Theoretical Particle Physics

The goal of particle theory is to produce a theory which unifies all forces and particles; to demonstrate how this can account for all known phenomena; and to produce predictions testable at colliders such as LHC. Our group is engaged with all aspects of this endeavour--particularly in string, BSM and collider phenomenology and lattice and analytic QCD. String theory is the most popular candidate for such a theory; it proposes that the basic objects are strings, and provides the only known consistent framework containing both quantum mechanics and gravity. The aim of string phenomenology is to obtain from this general theory (which resides in 10 dimensions) a realistic model in 4 dimensions consistent with known results and providing predictions which can be tested at e.g. LHC. In this process the extra 6 dimensions are 'compactified'--the 6 dimensional space becomes very small and unobservable. Remarkably, this process of compactification can explain many features of the observable world through the interplay of the 6-dimensional and 4-dimensional geometry. The goal is to obtain a theory of the form of the Standard Model by a judicious choice of compactification. An intermediate stage in this process is usually a supersymmetric version of the Standard Model, which we expect would reveal itself at high energies, Beyond the Standard Model (BSM)--but perhaps accessible at LHC. Supersymmetry postulates that the two fundamental types of particle in nature, bosons (which transmit forces) and fermions (which constitute matter), are related; the main attraction of the concept is that it explains the relatively tiny masses possessed by fundamental particles. If supersymmetry is unbroken then every boson would be accompanied by a fermion of the same mass. Since this is not observed, supersymmetry must be broken. The challenge of BSM phenomenology, one which we are addressing, is to explain how this happens while retaining the solution of the hierarchy problem. In fact our hope is that the details of the solution may as a by-product explain the exact pattern of all particle masses, including recently discovered neutrino masses. Any particular BSM theory will retain an imprint of its origin in string theory and it will be important to distinguish this in collider experiments such as at LHC. To this end it will be important to have very precise predictions for LHC processes, as one distinguishing feature of BSM theories may be an extremely small indirect effect on ordinary Standard Model processes. Group members are in the vanguard of very high precision calculations for collider phenomenology. Although the interactions in a collider are between fundamental particles such as quarks and electrons, individual quarks are never observed as reaction products; they are only seen combined as hadrons, bound states of two or three quarks, bound together by the strong interactions according to the quantum chromodynamics (QCD). This phenomenon is called quark confinement. Consequently it becomes vital to understand confinement and the strong interactions; both in order to reconstruct the interactions in colliders from the reaction fragments, and more generally to have a clear understanding of the way in which the Standard Model determines the behaviour of the nuclei making up matter. This requires novel techniques as most particle physics computations are based on the interactions being low in strength. The most effective such technique is lattice QCD, which seeks predictions from numerical simulations approximating spacetime by a lattice of discrete points. Our group is involved in the use of high performancecomputing for this purpose and is especially interested in lattice simulations of hadrons. Other group members are interested in the infra-red structure of QCD which has bearings on quark confinement.