Observation of the associated production of a single top quark and a W boson in pp collisions at sqrt(s) = 8 TeV

TL;DR

This CMS study reports the first observation of associated production of a single top quark and a W boson (tW) in pp collisions at $\sqrt{s}=8~\text{TeV}$, using $12.2~\text{fb}^{-1}$ of data. The dilepton-channel selection with a $b$-tagged jet is analyzed with a boosted decision tree (BDT) multivariate classifier to separate tW from the dominant $t\overline{t}$ background, complemented by cross-check analyses based on a single-variable fit and event counts. A simultaneous fit to the three dilepton final states yields an observed significance of $6.1\sigma$ and a cross section of $23.4\pm5.4~\text{pb}$, consistent with the SM prediction of $22.2\pm0.6~(scale)\pm1.4~(PDF)~\text{pb}$. From this, $|V_{tb}| = 1.03\pm0.12~(\text{exp})\pm0.04~(\text{th})$, with a 95% C.L. lower bound $|V_{tb}|>0.78$. The result provides a direct test of electroweak single-top production and constrains non-SM Wtb couplings.

Abstract

The first observation of the associated production of a single top quark and a W boson is presented. The analysis is based on a data set corresponding to an integrated luminosity of 12.2 inverse femtobarns of proton-proton collisions at sqrt(s) = 8 TeV recorded by the CMS experiment at the LHC. Events with two leptons and a jet originating from a b quark are selected. A multivariate analysis based on kinematic and topological properties is used to separate the signal from the dominant t t-bar background. An excess consistent with the signal hypothesis is observed, with a significance which corresponds to 6.1 standard deviations above a background-only hypothesis. The measured production cross section is 23.4 +- 5.4 pb, in agreement with the standard model prediction.

Figures (3)

• Figure 1: The number of loose jets in the event and the $p_{\mathrm{T}}$ of the system ($p_{\mathrm{T}}\xspace^{\text{sys}}$) composed of the jet, leptons, and $E_{\mathrm{T}}^{\text{miss}}$, in the signal region ($1j1t$) for all final states combined. Shown are data (points) and simulation (histogram). The hatched band represents the combined effect of all sources of systematic uncertainty.
• Figure 2: The BDT discriminant, in the signal region ($1j1t$) and control regions ($2j1t$ and $2j2t$) for all final states combined. Shown are data (points) and simulation (histogram). The hatched band represents the combined effect of all sources of systematic uncertainty.
• Figure 3: Event yields in data and simulation for events passing additional requirements from the cross-check analyses. Yields are shown in the $1j1t$ signal regions and $2j1t$ and $2j2t$ control regions for a combination of all three final states, with the simulation scaled to the outcome of the statistical fit from the event-count analysis. The hatched band represents the combined effect of all systematic uncertainties on the event yields.