Parameterisation of electromagnetic showers and search for large extra dimensions in pp collisions at $\sqrt{s}= 14 TeV$

For the start-up of the future proton-proton collider LHC at CERN in 2008 a multitude of simulated events are needed in order to understand the detectors and perform physics analyses at a centre-of-mass energy of 14 TeV. The solution presented here, namely the usage of shower parameterisation, can speed-up this simulation considerably without sacrificing too much precision. A package based on this concept, GFlash, was implemented in the general simulation framework of Geant4. It has been successfully integrated in the CMS detector simulation and has proven to work with an impressive speed and acceptable accuracy. In the second part of the thesis, the discovery potential of large extra dimensions (ADD model) with the CMS detector in the photon + missing transverse energy final state was studied. A normalisation method is proposed to measure the main background from data. The analysis shows that via this channel a 5 sigma discovery can be even made in the first year, depending on the model parameters.