Search for extra dimensions in diphoton final states with the ATLAS detector at the Large Hadron Collider

I present a search for the extra dimensions in the ATLAS detector at the Large Hadron Collider. The extra-dimension theory proposes higher dimensional space in the context of several models. The gravitational force carrier graviton can propagate in the extra dimension(s). The graviton decays into diphoton final states which can be detected in the ATLAS experiment. The excellent calorimeter resolution provides a high performance for the identification of the photons. In this dissertation I analyzed the ATLAS data taken with 7 TeV proton-proton collisions in 2010. Using the earlier 3.1 pb-1, I tested diphoton physics in a search for One Universal Extra Dimension in events with diphoton + missing transverse energy. This model with deltaR = 20, M_D = 5 TeV and N = 6 was excluded for a curvature of 1/R < 729 GeV at 95% C.L.. Then I performed a search for the Randall Sundrum (RS) graviton decaying into diphoton resonances with the whole data taken in 2010 (36 pb-1). I excluded the RS graviton with M_G<550 GeV for a coupling k/MPl=0.02 and M_G<940 GeV for k/MPl=0.1 at 95% C.L. at 7 TeV center of mass energy.