High Energy Physics

The warped geometry present in Randall-Sundrum (RS) models provides an elegant means by which to generate stable scale hierarchies. Given the famous hierarchy problem of the Standard Model, and the relatively small number of known mechanisms which may solve it, the RS model has deservedly received a lot of attention. However the construction of a completely realistic RS model remains difficult and requires a number of modifications beyond the minimal framework. In this talk I will give an overview of the RS model, the difficulties encountered in realistic implementations and the types of modifications necessary to address them.

As LHC era is coming close, all sorts of ideas about physics beyond the standard model are being explored. It remains possible that a strong-coupling chiral theory could appear at TeV scale. When it comes to strongly coupled theories, lattice is still the most reliable and straightforward regularization method. But defining a chiral gauge theory on the lattice is formidable on its own. In this talk, I will present some most recent theoretical developments in attempt to tackle this problem, and explain some general theorems we proved for generic chiral gauge theories on lattice. These results should be useful in future studies in the field. I will also present some numerical results suggesting that the idea of constructing a chiral gauge theory by decoupling the mirror fermions using a high scale strong- coupling gauge symmetric phase suffers from severe constraints. I will end my talk by a brief outlook on how one may hope to reach a conclusive prove on the feasibility of this idea in general.

We study various aspects of power suppressed as well as exponentially suppressed corrections in the asymptotic expansion of the degeneracy of quarter BPS dyons in N=4 supersymmetric string theories. In particular we explicitly calculate the power suppressed corrections up to second order and the first exponentially suppressed corrections. We also propose a macroscopic origin of the exponentially suppressed corrections using the quantum entropy function formalism. This suggests a universal pattern of exponentially suppressed corrections to all four dimensional extremal black hole entropies in string theory.

I will discuss the various sources of non-Gaussianity (NG) in a class of multi-field models of inflation. I will show that there is both an intrinsic and a local contribution to the NG although they both have the same shape. It is also possible in this class of models that the dominant part to the 3-pt function comes from loop diagrams. These models are of the hybrid type and while they occur naturally in string theory, the conditions for the NG to be important are not generic.

We report on recent progress in understanding string compactifications to four dimensions, preserving minimal supersymmetry. We develop a general formalism to construct the kinetic terms of the low energy degrees of freedom. At strong warping, new light Kaluza-Klein modes appear, which change the effective action for the complex and Kahler moduli. We explain how to determine these new fields starting from 10d, and find their couplings to the zero mode sector.

In two dimensional CFTs the Zamolodchikov\'s c-theorem is fundamental in that it shows that the number of degrees of freedom decreases along the renormalization group flow. I will give a short history of and discuss recent developments in the quest to find its four-dimensional analogue using the central charges a & c.

We are currently in the throes of a potentially huge paradigm shift in physics. Motivated by recent developments in string theory and the discovery of the so-called \'string landscape\', physicists are beginning to question the uniqueness of fundamental theories of physics and the methods by which such theories might be understsood and investigated. In this colloquium, I will give a non-technical introduction to the nature of this paradigm shift and how it developed. I will also discuss some of the questions to which it has led, and the nature of the controversies it has spawned.

The efficient computation of scattering amplitudes in quantum field theory has many important applications, ranging from the computation of QCD backgrounds at the LHC to the study of the perturbative finiteness of N=8 supergravity. \'On-shell methods\' are a crucial ingredient in the computation of gauge theory and gravity amplitudes because they are far more efficient than traditional Feynman diagram techniques. I give an introduction to the basic concepts used in this field. I explain one particularly elegant method, the MHV vertex expansion, and outline how we recently proved the validity of this expansion in N=4 Super Yang-Mills Theory.

I discuss our recent investigations into 2+1 dim Chern-Simons theories with gravity duals that have reduced supersymmetry. Many new phenomena such as fractional statistics arise in 2+1 dim field theory that make this duality interesting and subtle. I focus on our work involving an example of such a duality with minimal supersymmetry and propose a field theoretic dual for a long known vacuum of gauged supergravity on AdS_4. I also argue that 2+1 dim duality might present a favorable landscape for constructing non-supersymmetric conformal fixed points at large but finite N.

One of the most challenging problems in theoretical physics today is the so called cosmological constant problem. While current observations are consistent with the prediction of GR with an unexplainable tiny cosmological constant, it remains possible that it\'s the deviation of the law of gravity at large distance from Einstein\'s theory that resolves the puzzle. In this talk, I will briefly review some of the theoretical attempts we made along this line, in particular, the so called \'classically constrained gravity\' and its implications in quantum cosmology. I will also present some most recent study on massive spin-2 particles in De Sitter space, and describe a model, initially motivated by DGP theory, which allows one to explore the Higuchi forbidden mass range of the graviton on the De Sitter background.