Measurement of the electron charge asymmetry in inclusive W production in pp collisions at √s = 7 TeV in the CMS experiment.

Abstract

In this thesis, two measurements of the electron charge asymmetry in inclusive W boson production with the CMS detector are presented. The measurements are obtained from proton-proton collision data with sqrt(s) = 7 TeV. The first measurement is performed with data corresponding to an integrated luminosity of 36 pb-1 collected by the CMS detector in 2010 and the second one uses data corresponding to 840 pb-1 collected during the first half of 2011. In proton-proton collisions, more W+ bosons are produced relative to W- due to the prevalence of up-type quarks with respect to down-type valence quarks in the proton. A measurement of this asymmetry as a function of the boson rapidity can provide valuable information on the u/d ratio within the proton. Since the boson rapidity cannot be directly measured due to the longitudinal momentum carried by the undetected neutrino, the asymmetry is measured as a function of the pseudorapidity of the charged lepton, in this case the electron, from the W decay. In both measurements, events are selected by requiring a single electron with tight selection criteria on quality of identification and measurement of the energy of the electron. The signal yield is extracted using extended maximum likelihood fits to the missing transverse energy spectrum using a set of reference template shapes for the signal, electroweak background and QCD background. The templates are obtained using Monte Carlo simulation, corrected with information from collision data events (for the signal and electroweak backgrounds), and a control sample of events obtained from an inverted selection (for the QCD background). In the first measurement the charge asymmetry is measured in 6 bins of the absolute value of the electron’s pseudorapidity and compared with predictions from theory. The statistical error ranges from 0.006 to 0.010. The increased amount of data in the second measurement allows the results to be presented in 11 bins of the absolute value of the electron’s pseudorapidity. The statistical error has been reduced and ranges from 0.003 to 0.004 and the global error is in the range 0.006 to 0.014.

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