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 toverline{t}Z and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for tℓ+ℓ−q production, including non-resonant dilepton pairs with {m}_{{mathrm{ell}}^{+}{mathrm{ell}}^{-}} > 30 GeV, is 97 ± 13 (stat.) ± 7 (syst.) fb, consistent with the Standard Model prediction.
Highlights
Background estimationTwo classes of backgrounds are considered: processes in which three or more prompt leptons are produced, such as diboson production or the associated production of a top-quark pair and a boson (W, Z or H ); and processes with only two prompt leptons in the final state and one additional non-prompt or fake lepton that satisfies the selection criteria
Inelastic collisions were simulated using Pythia 8.186 [16] with the A3 set of tuned parameters [17] and the NNPDF2.3 LO [18] set of parton distribution functions (PDFs), and overlaid on the signal and background Monte Carlo (MC) samples
Such non-prompt or fake leptons can originate from decays of bottom or charm hadrons, from jets misidentified as electrons, leptons from kaon or pion decays, or electrons from photon conversions
Summary
The ATLAS detector [4] at the LHC covers nearly the entire solid angle around the collision point. It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting toroidal magnets. The ATLAS detector [4] at the LHC covers nearly the entire solid angle around the collision point.1 It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting toroidal magnets. The high-granularity silicon pixel detector covers the vertex region and typically provides four measurements per track, the first hit being normally in the insertable B-layer installed before Run 2 [5, 6]. It is followed by the silicon microstrip tracker, which usually provides eight measurements per track. The first-level trigger selects events from the 40 MHz bunch crossings at a rate below 100 kHz, which the high-level trigger further reduces to record events to disk at about 1 kHz
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