Study of Single Top Quarks in Association with Vector Bosons

The search for single top production in association with a massive electroweak vector bosonusing data collected by the CMS detector at the Large Hadron Collider is presented. Twoanalyses are discussed: the search for a single top produced in association with a W boson(tW production) and the search for t-channel single top production with a radiated Zboson (tZq production). Both analyses make use of proton-proton collision data at acentre-of-mass energy of 8 TeV, and focus on the leptonic decays of the vector bosons. Acut and count based approach is employed for the tW search, searching for a final statecontaining two leptons, two neutrinos and a jet originating from the decay of a b-quark.The main backgrounds to be understood and controlled in this analysis are those arisingfrom the production of top quark pairs and Z bosons with one or more jets. Using a set ofdata corresponding to an integrated luminosity of 12.2 fb−1 , a production cross section of33.9 ± 8.6 pb was measured for the process. This corresponds to an observed (expected)signal excess of 3.6σ (2.8+0.9 σ) over the background-only hypothesis.−0.8The tZq search was performed on events containing three leptons, one neutrino and twoor three jets, at least one of which originated from the decay of a b-quark. A multivariateanalysis based on the kinematic properties of the selected events is used to separate thesignal from the main backgrounds: WZ diboson production and the production of a topquark pair in association with a Z boson. Additionally, backgrounds arising from topquark pair production and the production of a Z boson with additional jets are estimatedfrom data. Using a set of data corresponding to an integrated luminosity of 19.7 fb−1 , aproduction cross section of 783+1000 fb was measured for the process. This corresponds to−543an observed (expected) signal excess of 3.432σ (1.389 ± 0.005σ) over the background-onlyhypothesis. This constitutes the first evidence for this rare Standard Model process at theLHC.