Particle physics is the study of the most elementary constituents of matter, the basic forces of nature by which they interact, and their role in the early Universe. The study of particle physics relies on experiments ranging from huge particle accelerators, to deep underground laboratories, to orbiting satellites, to particle simulations using the most powerful computers.
The next generation of such experiments (e.g. the large experimental facilities at the LHC at CERN) is poised to unlock some of the deepest questions such as why is the Universe so large, and how did it begin and how will it end? What is the nature of dark matter and structure in the Universe? What is the origin of particle masses? Can the forces of nature be unified? Are there extra space dimensions? Why are there three families of matter particles? Can the equations describing the strong interaction be cracked?
With the aid of the new data the theoretical physics group at Southampton aims to shed light on such fundamental questions. We have experts who are currently involved in trying to answer all of these questions, using the latest ideas in lattice theory, quantum field theory, string theory, supersymmetry, and cosmology.
The following list reflects the current major interests of members of the group.
Lattice Quantum Chromodynamics (QCD)
We work with the UKQCD collaboration of seven British universities and with the US-based Riken-Brookhaven-Columbia (RBC) Collaboration and the KEK group in Japan, exploiting our IBM BlueGene/Q 1.26 Pflop/s supercomputer. We have a strong interest in kaon physics, heavy quark (b and charm) physics and g-2. We calculate quantities needed to test the Standard Model, in particular the picture of quark flavour-mixing and CP violation, but also other quantities that allow to improve and check our understanding of QCD itself. Research on related theoretical techniques like effective field theories or algorithmic developments complements numerical work.
Results from BaBar and Belle and LHCb, ongoing work at Fermilab drive our interest in b-quark phenomenology, especially CP-violation and the CKM matrix. Our work includes developing and exploiting our factorisation formalism for two-body hadronic decays such as B -> pi pi and B -> pi K, and extracting the CKM matrix element Vub from combined experimental and theoretical results for exclusive semileptonic B -> pi decays. We have active links with the LHCb group at RAL.
Weak Interaction Corrections
Weak interaction corrections to processes dominated by strong forces are becoming increasingly relevant as present and future accelerators (RHIC, Tevatron, LHC and a LC) probe higher energies with greater precision. Calculations of these effects within the Standard Model are currently being carried out within our group. For example, large weak interaction effects have been identified in jet, Z boson and photon production.
We are also engaged in phenomenological studies of the physics potential of present and future high energy particle accelerators in performing tests of the Higgs sector of the Standard Model and of its minimal and non-minimal Supersymmetric extensions, by using numeric calculational methods including full event simulation through Monte Carlo techniques, as implemented in the HERWIG program. Such MC programs are crucial tools needed to analyse present and future collider data, bringing the Southampton group in direct contact with the experiments at BNL, FNAL and CERN. These activities are carried out in the framework of the NExT Institute.
Beyond the Standard Model
This area of our research is concerned with addressing the following unresolved puzzles of the Standard Model: The origin of mass (the origin of the weak scale, its stability under radiative corrections, and the solution to the hierarchy problem); The problem of flavour (the problem of the undetermined fermion masses and mixing angles (including neutrino masses and mixing angles); The question of unification (the question of whether the three known forces of the standard model may be related into a grand unified theory, and whether such a theory could also include a unification with gravity). The approaches we develop are based on Supersymmetric Grand Unified Theories (SUSY GUTs) with extra Family Symmetries or String-Inspired models involving D-branes embedded in Extra Dimensions. We are interested in the experimental consequences of these theories at experiments such as the forthcoming Large Hadron Collider or the many Neutrino Experiments.
Particle Physics and Cosmology
This area of our research is concerned with addressing the following unresolved puzzles of the Cosmological Standard Model: The origin of dark matter and dark energy (the embarrassing fact that 20 per cent of the matter and 75 per cent of the energy of the Universe are in a form that is presently unknown); The problem of matter-antimatter asymmetry (the problem of why there is a tiny excess of matter over antimatter in the Universe, at a level of one part in a billion, without which there would be no stars, planets or life); The question of the size, age, flatness and smoothness of the Universe (the question of why the Universe is much larger and older than the Planck size and time, and why it has a globally flat geometry with a very smooth cosmic microwave background radiation). The approaches we develop are based on ideas of Inflationary Cosmology and Leptogenesis which are related to the latest ideas of physics Beyond the Standard Model (see above).
High Temperature and Density QCD
RHIC (Brookhaven) and ALICE (CERN) have renewed interest in QCD at high temperature and density, where the quark gluon plasma shows signs of being a perfect fluid. There is also interest in the idea that cool and dense QCD (as in neutron stars) may be a colour superconductor. We study these phases using thermal models and through weakly coupled string theory duals to strongly coupled gauge theories. The latter, using anti-de-Sitter black holes and their quasi-normal modes, allow the computation of transport coefficients in the plasma as well as computing properties of mesons melting in the background thermal bath.
Quantum Gravity and Exact Renormalisation Group
One possibility for combining gravity and quantum mechanics into a complete predictive framework could be through a suitable non-trivial ultra-violet fixed point. This scenario is known as "asymptotic safety". We are investigating this possibility through non-perturbative approximations allowed by the exact renormalisation group.
Strings and Branes
We study some of the thorniest outstanding issues in quantum theories including strong interactions beyond perturbation theory (such as occur in QCD) and quantum gravity.
We work on dualities between gauge theories and string theories, including holography. Our group has developed the method of holographic renormalization, which is the gravitational counterpart of renormalization in QFT. The method is essential for obtaining well-defined rules for computations in gravity/gauge duality and moreover it explicitly shows how spacetime is reconstructed from gauge theory data. Further interests cover black holes, applications of holographic methods to condensed matter systems and extending the holographic methods to cosmology. Recently we have used the AdS/CFT correspondence and its deformations to understand new descriptions of confinement and mass generation in QCD-like theories.
We also study other approaches to quantum gravity, including recent work on asymptotically UV safety using exact renormlization group methods.