CMS-PAS-TOP-20-002

CMS-PAS-TOP-20-002
Measurement of tW production in the semileptonic channel in pp collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
March 2021

Abstract: A measurement of the associated production of a single top quark and a W boson in the final states with an electron or muon and jets using pp collisions with $\sqrt{s}= $ 13 TeV collected by the CMS detector at the CERN LHC is presented. The data used correspond to an integrated luminosity of 35.9 fb$^{-1}$. This result is the first observation of the tW process in the final states containing a muon or electron and jets, with an observed significance clearly exceeding 5 standard deviations. The measured signal strength is $\mu = $ 1.24 $\pm$ 0.18, consistent with unity. The inclusive cross section is determined to be 89 $\pm$ 4 (stat) $ \pm$ 12 (syst) pb.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order Feynman diagrams for single top quark production in the tW mode. The charge-conjugate modes are implicitly included.
png pdf	Figure 2: Feynman diagrams for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. The charge-conjugate modes are implicitly included.
png pdf	Figure 2-a: Feynman diagrams for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. The charge-conjugate modes are implicitly included.
png pdf	Figure 2-b: Feynman diagrams for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. The charge-conjugate modes are implicitly included.
png pdf	Figure 2-c: Feynman diagrams for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. The charge-conjugate modes are implicitly included.
png pdf	Figure 3: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-a: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-b: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-c: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-d: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-e: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.
png pdf	Figure 3-f: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from top to bottom) 3j, 2j and 4j regions. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the total uncertainty in each bin. The signal and backgrounds have been scaled with the results of the fit.

Tables
png pdf	Table 1: The total number of events passing event selection in each defined jet topology region for the analysis and their associated statistical uncertainties. The event yields are given for the tW signal and all major backgrounds for both the muon (upper) and electron (lower) channels. The estimations of QCD multijet and W+jets backgrounds include data-based estimates.
png pdf	Table 2: Descriptions of the variables used to train and evaluate the BDT, ranked in order of importance in the final result. The same variables are used in both muon and electron channels.
png pdf	Table 3: Uncertainty in the signal strength from each source of systematic uncertainty for the combination of electron and muon channels. The table is divided between normalization, experimental and theoretical uncertainties. Uncertainties arising from the limited size of the MC samples are included in the statistical uncertainty.

Summary

In this note, the first experimental investigation into the tW process using the semileptonic channel by the CMS Collaboration is presented. The inclusive cross section is extracted using a binned likelihood fit of the discriminant from a multivariate analysis designed to separate the signal from the dominant $\mathrm{t\bar{t}}$ background. The analysis is performed using pp collision data at a centre-of-mass energy of 13 TeV recorded by the CMS detector at the LHC corresponding to an integrated luminosity of ${\mathcal{L}}$.

This result is the first observation of the tW process in the final states containing a muon or electron and jets, with an observed significance clearly exceeding 5 standard deviations. The measured signal strength is $\mu = $ 1.24 $\pm$ 0.18, consistent with unity. The inclusive cross section is determined to be 89 $\pm$ 4 (stat) $ \pm$ 12 (syst) pb.

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