Particle Physics

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.

The cosmological constant problem and the compatibility of gravity with quantum mechanics are the two most pressing problems in all of gravitational theory. While string theory nicely addresses the latter, it has so far failed to provide any compelling solution to the former. On the other hand, while conformal gravity nicely addresses the cosmological constant problem [by naturally quenching the amount by which the cosmological constant gravitates rather than by quenching the cosmological constant itself -- Mannheim, Prog. Part. Nuc. Phys. 56, 340 (2006)], the fourth order derivative conformal theory has long been thought to possess a ghost when quantized. However, it has recently been shown by Bender and Mannheim [Phys. Rev. Lett. 100, 110402 (2008)] that not only do theories based on fourth order derivative equations of motion not have ghosts, they actually never had any to begin with, with the apparent presence of ghosts being due entirely to treating operators which were not Hermitian on the real axis as though they were. When this is taken care of via an underlying PT symmetry that such theories are found to possess, there are then no ghosts at all and the theory is fully unitary. Conformal gravity is thus advanced as a fully consistent four-dimensional alternative to ten-dimensional string theory.

We discuss a quantum corrected inflation scenario driven by a generic GUT or Standard Model type particle model, whose scalar field playing the role of an inflaton has a strong non-minimal curvature coupling. We show that currently widely accepted bounds on the Higgs mass falsify the suggestion of [arXiv:0710.3755] (the work underestimating the role of radiative corrections) that the Standard Model Higgs boson can serve as the inflaton. However, if the Higgs mass could be raised to 216 GeV, then the Standard Model could generate the inflationary scenario matching the CMB data with $n_ssimeq 0.93$ and a very low tensor to scalar perturbation ratio $rsimeq 0.0005$.

In SUGRA flavour models, a total sequestering is not possible and an irreducible amount of flavour and CP violation is essentially unavoidable, which renders many flavour models testable in the near future experiments.

WIMP dark matter candidates chi^0 have interesting signatures for direct and indirect detection in regimes where there is a near degeneracy with a heavier charged state chi^{pm}, as occurs for example along the boundary of the coannihilation strip in the CMSSM. For small splittings of O(10) MeV, the scattering of WIMPs off nuclei may be dominated by inelastic recombination processes mediated by the formation of (chi^- N) bound states, leading for example to a distinct signature for direct detection. I will discuss these and other resonant processes that distinguish the detection signatures of this class of WIMP scenarios.

Working in the weak tachyon region of a condensing tachyon background, we find the modified equations of motion for massless strings with conformal perturbation theory. We then estimate the backreaction on the background dilaton. In large (supercritical) dimensions, we find that the backreaction can be significant in a large region of spacetime.