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In this thesis we present the first measurement of the W production differential cross section in the electron decay channel as a function of transverse momentum of W bosons. The analysis is based on a data sample of proton-proton collisions at the centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 18.4 ± 0.5 pb−1, collected during a special low-luminosity running of the Compact Muon Solenoid detector at the CERN Large Hadron Collider in the year 2012.
The W boson candidates are reconstructed by their leptonic decay channel with an electron and a neutrino in the final state. Electron candidates within the detector fiducial volume of ∣η ∣ < 2.5 (pseudorapidity) and pT > 25 GeV (transverse momen- tum on the plane perpendicular to the beam line) are used. The W bosons are identi- fied by the presence of a high-pT electron that satisfies the electron selection criteria, while rejecting the candidates from background processes. Studies on electron se- lection criteria, selection efficiencies, and effects of energy scale and resolution are presented. The electron efficiencies are measured as a function of η and pT based on the tag-and-probe method, using the Z bosons in the invariant mass window of 60 to 120 GeV. Differences of the electron efficiencies in data and Monte Carlo are corrected in the analysis. Electron energy scale and resolution effects are studied and corrections are applied to match the data and Monte Carlo.
The number of W signal candidates are extracted by using an unbinned maximum- likelihood fit to the missing transverse energy distribution, which is the sum of the W boson signal, the QCD background, and other electroweak background contri- butions. The background from QCD multijet events is estimated by using a data driven technique. Estimation of the backgrounds from other electroweak and top backgrounds are based on Monte Carlo simulations.
Reconstructed p^W_T distribution is corrected for detector resolution and final state radiation effects. This is achieved by a two-step unfolding procedure, which is based on a singular value decomposition method.
The measured W boson transverse momentum distributions are compared with the expectations from the best available theoretical predictions: (1) RESBOS, a next- to-next-to-leading logarithmic/next-to-leading order quantum chromodynamics calculation with resummation, (2) POWHEG, a next-to-leading order quantum chromodynamics calculation, and (3) FEWZ, a next-to-next-to-leading-order quantum chromodynamics calculation. The predictions of all three calculations are in agreement with the experimental measurement within uncertainties, but can deviate from the data in some regions by up to 20%.
Thesis Advisor: Prof. Dongchul Son