producing a quark-gluon plasma which reaches thermal equilibrium quickly and affects the behaviour of energetic particles passing through it. To understand the properties of this strongly coupled fluid we will combine the gauge-string duality technique with other techniques. There are intriguing connections to the thermodynamics of black holes which we will explore.

Extensions of the Standard Model often predict new particles that cannot be detected directly at the LHC but which can affect astrophysical & cosmological processes. An example is the axion-like-particle which arises naturally in most string theories and whose presence may be seen in astrophysical signals. An important part of our proposal is to establish in detail the signatures of such particles and devise new strategies using novel quantum sensors for detecting them in 'table top' experiments as well. We will also use high energy cosmic neutrinos to probe new phenomena well beyond the energies achievable at the LHC.

String theories are a promising candidate for unifying the SM with gravity. However generally they contain many states not observed in Nature; we will continue our programme to find those consistent with the SM and calculate the 'Yukawa couplings' that correspond to observed particle masses. We will explore mathematical models such as 'causal dynamical triangulations' to see if these can capture the physics of quantum gravity.

Planned Impact

As well as the significant academic impact, described in the Academic Beneficiaries section above, this programme also delivers demonstrable contributions to the economy and society.

Our work to find tiny signals in large, complex datasets, such as those streaming from the Large Hadron Collider, has many potential applications - for example, development of improved methods in data analysis is of benefit to industries using predictive analytics or searching for equally tiny signals in complex data. Artificial intelligence and its application to machine learning have been recognised as key fields for driving future economic growth. Our QCD and Strings research areas will adopt and refine machine learning methods, creating novel datasets that can be used to inform development of the technique, and training people with the skills needed by AI industry.

The investigator team's involvement in STFC & EPSRC's Quantum Sensors for Fundamental Physics programme, backed by a dedicated core post to support the UK-wide quantum science community, ensures outcomes from our theoretical programme will inform that £40M strategic UK investment. This provides benefits not only at the cutting edge of academic research but, further downstream, to the development of potential spin offs into industrial and societal applications, such as ground sensing.

Key outputs of this theoretical programme are the highly-skilled early career researchers developed within the supportive and synergetic environment our cross-departmental approach to problem solving creates. Whilst many of our PDRAs and graduate students will strengthen academia and physics-related industries as their careers develop, the UK's wider future workforce also requires critical and analytical thinkers, particularly to retain UK strength in areas of economic importance such as data analytics, software development and finance. Our excellent track record of training graduates and PDRAs who are highly sought after by such companies demonstrates a valuable strand of economic impact for our programme.

Strengthening the UK's international connections is becoming ever more important for a stable economic future. Oxford's Theoretical Particle Physics programme has proven to be of ambassadorial impact for the UK, with initial scientific connections leading to wider collaborative activities in other areas - creation of the India-Oxford programme through the efforts of Co-I Sarkar being a particular example. Such connections facilitate inward investment into the UK and create opportunities for cultural and political exchange. Our internationally-leading investigator team will continue to leverage such connections for societal and economic benefit - both for the UK and the partnering nations.

A detailed engagement programme, inspired by our five research areas, will enthuse young people and the public in the value of science as a cultural endeavour. Through targeted events, exhibitions, interactive and online materials, we will not only enhance understanding of the beauty of fundamental physics and its power to describe the world around us, but also demonstrate its economic importance to areas such as high-performance computing and instrument design. Elements of the engagement programme, targeted particularly at groups under-represented in scientific careers and those less likely to progress to university level study, aim to improve inclusion and social mobility. Other activities, working with science teachers, will ensure that classroom teaching benefits from the inspirational value of our cutting-edge research.

Investigator and PDRA engagement with senior policy and decision makers, brokered through opportunities such as the Royal Society Pairing Scheme and STEP for Britain, will ensure that the importance of STFC's long term funding of fundamental research to the health of the nation is kept at the forefront of funding decisions.


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Title Two-body cuts 
Description Code that implements a variety of cuts for selecting two-body kinematics at hadron colliders, including the cuts from arXiv:2106.08329. Also to be found at 
Type Of Technology Software 
Year Produced 2021 
Open Source License? Yes  
Impact This code offers an approach to the analysis of certain collider data that will make it significantly easier to interpret the results of the analysis within the framework of the Standard Model and beyond. 
Description Live webcast with IceCube `winterovers' at South Pole 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact In this event titled "90 degrees south: detecting invisible messengers of the cosmos" we invited school students and our alumni to "Join us on a journey to the South Pole" to learn about the IceCube Neutrino Observatory. This extraordinary `telescope' uses the Antarctic icecap to see neutrinos from outer space and an international collaboration of over 300 physicists from 53 institutions in 12 countries use IceCube's findings to further our understanding of the Universe.

Head of Particle Physics at Oxford Professor Daniela Bortoletto and Professor Subir Sarkar, who leads on the IceCube experiment in Oxford, linked up live with Josh Veitch-Michaelis and Martin Wolf, IceCube's current 'winterovers' - the two people who stay at the South Pole to maintain IceCube during the winter months when it isn't possible to leave. They were joined by Dr Jim Madsen, Executive Director Wisconsin IceCube Particle Astrophysics Center and Gwenhaël de Wasseige, MSCA Individual Fellow, Laboratoire Astroparticule et Cosmologie, to share their work and talk all things neutrino.
Year(s) Of Engagement Activity 2021
Description Oxford University Physics Society: Beyond the cosmological standard model 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Undergraduate students
Results and Impact Abstract: The 'standard model' of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present - as was inferred from the Hubble diagram of Type Ia supernovae. There now exists a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these 'standard(isable) candles' indeed indicate cosmic acceleration. We find, rather surprisingly, that the inferred acceleration is mainly along the direction we seem to be moving with respect to the cosmic microwave background. It cannot therefore be due to the Cosmological Constant (aka vacuum energy), even though a variety of observations are supposed to have meanwhile confirmed its presence. This is an exciting time to be a cosmologist!

There was a lively Q&A session and the YouTube video has had over 2.5k views.
Year(s) Of Engagement Activity 2020