CMS-TOP-20-002

CMS-TOP-20-002 ; CERN-EP-2021-131
Observation of tW production in the single-lepton channel in pp collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
3 September 2021
JHEP 11 (2021) 111
Abstract: A measurement of the cross section of the associated production of a single top quark and a W boson in final states with a muon or electron and jets in proton-proton collisions at $\sqrt{s} = $ 13 TeV is presented. The data correspond to an integrated luminosity of 36 fb$^{-1}$ collected with the CMS detector at the CERN LHC in 2016. A boosted decision tree is used to separate the tW signal from the dominant $\mathrm{t\bar{t}}$ background, whilst the subleading W+jets and multijet backgrounds are constrained using data-based estimates. This result is the first observation of the tW process in final states containing a muon or electron and jets, with a significance exceeding 5 standard deviations. The cross section is determined to be 89 $\pm$ 4 (stat) $\pm$ 12 (syst) pb, consistent with the standard model.
Links: e-print arXiv:2109.01706 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order Feynman diagrams for single top quark production in the tW channel. Charge conjugate states are implied.
png pdf	Figure 1-a: Leading-order Feynman diagram for single top quark production in the tW channel. Charge conjugate states are implied.
png pdf	Figure 1-b: Leading-order Feynman diagram for single top quark production in the tW channel. Charge conjugate states are implied.
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. Charge conjugate states are implied.
png pdf	Figure 2-a: Feynman diagram for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. Charge conjugate states are implied.
png pdf	Figure 2-b: Feynman diagram for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. Charge conjugate states are implied.
png pdf	Figure 2-c: Feynman diagram for tW single top quark production at next-to-leading order that are removed from the signal definition in the DR scheme. Charge conjugate states are implied.
png pdf	Figure 3: The ${p_{\mathrm {T}}}$ of the selected muon (left) and electron (right) in the signal region of their respective channels. The signal and backgrounds have been scaled with the results of the final fit. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the statistical uncertainty from the limited size of the simulated samples for each bin.
png pdf	Figure 3-a: The ${p_{\mathrm {T}}}$ of the selected muon in the corresponding signal region. The signal and backgrounds have been scaled with the results of the final fit. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the statistical uncertainty from the limited size of the simulated samples for each bin.
png pdf	Figure 3-b: The ${p_{\mathrm {T}}}$ of the selected electron in the corresponding signal region. The signal and backgrounds have been scaled with the results of the final fit. The lower panel shows the ratio of observed data to the prediction for signal and background. In both panels the hatched regions show the statistical uncertainty from the limited size of the simulated samples for each bin.
png pdf	Figure 4: BDT discriminant in the signal region for the muon (left) and electron (right) channels for the (from upper to lower) 3j, 2j and 4j regions. The upper 3j region is considered the nominal signal region, while the remaining 2j and 4j regions are considered control regions, enhanced in W+jets and QCD multijet, and ${\mathrm{t} {}\mathrm{\bar{t}}}$ background events, respectively. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-a: BDT discriminant in the signal region for the muon channel for the 3j region. The 3j region is considered the nominal signal. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-b: BDT discriminant in the signal region for the electron channel for the 3j region. The 3j region is considered the nominal signal. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-c: BDT discriminant in the signal region for the muon channel for the 2j region. The 2j region is considered a control region, enhanced in W+jets and QCD multijet. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-d: BDT discriminant in the signal region for the electron channel for the 2j region. The 2j region is considered a control region, enhanced in W+jets and QCD multijet. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-e: BDT discriminant in the signal region for the muon channel for the 4j region. The 4j region is considered a control region, enhanced in ${\mathrm{t} {}\mathrm{\bar{t}}}$ background events. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.
png pdf	Figure 4-f: BDT discriminant in the signal region for the electron channel for the 4j region. The 4j region is considered a control region, enhanced in ${\mathrm{t} {}\mathrm{\bar{t}}}$ background events. The shape of the discriminant for the tW signal multiplied by 10 is overlayed. The signal and backgrounds have been scaled with the results of the fit. 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 of the prediction.

Tables
png pdf	Table 1: The total number of events passing event selection in each analysis region 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 estimation of the QCD multijet background comes from a data-based method, whilst the W+jets background is estimated from simulation and corrected using data. All other processes are estimated from simulation only. The single t background is comprised of the $t$- and $s$-channel single top quark processes.
png pdf	Table 2: Relative uncertainty in the measured cross section from each source of systematic uncertainty for the combination of the muon and electron channels. The table is divided into experimental, normalization, and theoretical uncertainties. Uncertainties arising from the limited size of the simulated samples are included in the statistical uncertainty.

Summary

The first observation of the associated production of a single top quark and a W boson in the single-lepton channel containing a muon or electron and jets is presented. The cross section is extracted using a binned likelihood fit of the discriminant from a boosted decision tree designed to separate the signal from the dominant top quark and antiquark pair background. The analysis is performed using proton-proton 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 36 fb$^{-1}$.

The cross section is 89 $\pm$ 4 (stat) $\pm$ 12 (syst) pb, with a significance exceeding 5 standard deviations, which is compatible with both the standard model predictions at approximate next-to-next-to-leading order in quantum chromodynamics of 71.7 $\pm$ 1.8 (scale) $\pm$ 3.4 (PDF) pb and at approximate next-to-next-to-next-to-leading order of 79.5$^{+1.9}_{-1.8}$ (scale)$^{+2.0}_{-1.4}$ (PDF) pb.

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