First Observation of the Production of a Single Top Quark in Association with a W boson

A search for the production of a single top quark produced in association with a W boson is presented. The analysis is based on data collected by the Compact Muon Solenoid detector at the Large Hadron Collider, located at CERN in Geneva, Switzerland. Data are recorded in 2011 and 2012 in proton-proton collisions at center of mass energies of 7 and 8 TeV. The analysis is performed in the dilepton final state, selecting events with two leptons and a jet originating from a b quark. A multivariate analysis is used to distinguish the signal from the dominant t ̄ t background, based on kinematic and topological differences between the two processes. In the analysis of 12.2 fb − 1 of data from 2012, the first ever observation of the single top tW associated production channel was made with a significance of 6.1 standard deviations above a background-only hypothesis. Using the full 2012 dataset, corresponding to an integrated luminosity 19.7 fb − 1 of proton-proton collisions at √ s = 8 TeV, the cross section and charge ratio of the tW channel are measured. The total production cross section for the tW signal is measured to be 25 . 8 +3 . 8 − 3 . 7 pb, in agreement with the theoretical prediction. The ratio between the cross sections of top and anti-top quarks in the tW channel is measured to be R = 0 . 73 +0 . 24 − 0 . 22 , compared to a theoretical prediction of R = 1 .