Research in Particle & Astroparticle 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 and the origin of neutrino mass, 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. Supersymmetry - a generalized spacetime symmetry under which fermions and bosons can be transformed into each other, and in which Standard Model particles are accompanied by "superpartners" - has been the most prominent over the past 2-3 decades. The reasons for its popularity 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 its elegant symmetry-based mathematical structure. However, there is as yet no evidence for supersymmetry near the weak scale, and the recent exploration by the LHC has imposed strong constraints. In turn, over the past decade there has been a growing focus on simpler scenarios involving dark sectors, motivated by the driving empirical need to explain dark matter and neutrino mass. 

In parallel, our knowledge of the evolution of the universe has taken remarkable strides in recent years. While the gravitational force is known to provide a very accurate description of the formation of large scale structure, like galaxy clusters, at various epochs the interplay with particle physics has been crucial. Indeed, our current knowledge of the Cosmic Microwave Background Radiation (CMB), combined with other data such as the apparent acceleration of distant supernovae, suggests that the universe is well described by an early period of inflation, followed by a hot big bang, and most recently a new period acceleration. Moreover, the constituents are predominantly not ordinary matter, but dark matter, and an even stranger substance known as dark energy, which is driving the current accelerating phase, and in fact forms around 70% of the current matter density in the standard cosmological model. This may well be in the form of Einstein's famous cosmological constant. Particle physics plays an important role in describing the period of Big Bang Nucleosynthesis, and can also provide new candidates for dark matter. The latter aspect has led to a rapid expansion in the field now known as astroparticle physics.

Our group has studied new physics scenarios from various directions. The exploration of particle dark matter candidates and dark sectors has been central over the past decade. New sources of CP violation, required to explain the matter-antimatter asymmetry, have also been a focus of attention. In this area, energy precision experiments searching for particle electric dipole moments provide complementary information to high energy collider searches.