# 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

J Casalderrey-Solana, A Starinets*

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 string theory 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

J Casalderrey-Solana, U Haisch, J March-Russell, G Ross+, S Sarkar, G Zanderighi*

There is an on-going programme of research in phenomenology with the dual aims of testing the Standard Model and identifying new phenomena. Analysis of the new precision data on heavy quark systems from B-factories is proceeding together with the implications for quark and lepton masses, mixing and CP violation. Complementary to this, studies are proceeding of the phenomenology of neutrino oscillations and the implications for neutrino mass, leptonic mixing and CP violation. Neutrino interactions at ultra-high energies is also of interest, in the context of astroparticle experiments.

A major focus is theoretical studies of collider physics at the LHC. The goal is to improve the accuracy of the description of processes involving many particles in the final state (light and heavy quarks that form jets, Z, W or the Higgs boson). These processes constitute either signals or important backgrounds for Higgs and new-physics searches, and are also of interest because they allow to test indirectly the structure of the Standard Model (for instance they are sensitive to the presence of anomalous gauge-boson couplings). An accurate description is achieved by performing next-to-leading order (NLO) calculations using novel unitarity-based techniques and/or by resumming logarithmically-enhanced contributions to all orders in perturbation theory. Particular emphasis is put on studying the phenomenological implications of these calculations and on direct comparison with LHC data.

### Physics beyond the Standard Model

J Conlon, U Haisch, 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 leading experiments such as the Pierre Auger Observatory, IceCube Neutrino Observatory and the Cherenkov Telescope Array.

* Main contact + Emeritus

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