Early Physics at the LHC using the CMS Electromagnetic Calorimeter

The Compact Muon Solenoid (CMS) is a general purpose detector at the Large Hadron Collider (LHC) at CERN. It has been designed to study proton-proton col- lisions at a centre of mass energy of 14 TeV. CMS has constructed an electromagnetic calorimeter (ECAL) with extremely good energy resolution using scintillating lead tungstate crystals. Radiation studies performed on samples of these crystals are presented in this thesis. Results obtained from operating large parts (supermodules) of the ECAL in a test beam are also presented and compared to simulation. Methods of measuring electron efficiencies with data have been developed, tested and used in the measurement of W and Z boson cross sections in electron decay modes. Samples of ECAL crystals produced by the Shanghai Institute of Ceramics were tested during 2005 for radiation hardness by measuring the drop in light yield caused by irradiation. Results showed crystals were radiation hard enough for use in CMS. The ability to monitor crystals in situ and crystal non-uniformity have also been investigated. During the summer of 2006, nine ECAL supermodules were placed in a test beam from the Super Proton Synchrotron. The variation in the fraction of energy contained in reconstructed ECAL clusters versus pseudorapidity was studied and the comparison between data and simulation is presented. The depth of an electromagnetic shower is an input parameter in the CMS electron reconstruction algorithm. A study to optimise this value and validate part of the electron reconstruction chain is documented. Tag and probe methods have been developed to measure online and offline electron efficiencies from data. Methods of estimating the background and subtracting it have been successfully employed. These efficiencies are used in the measurement of the inclusive W and $\gamma^{*}/Z$ production cross sections in the electron decay channel, assuming 10 pb-1 of proton-proton data.