Sharif Digital Repository

Double Field Theory tries to incorporate T-duality, one of fundamental features of string theory, as a symmetry of a field Theory on a doubled space. It sets fields depend on winding mode conjugate coordinates as well as conventional space-time coordinates. In this review after presenting a brief introduction to string theory we investigate O(d,d,ℤ) duality group. Subsequently, we focus on Double Field Theory construction putting our attention on massless sector of string theory. We investigate on 2D nonlinear sigma model action in DFT and its gauge algebra. We follow this review by discussing constraints of DFT and Double Geometry. At the end we present some unsolved questions in this... 

Studying near-horizon space-time for Reissner-Nordstrom black hole when taking extremal limit, we see how the region between the inner and outer event horizons appears as a part of , while it seems to shrink when looking into the geometry of a bit far from each of two horizons. By a careful investigation of radial geodesics in this limit, it is shown that specific families of observers starting their motion form anywhere so close to outer horizon, always pass this region in a constant proper time; however, observers coming from somewhere, further, pass it in as extremely short time as it sounds this region is hardly tangible  

The study of quantum effects for black holes indicates that they can thermally radiate different particles.In this thesis, we investigte the radiation of charged spin zero particles and show that these particles can interact with each other and as a result produce photons.These produced photons make a correction to the emitted photons from the black hole. By caculating this correction, we conclude that interactions have a small effect on the radiation spectrum of the photon  

Supersymmetry breaking mechanism is an unanswered question. Which is not allowed to break at tree-level in MSSM due to supertrace restrictions. Most theories suppose it’s broken in a dark section, much higher energies than electro-weak and its impacts as soft SUSY breaking terms emerged in Lagrangian. Here, in a different approach, an extension of MSSM with a U(1) and Stueckelberg superfield along with nonlinear gauge transformation, provides an additional condition to present a tree-level SUSY breaking mechanism. Mass mixing term in Stueckelberg’s Lagrangian or kinetic mixing are two methods of such an extension. The particles mass terms, solving the Higgsino mass parameter problem µ,...