CMS-TOP-16-020

CMS-TOP-16-020 ; CERN-EP-2017-296
Measurement of the associated production of a single top quark and a Z boson in pp collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
8 December 2017
Phys. Lett. B 779 (2018) 358
Abstract: A measurement is presented of the associated production of a single top quark and a Z boson. The study uses data from proton-proton collisions at $\sqrt{s} = $ 13 TeV recorded by the CMS experiment, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Using final states with three leptons (electrons or muons), the tZq production cross section is measured to be $\sigma ({\mathrm{p}}{\mathrm{p}}\to\mathrm{t}\mathrm{Z}\mathrm{q}\to\mathrm{W}\mathrm{b}\ell^+\ell^-\mathrm{q}) = $ 123$^{+33}_{-31}$ (stat) $^{+29}_{-23}$ (syst) fb, where $\ell$ stands for electrons, muons, or $\tau$ leptons, with observed and expected significances of 3.7 and 3.1 standard deviations, respectively.
Links: e-print arXiv:1712.02825 [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 $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagrams. The lower right-hand diagram represents the non-resonant contribution to the $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ process.
png pdf	Figure 1-a: Leading-order $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagram.
png pdf	Figure 1-b: Leading-order $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagram.
png pdf	Figure 1-c: Leading-order $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagram.
png pdf	Figure 1-d: Leading-order $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagram.
png pdf	Figure 1-e: Leading-order $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ production diagram.
png pdf	Figure 1-f: Leading-order diagram representing the non-resonant contribution to the $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ process.
png pdf	Figure 2: Normalized distributions of the BDT output for signal (thick lines) and backgrounds (thin lines) from simulation for the 1bjet (left) and 2bjets (right) regions. The discriminators including and excluding MEM variables in the BDT training are shown, respectively, as dashed and solid lines. Contributions from the four considered channels are included in the signals and backgrounds.
png pdf	Figure 2-a: Normalized distributions of the BDT output for signal (thick lines) and backgrounds (thin lines) from simulation for the 1-bjet region. The discriminators including and excluding MEM variables in the BDT training are shown, respectively, as dashed and solid lines. Contributions from the four considered channels are included in the signals and backgrounds.
png pdf	Figure 2-b: Normalized distributions of the BDT output for signal (thick lines) and backgrounds (thin lines) from simulation for the 2-bjets region. The discriminators including and excluding MEM variables in the BDT training are shown, respectively, as dashed and solid lines. Contributions from the four considered channels are included in the signals and backgrounds.
png pdf	Figure 3: Data-to-prediction comparisons in the 1bjet region (signal-enriched, upper row) and in the 2bjets region (lower row) for the largest CSVv2 discriminant value among all selected jets (left), the logarithm of the MEM score associated to the most probable $ {\mathrm{t} \mathrm{Z} \mathrm{q}} $ kinematic configuration (centre), and the $\Delta R$ separation between the b quark and the recoiling jet (right). The distributions include events from all final states. Underflows and overflows are shown in the first and last bins, respectively. The predictions correspond to the normalizations obtained after the fit described in Section xxxxx. The hatched bands include the total uncertainty on the background and signal contributions. The pulls in the distributions are shown in the bottom panels.
png pdf	Figure 4: Data-to-prediction comparisons in the 0bjet region for the $\eta $ (left) and ${p_{\mathrm {T}}}$ (centre) distributions of the recoiling jet, and for the asymmetry of the top quark decay lepton (right). More details are given in the caption of Fig. yyyyy.
png pdf	Figure 5: Template distributions used for signal extraction. Left: BDT discriminator in the 1bjet region; centre: BDT output in the 2bjets control region; right: ${m_\mathrm {T}^\mathrm{W}} $ in the 0bjet control region. More details are given in the caption of Fig. yyyyy.

Tables
png pdf	Table 1: Observed and post-fit expected yields for each production process in the 1bjet region. The yields of columns 2-5 correspond to each channel, and column 6 displays the total for all channels. The last column displays the ratio between post-fit and pre-fit yields.

Summary

The associated production cross section of a single top quark and a Z boson was measured using data from pp collisions at 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The measurement uses events containing three charged leptons in the final state. Evidence for $ {\mathrm{t}\mathrm{Z} \mathrm{q}} $ production is found with an observed (expected) significance of 3.7\,(3.1) standard deviations. The cross section is measured to be $\sigma({\mathrm{t}\ell^+\ell^- \mathrm{q}} )= $ 123$^{+33}_{-31}$ (stat) $^{+29}_{-23}$ (syst) fb, for ${m_{\ell^+\ell^-}} > $ 30 GeV, where $\ell$ stands for electrons, muons, or $\tau$ leptons. This value is compatible with the next-to-leading-order standard model prediction of 94.2 $\pm$ 3.1 fb.

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