Measurement of the W->enu Cross Section with Early Data from the CMS Experiment at CERN

The Compact Muon Solenoid (CMS) is a general purpose detector designed to study proton-proton collisions, and heavy ion collisions, delivered by the Large Hadron Collider (LHC) at the European Laboratory for High Energy Physics (CERN). This thesis describes a measurement of the inclusive W → eν cross section at 7 TeV centre of mass energy with 2.88 ± 0.32 pb−1 of LHC collision data recorded by CMS between March and September 2010. W boson decays are identified by the presence of a high-pT electron that satisfies selection criteria in order to reject electron candidates due to background processes. Electron selection variables are studied with collision data and found to be in agreement with expectations from simulation. A fast iterative technique is developed to tune electron selections based on these variables. Electron efficiency is determined from simulation and it is corrected from data using an electron sample from Z decays. The number of W candidates is corrected for remaining background events using a fit to the missing transverse energy distribution. The measured value for the inclusive W production cross section times the branching ratio of the W decay in the electron channel is: σ(pp → W +X)×BR(W → eν) = 10.04±0.10(stat)±0.52(syst)±1.10(luminosity) nb, which is in excellent agreement with theoretical expectations.