Search for new physics in diphoton and $E^{miss}_{T}$ final states using the ATLAS detector at the LHC

A search for new physics in diphoton events with large missin g transverse momen- tum with the ATLAS detector is presented in this thesis. The 2 011 dataset of 4.8fb − 1 of proton-proton collisions provided by the LHC at a center- of-mass energy of √ s = 7TeV is analyzed. By making use of three signal regions, an optimized sensitiv ity of the event selection is achieved in different parameter ranges of physics models b eyond the Standard Model (SM). The SM background is estimated mainly in data con trol regions. No excess of events above the SM expectation is found in any sign al region. Hence, 95% confidence level (CL) exclusion limits on the production cross section and on masses of new physics particles are derived. The results are interpreted in scenarios with gauge-mediat ed SUSY breaking (GMSB), namely the general gauge mediation (GGM) scenario with a bin o-like lightest neu- tralino ( ̃ χ 0 1 ) and the SPS8 scenario of the (minimal) GMSB model. In the GGM scenario, gluino (squark) masses, which are required to be g reater than the lightest neutralino mass, are excluded below 1 . 10TeV (0 . 91TeV) at 95% CL, for masses of the lightest neutralino greater than 50GeV. In the GMSB SPS8 scenario, masses of the lightest neutralino (chargino) are excluded at 95% CL be low 302GeV (582GeV) corresponding to an 95% CL exclusion of the SUSY breaking sca le Λ < 208TeV. Furthermore, the results are interpreted in the context of u niversal extra dimensions (UED) with one additional space dimension. Scenarios with a compactification scale R − 1 < 1410GeV are excluded at 95% CL.