Sharif Digital Repository

Since 1980’s, String Theory is considered as one of the main candidates for the unified theory of nature. One of the important features of this theory, is describing the Gravity as a renormalizable quantum theory. An interesting and important problem is the interaction of the string with classical fields in spacetime such as gravity. The conclusions which can be deduced may be very different from the standard results in case of absence of these classical fields.On the other side, for giving a solution to one of the unsolved problems in standard model of particle physics, the hierarchy problem, Lisa Randall and Raman Sundrum in1999, suggested a model which assumes that spacetime may has... 

Models wih extra dimensions have been among the most important models built for solving the problems of the standard model. In the standard model, Higgs mechanism is used to ex-plain electroweak symmetry breaking. One of the theoretical problems of the standard model,the hierchy problem, is about the Higgs mass being unnaturally small and models with extra dimensions have provided solutions to solve the problem. Moreover, extra dimensions pro-vide tools using which several models have been developed to explain electroweak symmetry breaking. In this thesis, acher reviewing the electroweak symmetry breaking sector of the stan-dard model, and the hierarchy problem, some models of electroweak...