My research area is the intersection of particle physics and cosmology, known as particle cosmology or astro-particle physics.

My past and present research interests 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, flat-direction modification), 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:

(i) Thermal WIMP Dark matter and the baryon-to-dark matter ratio

One of the puzzles of the observed Universe is the similarity of the densities of baryons and of dark matter. The ratio of baryons to dark matter is observed to be ~ 1/5. In most theories of the origin of baryons, there is no relation between the baryon density and the dark matter density. One possibility to explain the similarity is to have a common origin for the two densities, such as a particle which decays to an equal asymmetry in baryons and dark matter. In this case the dark matter particle mass must typically be light, a few GeV. However, such an explanation completely ignores the well-known similarity of the observed dark matter density and a thermal relic density of weakly-interacting massive particles (WIMPs), the so-called “WIMP miracle”. Ideally, an explanation for the similarity of the baryon and dark matter densities should relate the baryon density to a thermal relic WIMP density, so preserving the “WIMP miracle” as an explanation of dark matter. We have proposed a completely new framework to achieve this, which we have called “Baryomorphosis”. [“Baryomorphosis: Relating the Baryon Asymmetry to the “WIMP Miracle”, arXiv:1009.3227; “Simultaneous Generation of WIMP Miracle-like Densities of Baryons and Dark Matter”, arXiv:1108.4653 [hep-ph], Phys. Rev. D 84 (2011) 103514.] In this approach, an initially large baryon asymmetry is modified -- hence “Baryomorphosis” rather than “Baryogenesis” -- by new baryon number violating annihilation processes to become a thermal relic WIMP-like density. The model predicts new particles with long lifetimes which have baryon number violating decays and which may be produced at the LHC.

An alternative approach is to relate the baryon and dark matter densities via the “anthropic principle”, the idea that our existence as observers depends on the conditions in the region (or “domain”) of the Universe we inhabit. In particular, our existence may be sensitive to the density of dark matter and baryons in galaxies. To make such an explanation possible, we need a model of dark matter and of baryogenesis which can generate domains with different densities. I am presently investigating the possibility that Affleck-Dine baryogenesis can generate domains with different baryon density [“Anthropically Selected Baryon Number and Isocurvature Constraints”, arXiv:1207.2135 [hep-ph]]. Future research will combine this with axion dark matter to produce a model in which both the baryon and dark matter densities are naturally varying on length scales much larger than the region of the Universe we inhabit.

(ii) The cosmology of supersymmetric particle physics models

The Minimal Supersymmetric Standard Model (MSSM) is widely expected to be the next stage of particle theory beyond the Standard Model. This belief is based on the stability of the weak scale with respect to quantum corrections and on the near perfect unification of the gauge couplings in the MSSM. An outstanding question is the nature of supersymmetry breaking, which has direct implications for both collider phenomenology and dark matter. One possibility is Gauge-Mediated SUSY Breaking (GMSB), where SUSY-breaking in a hidden sector is transmitted to the observable MSSM sector via messenger particles which carry Standard Model gauge charges. In GMSB models the dark matter candidate is expected to be the gravitino, the SUSY partner of the graviton.

Together with PhD student Francesca Doddato, I am presently engaged in a study of MSSM flat directions and Affleck-Dine baryogenesis in the context of GMSB models, studying the role of Q-ball decay in gravitino dark matter production. [“Affleck-Dine Baryogenesis, Condensate Fragmentation and Gravitino Dark Matter in Gauge-Mediation with a Large Messenger Mass”, arXiv:1101.5328 [hep-ph], JCAP 1106 (2011) 008; “New Q-ball Solutions in Gauge-Mediation, Affleck-Dine Baryogenesis and Gravitino Dark Matter”, arXiv:1111.2305 [hep-ph], JCAP 1206 (2012) 031.] This provides a model which can account for both dark matter and the baryon asymmetry without requiring any new fields beyond those in the MSSM.

(iii) Generalizations of Higgs Inflation and the cosmology of minimal extensions of the Standard Model

It has been proposed that inflation could be explained by the Higgs scalar [F.L.Bezrukov and M.Shaposhnikov,``The Standard Model Higgs boson as the inflaton'', arXiv:0710.3755 [hep-th], Phys. Lett. B 659 (2008) 703]. Previously this was viewed as impossible, as the Higgs self-coupling is too strong to produce successful inflation. However, inflation via the Higgs is possible if the Higgs scalar is non-minimally coupled to gravity. This opens up the remarkable possibility that all of cosmology could be explained by weak scale particles accessible to the LHC. Given that the Higgs boson is the only scalar field which is believed to exist with any certainty, it is clearly essential to precisely establish the conditions under which the Higgs can successfully produce inflation and generate the primordial density perturbations.

However, in order to completely account for cosmology, the Standard Model must be extended to include a cold dark matter candidate. In collaboration with Rose Lerner, I have proposed a version of Higgs Inflation in which both cold dark matter and inflation are explained by a single particle. ["Gauge singlet scalar as inflaton and thermal relic dark matter", arXiv:0909.0520 [hep-ph], Phys.Rev.D80 (2009) 123507; Distinguishing Higgs Inflation and its Variants, arXiv:1104.2468 [hep-ph], Phys.Rev. D83 (2011) 123522.] This particle is a gauge singlet scalar, which is the minimal extension of the Standard Model which can account for thermal relic weakly-interacting dark matter. This interacts with the Standard Model via the "Higgs portal", the unique renormalizable interaction between scalar particles and the Standard Model. The Higgs portal gauge singlet scalar dark matter model was first proposed by Silveira and Zee [V.Silveira and A.Zee,``Scalar Phantoms'', Phys. Lett. B 161 (1985) 136] and independently by JMcD in 1994 ["Gauge singlet scalars as cold dark matter", Phys.Rev. D50 (1994) 3637]. The model is now regarded as one of the canonical models for cold dark matter. The new model proposes that the gauge singlet scalar can also serve as the inflaton, by non-minimally coupling it to gravity in a manner analogous to Higgs Inflation. This new model, dubbed "S-inflation", therefore unifies for the first time thermal relic dark matter with inflation. Our on-going research is focused on developing this model, in particular elucidating the process of reheating in the model and studying the characteristic cosmological signatures for S-inflation as compared with Higgs Inflation.

S-inflation can be viewed as one component in a complete weak-scale theory of cosmology and particle physics, once a baryogenesis mechanism (most likely electroweak baryogenesis) and an explanation for neutrino masses are included. This is a longer-term goal, which could also include gauge coupling unification via additional weak scale particle mulitplets and the embedding of the model in a Grand Unified Theory.

Non-minimal coupling of scalar particles to gravity encounters a barrier in the form of breakdown of tree-level perturbative unitarity in high-energy scalar particle scattering processes. This is a violation of quantum mechanics which indicates that the theory is incomplete i.e. new particles or interactions must be added to restore unitarity in high-energy scattering. The 'ultra-violet (UV) completion' may modify or even rule out Higgs Inflation-type models. Rose Lerner and I have studied the nature of perturbative unitarity violation in “Higgs Inflation and Naturalness (with Rose Lerner), arXiv:0912.5463 [hep-ph], JCAP 04 (2010) 015” and the possibility of a UV completion which preserves a form Higgs Inflation in “A Unitarity-Conserving Higgs Inflation Model (with Rose Lerner), arXiv:1005.2978 [hep-ph], Phys. Rev. D82 (2010) 103525.” An alternative to UV completion is that strong coupling can restore unitarity in high-energy scattering processes, since perturbation theory generally breaks down before the energy of tree-level violation is reached. Whether unitarity violation is a real effect and whether strong coupling is consistent with Higgs Inflation are as yet unsolved problems. [Unitarity-Violation in Generalized Higgs Inflation Models (with Rose Lerner), arXiv:1112.0954 [hep-ph].]