Electron identification with the CMS detector and applications for Higgs and W boson searches

The efficient identification of high energy electrons at the Large Hadron Collider (LHC) is of primary importance for the study of many physics processes, like the search for the Higgs boson and other Standard Model and beyond Standard Model processes. For the measurement of the electron energy, the CMS exp eriment uses a high precision Electromagnetic Calorimeter (ECAL). It is divided into a barrel and two end-caps composed of lead tungstate crystals, which are read by photo detectors. The integration of the ECAL readout electronics into the structure of the ECAL, the testing strategy, and the current performance of the ECAL electronics are presented in detail. In February 2010, 99.6% of the 75 848 ECAL readout channels were found to be fully operational. The identification of isolated electrons with the CMS detector is discussed, focusing on the reduction of the fake rate, i.e. the rate of jets or photons that are miss-identified as electrons. Tools for the electron selection are introduced, and a simple and effective electron selection for isolated high energy electrons in the ECAL barrel is presented. The study of the electron selection based on Monte Carlo simulations results in a fake rate of only 10−4 and an efficiency to select genuine electrons of 70%. The electron selection is applied to a study of the search of the Higgs boson in the channel Higgs → W+ W− → e+ νe− ν¯ and for a Higgs mass in the range 155-180 GeV. In particular, the background process W± X → e± νX is investigated in detail, where X is a jet or a photon that is miss-identified as an electron. Using the proposed electron selection, this background can be reduced well below the irreducible backgrounds. Finally, the developed electron selection is used in a study of the selection of W± → e± ν events with low background contributions. In addition, a new method to measure the transverse momentum spectrum of the selected W bosons is proposed. The measurement of this spectrum can constrain the gluon distribution functions of the proton at the LHC energies.