# Particle Phenomenology

Since the early 1970's, a theoretical framework for particle physics based on quantum field theory with local (or gauge) symmetry has condensed into what is now called the "Standard Model". It rests on gauge theories of the strong, weak and electromagnetic interactions, and has survived increasingly more precise tests. Currently, it provides a very accurate description of the elementary particles of nature, namely electrons, quarks, photons, and gluons, etc. from energy scales of a few eV to around 100 GeV. The latter scale is where the Standard Model predicts the spontaneous breakdown of the electroweak gauge symmetry, which is responsible for providing mass to the quarks and leptons, and rendering the weak interactions genuinely "weak". The details of electroweak symmetry breaking are now being explored at the Large Hadron Collider (LHC) at CERN. In 2012, the discovery of a particle fitting the description of the Standard Model Higgs boson was announced, providing the final unobserved component of the Standard Model. Further exploration of the properties of this particle are ongoing.

The success of the Standard Model notwithstanding, there are still many unanswered questions. These range from the nature of dark matter and dark energy, to the origin of the matter-antimatter asymmetry and of neutrino mass, just to name a few. There are also theoretical puzzles, such as the tuning required to understand a Higgs mass at the weak scale - known as the hierarchy problem, and the remarkable running of the gauge couplings, and patterns of the matter multiplets, which suggests a possible unification of the gauge group at high scales. Many of these issues provide motivation for believing that the Standard Model is an effective field theory, and will be incorporated into something more fundamental at shorter distance scales.

A number of different paradigms have been studied for the new physics that may emerge above the electroweak scale. These include "supersymmetry", "large extra dimension", and various other scenarios. Supersymmetry is a generalized spacetime symmetry under which fermions and bosons, despite their apparently very different characteristics can be transformed into each other. The introduction of supersymmetry requires that the number of fields of the Standard Model essentially becomes twice as large, as the Standard Model particles are accompanied by "superpartners". The reasons why supersymmetry has to a great extent been the leading candidate for physics beyond the Standard Model are primarily theoretical, having to do with its ability to stabilise the Higgs sector from quantum corrections, the emergence of a precise unification scale (near the Planck scale), and the fact that it can also provide a suitable candidate for the dark matter in the universe. It is also a very powerful and elegant mathematical structure, as the interactions between particles are to some extent fixed by this symmetry. However, there is as yet no evidence for supersymmetry near the weak scale, and the recent exploration by the LHC has imposed strong constraints.

Our group has studied new physics scenarios from various directions. New sources of CP violation, required to explain the matter-antimatter asymmetry, have been a focus of attention. Low energy precision experiments searching for particle electric dipole moments provide complementary information to high energy collider searches. Searches for particle dark matter have also been a focus of recent attention as described separately on this webpage.