Dr John McDonald
Room: C21 Physics Building
Tel: +44 1524 592845
UK: (01524) 592845
Fax: +44 1524 844037
UK: (01524) 844037
My research area is the intersection of particle physics and cosmology, known as particle cosmology or astro-particle physics.
My past research activities include the cosmology of the minimal supersymmetric standard model (MSSM) and its extensions (flat directions, Affleck-Dine baryogenesis and leptogenesis, Q-ball formation and decay, dark matter, curvatons and right-handed sneutrinos), supersymmetric hybrid inflation models (reheating and oscillon formation, modification by RH sneutrinos), dark matter models (thermal relic MSSM neutralino and gauge singlet scalar dark matter, RH sneutrino dark matter) and electroweak baryogenesis.My recent research focuses on the following themes:
Recent data from the WMAP (Wilkinson Microwave Anisotropy Probe) satellite has shown that the observed cosmic microwave background has a spectral index significantly lower than that predicted by most inflation models. This is true in the case of supersymmetric (SUSY) hybrid inflation models, which are regarded as a strongly favoured class of inflation model. A possible interpretation of this discrepancy is that it is due to the influence of other particles. Together with my student Chia-Min Lin, I have been investigating the effect of MSSM flat direction fields (in particular right-handed [RH] sneutrinos) on SUSY hybrid inflation models. We have established that the spectral index can naturally be lowered to be consistent with WMAP observations. Future research aimed at understanding the effect of such fields on the cosmic string density produced at the end of inflation, which is typically too large in conventional SUSY hybrid inflation models.
A longer-term research programme is the investigation of reheating in SUSY inflation models. An important feature of inflation models is the process by which they transit from the cold inflationary era to the standard hot Big Bang model, known as reheating. In SUSY hybrid inflation models this is a complex process of non-linear scalar field dynamics known as tachyonic preheating. Understanding in detail how this process completes is a long-term research goal. In addition, there have recently been significant developments in the physics of reheating and gravitino production in supergravity models. Future research will investigate reheating and gravitino production in a variety of supersymmetric models with different supersymmetry-breaking mechanisms. A collaboration with Helsinki University has recently been established to study these issues.(ii) Dark Sector in Supersymmetric Models:
Only about 4% of the matter in the Universe corresponds to conventional atomic (baryonic) matter. 23% is in the form of dark matter which clusters to form the framework of galaxy formation. The remaining 73% is in the form of 'dark energy', which is responsible for the recent acceleration of the expansion of the Universe. Dark matter plus dark energy collectively form the 'dark sector'.
Supersymmetric particle physics theories provide natural candidates for dark matter. In order to prevent rapid decay of the proton, a symmetry known as R-parity [exact or weakly-broken] must be imposed. A side-effect of R-parity is that the Lightest Supersymmetric Particle (LSP) is absolutely stable. This provides a natural candidate for the dark matter particle. In particular, in the case of neutralino dark matter, the thermal relic density left over after the Big-Bang can have the right magnitude to naturally account for the observed dark matter. However, other candidates for dark matter exist, with different production mechanisms, including gravitinos, axinos, and RH sneutrinos.
One way to distinguish between DM candidiates is via their ability to account for the baryon-to-dark matter (BDM) ratio. Observation has shown that the ratio of the density in baryons to that in dark matter is about 1/6. However, the production mechanisms for baryons and dark matter are usually physically unrelated. It is therefore remarkable that their densities can be within an order of magnitude. A natural interpretation of this observation is that the dark matter and baryon densities have a similar cosmological origin. Requiring that the baryon asymmetry and the dark matter density have a related origin would provide a powerful principle by which to identify the correct particle physics theory, baryogenesis mechanism and dark matter particle. This knowledge would, in turn, allow us to probe more deeply into early Universe cosmology.
I have recently proposed that the BDM ratio can be explained by a dark matter condensate of RH sneutrinos combined with a baryon asymmetry generated via Affleck-Dine leptogenesis. In this model the densities of dark matter and baryons both originate from Bose-Einstein condensates of scalar particles which have closely related dynamics in the early Universe. Future research in this area will aim to understand in detail the phenomenology and cosmology of RH sneutrino dark matter, in particular the observational consequences for the CERN Large Hadron Collider (LHC) and ESA Planck cosmic microwave background satellite. In addition, alternative BDM scenarios and non-BDM dark matter models will be explored in order to assess the uniqueness of the RH sneutrino BDM model.
A complete description of the Dark Sector also requires an explanation of the dark energy and why its density is similar to that of dark matter, a feature known as the Coincidence Problem. This is particularly challanging in supersymmetric models due to the large mass of the scalar particles in supersymmetry. A long-term goal will be to investigate supersymmetric dark energy models which can address the Coincidence Problem.
I am the manager of the Physics with Astrophysics and Cosmology degree scheme. In 2007/8 I will be teaching the PHYS 213 (Maths II) and PHYS 461 (Cosmology III) courses and administering the PHYS 364 Cosmology Lab. I will also supervise two PHYS 451 MPhys projects.
Last modified: 4 April, 2007