Observation of the associated production of a top quark and a Z boson in pp collisions at s $$ \sqrt{s} $$ = 13 TeV wi
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Springer
Received: February 19, 2020 Accepted: June 10, 2020 Published: July 20, 2020
The ATLAS collaboration E-mail: [email protected] Abstract: Single top-quark production in association with a Z boson, where the Z boson decays to a pair of charged leptons, is measured in the trilepton channel. The proton-proton collision data collected by the ATLAS experiment from 2015 to 2018 at a centre-of-mass energy of 13 TeV are used, corresponding to an integrated luminosity of 139 fb−1 . Events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a b-hadron, are selected. The main backgrounds are from tt¯Z and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for t`+ `− q production, including nonresonant dilepton pairs with m`+ `− > 30 GeV, is 97 ± 13 (stat.) ± 7 (syst.) fb, consistent with the Standard Model prediction. Keywords: Hadron-Hadron scattering (experiments) ArXiv ePrint: 2002.07546
Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. 3 Article funded by SCOAP .
https://doi.org/10.1007/JHEP07(2020)124
JHEP07(2020)124
Observation of the associated production of a top √ quark and a Z boson in pp collisions at s = 13 TeV with the ATLAS detector
Contents 1
2 ATLAS detector
2
3 Data and simulation samples
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4 Object reconstruction
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5 Signal and control regions
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6 Background estimation
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7 Multivariate analysis
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8 Systematic uncertainties
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9 Results
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10 Conclusion
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A Validation regions
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The ATLAS collaboration
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1
Introduction
This paper reports on the observation of the electroweak production of a top quark or an anti-top quark associated with a Z boson (tZq) by the ATLAS collaboration using a data sample corresponding to an integrated luminosity of 139 fb−1 of proton-proton (pp) √ collisions at a centre-of-mass energy of s = 13 TeV. The final state is where the Z boson decays into electrons or muons, and the W boson, from the top-quark decay, decays into an electron or muon and an associated neutrino, and also includes the contribution from τ -lepton decays into electrons or muons. The requirement of three leptons in the final state maximises the signal significance relative to the backgrounds. Evidence for this process has previously been reported by the ATLAS collaboration [1] with a significance of 4.2 standard deviations using data collected in 2015 and 2016. The CMS collaboration [2] reported an observation with a measured cross-section uncertainty of 15% using data collected in 2016 and 2017. At leading order (LO) in the Standard Model (SM) both the single top-quark production and decay occur through the electroweak interaction. The main LO Feynman
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JHEP07(2020)124
1 Introduction
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(c) Figure 1. Example Feynman diagrams of the lowest-order amplitudes for the tZq process, correspond
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