Abstract

A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of sqrt{s} = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb−1. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.

Highlights

  • This paper presents a search for new phenomena in events with top quark pairs, in a final state with exactly one isolated charged lepton from the decay of an on- or off-shell W boson, jets, and a significant amount of missing transverse momentum, the magnitude of which is denoted by ETmiss

  • The ATLAS detector [47] at the Large Hadron Collider (LHC) is a multipurpose particle detector with almost 4π coverage in solid angle around the interaction point.3. It consists of an inner tracking detector (ID) surrounded by a superconducting solenoid providing a 2 T axial magnetic field, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS), which is based on three large air-core toroidal superconducting magnets consisting of eight coils each

  • This paper presents searches for direct stop pair production covering various regions of SUSY phase space and searches for a spin-0 mediator decaying into pair-produced dark-matter particles

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Summary

Introduction

This paper presents a search for new phenomena in events with top quark pairs, in a final state with exactly one isolated charged lepton (electron or muon, referred to as ‘lepton’) from the decay of an on- or off-shell W boson, jets, and a significant amount of missing transverse momentum (pTmiss), the magnitude of which is denoted by ETmiss This experimental signature may arise in Supersymmetry (SUSY) [1,2,3,4,5,6,7] or in models with a spin-0 mediator produced in association with top quarks [8, 9] and subsequently decaying into a pair of dark matter (DM) particles. Searches for spin-0 mediators produced in association with heavy-flavour quarks and decaying into a pair of DM particles have been reported by the ATLAS [29, 41] and CMS [42] Collaborations

Signal models and search strategy
ATLAS detector and data collection
Simulated event samples
Event reconstruction
Discriminating variables
Dileptonic ttreconstruction
Reconstruction of hadronic top decays
Backgrounds with mismeasured missing momentum
Signal regions
Dark matter
Backgrounds
Systematic uncertainties
10 Results
11 Interpretations
12 Conclusion
Full Text
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