Astrophysics

With LHC commissioned in just a few month ahead, all sorts of ideas about physics beyond the standard model are being explored intensively. A strong-coupling chiral theory appearing at TeV scale remains a possibility but also a very hard scenario to study. When it comes to strongly coupled theories, lattice regularization is by far the most reliable method. But defining exact chiral gauge theory on the lattice remains a difficult problem on its own. We show that the idea to use additional non-gauge, high-scale mirror-sector dynamics to decouple the mirror fermions without breaking the gauge symmetry might lead to a practically manageable solution. We demonstrate, using the exact lattice chirality, that partition functions of lattice gauge theories with vector like fermion representations can be split into \'light\' and \'mirror\' parts, and each contains a chiral representation. Such a splitting is only well defined when both sectors are separately anomaly free. We also prove that, the generating function and therefore the spectrum of an arbitrary chiral gauge theory is a smooth function of the background gauge field, if and only if the anomaly free condition is satisfied. We reached this conclusion by proving some very general properties of an arbitrary chiral gauge theory on lattice, and the results should be of importance for further studies in this field.

If light scalar fields are present at the end of inflation, their nonequilibrium dynamics can produce non-Gaussian density perturbations. Lattice field theory simulations show that this effect can be very strong in the massless preheating model. It is therefore an important factor in assessing the viability of inflationary models. I present a phenomenological model that can be used to calculate the perturbations analytically.

Within the context of F-theory compactified on Calabi-Yau fourfolds, we describe a class of string theory vacua which contain several features necessary in supersymmetric grand unified models of particle physics. Focussing on a simple class of local Calabi-Yau fourfolds, we explain how the matter content and superpotential in four dimensions are determined by a topological gauge theory. Along these lines, we present some minimal examples of GUT models.

The process of reheating in warped brane world models was initially thought to be quite efficient. However, the identification of long-lived Kaluza-Klein (KK) relics associated with isometries along the internal directions suggests that reheating may not be efficient, and may conflict with BBN and baryogenesis constraints. This talk discusses processes which may accommodate their decay and quantifies their expectant lifetimes, resulting in strong constraints on the parameters of the underlying theory. We also point out several shortcomings of other, recent investigations into the decay mechanisms of the KK relics.

The Standard Model (SM) of particle physics provides an excellent description of nearly every collider physics experiment performed to date. However, the SM is unable to explain the observed cosmology. Among its cosmological shortcomings, the SM cannot account for the dark matter or explain why there is more matter than anti-matter. A well-motivated way to extend the SM is supersymmetry. In the minimal supersymmetric extension of the the SM, the MSSM, new superpartner particles can make up the dark matter and generate the matter-antimatter asymmetry. These two requirements place strong constraints on the mass spectrum of superpartners in the MSSM. In this talk, we will describe this cosmologically motivated spectrum, and discuss some of the interesting signatures it could create at the upcoming LHC experiments.

We consider a Born-Infeld like action for gravity coupled to an external connection field. We show that the equation of state of this fluid interpolates between dark matter and dark energy. We also show that on galactic scales this system predicts asymptotically flat rotation curves. This action is motivated by looking at a regime where the metric vanishes, and replacing the big bang by a smooth transition between a topological manifold to a Riemannian manifold.

We present a simple mechanism for obtaining large-field inflation, and hence a gravitational wave signature, from string theory compactified on twisted tori. For Nil manifolds, we obtain a leading inflationary potential proportional to phi^(2/3) in terms of the canonically normalized field phi, yielding predictions for the tilt of the power spectrum and the tensor-to-scalar ratio, $n_sapprox 0.98$ and $rapprox 0.04$ with 60 e-foldings of inflation; we note also the possibility of a variant with a candidate inflaton potential proportional to phi^(2/5). The basic mechanism involved in extending the field range -- monodromy in D-branes as they move in circles on the manifold -- arises in a more general class of compactifications, though our methods for controlling the corrections to the slow-roll parameters require additional symmetries.

We study effects of the neutrino yukawa coupling on neutralino dark matter observables. We found that presence of the top-like neutrino yukawa coupling does significantly affect neutralino relic density in the regions.