CMS-PAS-TOP-16-020

CMS-PAS-TOP-16-020
Evidence for the standard model production of a Z boson with a single top quark in pp collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
September 2017

Abstract: Evidence for the standard model production of a Z boson in association with a single top quark is presented. The study uses a data sample of proton-proton collisions at $\sqrt{s}= $ 13 TeV recorded in 2016 by the CMS experiment, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Final states with three leptons (electrons or muons) and at least two jets are investigated. The corresponding measured tZq measured cross section is $\sigma (\mathrm{pp}\rightarrow\mathrm{tZq}\rightarrow\mathrm{Wb}\ell^+\ell^-\mathrm{q}) = $ 123$^{+44}_{-39}$ fb, where $\ell$ stands for electrons, muons, and taus, with an observed (expected) significance of 3.7 (3.1) standard deviations.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 779 (2018) 358. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-a: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-b: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-c: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-d: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-e: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 1-f: Leading order tZq production diagrams. The initial- and final-state quarks, denoted q and q', are predominantly first-generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (c) represents the non-resonant contribution to the tZq process.
png pdf	Figure 2: Normalised distributions of the BDT output for signal (thick lines) and backgrounds (thin lines). The discriminants including (excluding) MEM variables in the BDT training are shown as dashed (solid) lines for the 1 bjet (left) and 2 bjets (right) regions. Contributions from all channels are included.
png pdf	Figure 2-a: Normalised distributions of the BDT output for signal (thick lines) and backgrounds (thin lines). The discriminants including (excluding) MEM variables in the BDT training are shown as dashed (solid) lines for the 1 bjet (left) and 2 bjets (right) regions. Contributions from all channels are included.
png pdf	Figure 2-b: Normalised distributions of the BDT output for signal (thick lines) and backgrounds (thin lines). The discriminants including (excluding) MEM variables in the BDT training are shown as dashed (solid) lines for the 1 bjet (left) and 2 bjets (right) regions. Contributions from all channels are included.
png pdf	Figure 3: Data-to-prediction comparisons in the 1 bjet (signal-enriched) region (upper row) and in the 2 bjets region (bottom row) for the highest CSVv2 discriminator among all selected jets (left), the negative values of the logarithm of the MEM score associated to the most probable tZq kinematic configuration (center), and the $\Delta R$ separation between the b quark and recoiling jets (right). The distributions include events from all final states. Underflows and overflows are included in the first and last bins, respectively. The predictions correspond to the normalisations obtained after the fit described in Section xxxxx. The hatched bands include the total uncertainty on the background and signal samples.
png pdf	Figure 4: Data-to-prediction comparisons in the 0 bjet region for the $\eta $ (left) and ${p_{\mathrm {T}}}$ (centre) of the recoiling jet, and the additional lepton asymmetry (right). More details are given in the caption of Fig. yyyyy.
png pdf	Figure 5: Template distributions used for signal extraction. Left: BDT discriminant in the 1 bjet region; centre: BDT discriminant in the 2 bjets control region; ${m_{\rm T}^\mathrm{W}}$ in the 0 bjet control region.

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

Summary

The associated production cross section of a top quark and a Z boson is measured using data from pp collisions at 13 TeV collected by the CMS experiment. The measurement uses events containing three charged leptons in the final state. Evidence for tZq production is found with an observed (expected) significance of 3.7 (3.1). The cross section is measured to be $\sigma({\mathrm{t} \ell^+\ell^- \mathrm{q}} ) = $ 123$^{+33}_{-31}$ (stat) $^{+29}_{-23}$ (syst) fb, compatible with the NLO SM prediction of 94.2 $\pm$ 3.1 fb.

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