# Research Topics

### Conformal field theory and quantum gravity

Conformal symmetries and their extensions are a powerful tool in the study of field theories. Logarithmic conformal field theories have been a major activity for many years; at present we are working on the properties of these theories in the presence of boundaries. This is of relevance to systems as diverse as a recoiling D-brane and densely packed polymers. Quantum gravity is studied in two dimensions where it has many equivalent descriptions including, as a conformal field theory and in higher dimensions where conformal structures are used in understanding the foundations of the subject.

### Gauge-string duality, holography, AdS-CFT correspondence

A superstring theory technique known as gauge-string duality or AdS-CFT correspondence allows the study of strongly coupled quantum systems by relating them to classical gravity in higher-dimensional space-times. Gauge-string duality at finite temperature and density connects transport properties of strongly coupled plasmas (viscosity, thermal conductivity, diffusion constants) to excitation spectra of black holes. By computing transport coefficients for these theoretical models, interesting insights are gained into physics of strongly coupled hot and dense nuclear matter created in heavy ion collision experiments, *e.g.* at RHIC and LHC, as well as the physics of cold dense matter.

### String theory/phenomenology

Phenomenological applications of string theory are studied *i.e.* developing string theory realisations of Standard Model and Beyond-the-Standard-Model physics. This includes Standard Model constructions of string theory through compactification on Calabi-Yau manifolds with appropriate gauge bundle backgrounds. These can be automated using powerful techniques from algebraic geometry with an ability to perform computer scans over many possible models. It also involves the study of moduli stabilisation (the fixing of the extra-dimensional geometry) and mechanisms of supersymmetry breaking in string theory. Such studies are carried out both from the perspective of the low energy supergravity theory and also directly on the string worldsheet. We also study applications in cosmology, for example the construction of inflationary potentials in string theory.

### Lattice field theory

Many non-perturbative problems in field theories can only be addressed by computer simulation which involves replacing continuum space-time by a discrete lattice of space-time points. In the past we have addressed many of the outstanding problems in Quantum Chromodynamics (QCD) *e.g.* the masses of glueballs, their fate in the experimental mass spectrum, chiral symmetry breaking, the topological structure of the vacuum and the dynamics of confinement. Over the last decade the large-N behaviour of SU(N) gauge theories has been the main focus, given the simultaneous developments associated with gauge-gravity dualities. More recently we have attempted to learn something about the effective string theory that describes the dynamics of confining flux tubes, which again has coincided with analytic progress in the area. Finally, strongly coupled theories with an infrared conformal fixed point (*i.e.* physics that is very different from QCD) are a topical current interest as this may enable dynamical electro-weak symmetry breaking.

Using lattice techniques the properties of fluctuating random surfaces and of simplicial quantum gravity are studied from a number of points of view. The main aim is to elucidate the geometrical structure of the typical universes in the quantum ensemble. This is done using mainly analytical techniques (*e.g.* rigorous statistical mechanics, series expansions, matrix model calculations) and supplemented by numerical work where necessary.

### Phenomenology of electroweak and strong interactions

F Caola, L Harland-Lang, G Salam*, S Sarkar, L Tancredi,

There is an ongoing programme of research in phenomenology with dual aims of studying the interactions of the Standard Model and improving our ability to detect potential physics beyond the Standard Model.

Much of the group’s work is oriented towards hadron colliders and in particular the Large Hadron Collider (LHC) at CERN. The group has active interests in the majority of the areas of theoretical research that make it possible for such experiments to relate their data to the underlying Lagrangian of particle physics, for example to better elucidate the interactions of the Higgs boson. These interests include the determination of parton-distribution functions, accurate fixed-order calculations in perturbative quantum field theory, resummation of logarithmically enhanced perturbative corrections, effective-field-theory fits, Monte Carlo event generation, the phenomenology of central exclusive production, non-perturbative phenomena at colliders and theoretical work on methods to analyse particle collisions, including machine-learning approaches and jet-physics.

The group also has interests in neutrino interactions at ultra-high energies notably in the context of the IceCube experiment, and in the physics of heavy-ion collisions.

### Physics beyond the Standard Model

J Conlon, J March-Russell*, G Ross+, S Sarkar

A substantial part of our activity is the study of extensions of the Standard Model of the strong, weak and electromagnetic interactions. The possibilities being explored include superstring/M-theory, Grand Unification, supersymmetry and compactified theories involving large/warped new dimensions. The phenomenological implications of these theories is under investigation, including the unification of gauge couplings, the quark, charged lepton and neutrino masses and mixing angles, CP violation, supersymmetric particle production and the production of the Kaluza-Klein tower of states associated with new space dimensions. The implications of these ideas for modifications of gravity at both large and small scales is also of interest.

### Particle astrophysics and cosmology

J Conlon, J March-Russell, G Ross+, S Sarkar*

The cosmological and astrophysical implications of theoretical and experimental developments beyond-the-Standard-Model are studied. The observationally well-founded Big Bang cosmology is used to constrain theories of massive neutrinos, supersymmetric particles, technicolour states, Kaluza Klein states *etc* which may constitute the dark matter in galaxies. Of particular interest is whether dark and visible matter may be linked through the leptogenesis mechanism for the origin of the baryon asymmetry of the universe. The generation of primordial density perturbations which gave rise to the observed large-scale structure in the Universe is investigated in the framework of inflation, both in the context of field theory and string/M-theory. Observational tests of the 'standard' cosmological model are formulated, in particular dynamical probes of dark energy. Other interests include the cosmological implications of new large/warped space dimensions (in particular infrared modifications of gravity) and astrophysical tests of quantum gravity.

There is also work done on very high energy cosmic rays, gamma-rays and neutrinos (especially as a probe of new physics), in particular through involvement in experiments such as the IceCube Neutrino Observatory and the Cherenkov Telescope Array.

* Main contact + Emeritus

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ResearchActivities2018-19.pdf | 3.91 MB |

ResearchActivities2019-20.pdf | 12.42 MB |