Measurement of the ttbar Cross Section with Early LHC Collision Data at CMS

The Standard Model of particle physics is one of the most well tested theories in physics but is not believed to be a complete theory of everything. The Large Hadron Collider (LHC) particle accelerator will create the most energetic environment ever achieved in a laboratory producing proton-proton collisions which will allow physicists to study physics in a new energy regime. This work presents a study of the decay channel t tbar e nu + jets, carried out on the Compact Muon Solenoid (CMS) detector experiment. A study assessing the feasibility to measure the production cross section of tt events through this channel has been carried out at a centre-of-mass energy of 10TeV. Detailed studies of the background processes will be presented with particular attention to the background arising from converting photons. Techniques to measure the backgrounds in a data-driven manner have been investigated and the study predicts a measurement with a 27% statistical uncertainty and 20% systematic uncertainty, with an additional 10% error arising from the luminosity uncertainty. In March 2010 the LHC began producing collision at a revised centre-of-mass energy of 7TeV. Comparisons between collision data and expectations from simulation have been carried out, and investigations conducted on the background processes. Using the experience acquired from the 10TeV study an analysis is carried out and measurement presented for the tt production cross section at a centre-of-mass energy of 7TeV using 35.8 pb-1 of integrated luminosity. The final result with statistical and systematic errors is sigma(t tbar) = 145.0 +/- 35.5 (stat) +/- 46.4 (syst) +/- 5.8 (lumi)pb