CMS-PAS-TOP-18-008

CMS-PAS-TOP-18-008
Observation of single top quark production in association with a Z boson in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
December 2018

Abstract: The observation of the production of a single top quark in association with a Z boson (tZq) is reported. Events from proton-proton (pp) collisions at a center-of-mass energy of 13 TeV containing three charged leptons (either electrons or muons) and at least two jets are analyzed. The data, collected with the CMS detector in 2016 and 2017 and corresponding to an integrated luminosity of 77.4 fb$^{-1}$, are used. The observed significance of the tZq signal is well over five standard deviations, thus unequivocally excluding the background-only hypothesis. The measured tZq production cross section in the leptonic decay mode of the Z boson is $\sigma (\text{pp} \to \text{tZq} \to \text{t} \ell^{+} \ell^{-} \text{q})= $ 111 $_{-13}^{+13}$ (stat) $_{-9}^{+11}$ (syst) fb, in agreement with the standard model expectation. The increased integrated luminosity, a multivariate lepton identification, and a redesigned analysis strategy improve significantly the sensitivity of the analysis compared to previous measurements of tZq production.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 122 (2019) 132003. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Observed (points) and post-fit expected (shaded histograms) BDT distributions for events in SR-2/3j-1b (left), SR-4j-1b (middle), and SR-2b (right). The vertical bars on the points represent the statistical uncertainties in the data. The hatched regions show the total uncertainties in the background. The lower panels display the ratio of the observed data to the predictions, including the tZq signal, with light-blue and yellow bands, respectively, representing the statistical and total uncertainties in the predictions.
png pdf	Figure 1-a: Observed (points) and post-fit expected (shaded histograms) BDT distributions for events in SR-2/3j-1b. The vertical bars on the points represent the statistical uncertainties in the data. The hatched regions show the total uncertainties in the background. The lower panel displays the ratio of the observed data to the predictions, including the tZq signal, with light-blue and yellow bands, respectively, representing the statistical and total uncertainties in the predictions.
png pdf	Figure 1-b: Observed (points) and post-fit expected (shaded histograms) BDT distributions for events in SR-4j-1b. The vertical bars on the points represent the statistical uncertainties in the data. The hatched regions show the total uncertainties in the background. The lower panel displays the ratio of the observed data to the predictions, including the tZq signal, with light-blue and yellow bands, respectively, representing the statistical and total uncertainties in the predictions.
png pdf	Figure 1-c: Observed (points) and post-fit expected (shaded histograms) BDT distributions for events in SR-2b. The vertical bars on the points represent the statistical uncertainties in the data. The hatched regions show the total uncertainties in the background. The lower panel displays the ratio of the observed data to the predictions, including the tZq signal, with light-blue and yellow bands, respectively, representing the statistical and total uncertainties in the predictions.
png pdf	Figure 2: Observed and post-fit expected distributions in SR-2/3j-1b events for the two most important discriminating variables, the maximum dijet mass among all pairs of jets in the event (left), and the ${ \| \eta \|}$ of the recoiling jet (middle). The right plot shows the ${p_{\mathrm {T}}}$ of the Z boson, reconstructed from its leptonic decay products, for events with BDT discriminant values in excess of 0.5 in SR-2/3j-1b events.
png pdf	Figure 2-a: Observed and post-fit expected distributions in SR-2/3j-1b events for one of the two most important discriminating variables: the maximum dijet mass among all pairs of jets in the event.
png pdf	Figure 2-b: Observed and post-fit expected distributions in SR-2/3j-1b events for one of the two most important discriminating variables: the ${ \| \eta \|}$ of the recoiling jet.
png pdf	Figure 2-c: The plot shows the ${p_{\mathrm {T}}}$ of the Z boson, reconstructed from its leptonic decay products, for events with BDT discriminant values in excess of 0.5 in SR-2/3j-1b events.

Tables
png pdf	Table 1: Post-fit expected background and tZq signal event yields with their total uncertainties, and observed number of events in data in each of the signal regions.

Summary

In summary, we have reported on a search for single top quark production in association with a Z boson, tZq. The search uses the leptonic tZq decay mode and leads to the observation of tZq production with an observed significance of well over five standard deviations, thus unequivocally excluding the background-only hypothesis. The tZq production cross section in the leptonic decay mode of the Z boson is measured to be $\sigma (\text{pp} \to \text{tZq} \to \text{t} \ell^{+} \ell^{-} \text{q})= $ 111 $_{-13}^{+13}$ (stat) $_{-9}^{+11}$ (syst) fb, for dilepton masses above 30 GeV, in agreement with the standard model prediction.

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