Measurement of the single top production cross section in proton-antiproton collisions at 1.96 TeV

Abstract

This thesis describes a search for singly produced top quarks via an electroweak vertex in head-on proton-antiproton collisions at a center of mass energy of {radical}s = 1.96 TeV. The analysis uses a total of 2.3 fb{sup -1} of data collected with the D0 detector at Fermilab, corresponding to two different run periods of the Tevatron collider. Two channels contribute to single top quark production at the Tevatron, the s-channel and the t-channel. In the s-channel, a virtual W boson is produced from the aniquilation of a quark and an antiquark and a top and a bottom quarks are produced from the W decay. The top quark decays almost exclusively into a W boson and a bottom quark. Final states are considered in which the W boson decays leptonically into an electron or a muon plus a neutrino. Thus, at the detector level, the final state characterizing the s-channel contains one lepton, missing energy accounting for the neutrino, and two jets from the two bottom quarks. In the t-channel, the final state has an additional jet coming from a light quark. Clearly, a precise reconstruction of the events requires a precise measurement of the energy of the jets. A multivariate more » technique, Bayesian neural networks, is used to extract the single top signal from the overwhelming background still left after event selection. A Bayesian likelihood probability is then computed to measure the single top cross section. Assuming the observed excess is due to single top events, the measured single top quark production cross section is {sigma}(p{bar p} {yields} tb + X, tqb + X) = 4.70{sub -0.93}{sup +1.18} pb. The observed excess is associated with a p-value of (3.2 {+-} 2.3) x 10{sup -8}, assuming the background-only hypothesis. This p-value corresponds to an excess over background of 5.4 standard deviations for a Gaussian density. The p-value computed using the standard model signal cross section of 3.46 pb is (22.7 {+-} 0.6) x 10{sup -6}, corresponding to an expected significance of 4.08 standard deviations. « less