the contribution of the fake and non-prompt leptons. The residual prompt-lepton contributions in this region are estimated from MC simulation and subtracted from data, thus obtaining a “pure” sample of fake/non-prompt leptons.
In practice, it is necessary to estimate distributions of various observables of the fake/non-prompt leptons background instead of just the total yield. This is done by constructing the distributions from loose events in data by applying a per-event weight:
w= fake(real−c)
real−fake , (6.4)
wherec =1 for loose leptons that pass the tight criteria, andc=0 for loose leptons that do not pass tight criteria (loose not-tight).
In this analysis, therealandfake efficiencies used were measured in Ref. [178]. The efficiencies are parametrised as a function of multiple variables related to lepton kinematics and isolation:
• Electron efficiencies are parametrised as a function of electronpTand theETRcut as defined for thefixed-cutisolation in Sec.4.2.
• Muon efficiencies are parametrised as a function of muon pT, ETRcut and the distance to the closest jet.
Thefakeefficiency for electrons ranges from 18 % up to 92 % for high-pTelectrons and for muons from 4 % up to 94 % for high-pTmuons [178].
6.3 Event selection
The selection used in this analysis is based on the selection optimised in the search for heavy particles decaying intott¯pairs in [178], which includes a dedicated boosted region to reconstruct the hadronically decaying top quark as a large-Rjet and the leptonically decaying top quark from single lepton in event, a close-by small-Rjet, and missing transverse momentum.
Only events recorded under stable beam conditions and with all ATLAS detector sub-systems operations are considered. A reconstructed primary vertex is necessary and a single-electron or single-muon trigger must fire. The trigger selections are shown in Table6.2, and include selection on minimum lepton pT, identification WP and isolation. The trigger requirements during the 2016 data-taking period are tighter than in 2015. The differences arise from the increased instantaneous luminosity of the LHC and limits on the throughput of the ATLAS trigger and data acquisition system.
The electron identification WPs used in the triggers correspond to those described in Sec4.2and the isolation WPs are defined similarly as the fixed-cut track isolation in Sec4.2and4.3, but with a looser selection(2). The trigger requirements are imposed in both data and MC simulation.
The events are then required to pass the following selection criteria:
• At least onetightelectron ormediummuon withpT >30 GeV, passing thefixed-cutisolation selection, is required. These criteria are tighter than the trigger criteria, ensuring the trigger is fully-efficient in this kinematic regime. Events containing additional leptons withpT >25 GeV are rejected.
(2)The fraction of momentum carried by tracks in the cone surrounding the electron or muon candidatepvarRT cut/p`Tis allowed to be higher for looser isolation requirements of the triggers.
6. Measurement of signal efficiency of boosted top-quark andW-boson taggers
Table 6.2: Single-lepton triggers employed in the analysis, including theirpTthresholds, identification WP, isolation WP or any other criteria if applicable.
Electron triggers Muon triggers
Data period pTthreshold extra criteria pTthreshold extra criteria
2015 24 GeV mediumWP 20 GeV
60 GeV mediumWP 50 GeV
120 GeV looseWP
2016 and later 26 GeV tightWP, no|d0|/σ(d0)cut,
looseisolation 26 GeV mediumisolation
60 GeV mediumWP 50 GeV
140 GeV looseWP
• The events must haveETmiss > 20 GeV andETmiss+MTW > 60 GeV(3). This selection enhances the contribution of leptons originating fromW-boson decays.
• To ensure a topology consistent with semileptonically-decaying top quark, at least one small-R calorimeter jet with pT > 25 GeV and |η| < 2.5 is required, and must be close to the lepton;
∆R(jet, `) < 1.5.
• At least one R = 0.2 track jet in the event is required, that is identified as originating from B-hadron (b-tagged) based on theb-tagging selection described in Sec.4.4.5.
• At least one large-Rjet withpT > 200 GeV and |η| < 2.0 is required. It is expected, that the hadronically-decaying top quark is well separated from the semileptonically-decaying top-quark, hence the following angular separation criteria are required for the large-Rjet:
– Distance from the small-Rjet close to the lepton is required to be
∆R(small-Rjet,large-Rjet) >1.5.
– The transverse plane opening angle between the large-R jet and the isolated lepton
∆φ(large-Rjet, `) >2.3.
If multiple jets satisfy this selection, the highest-pTjet is considered.
The yields fortt, the individual backgrounds and the data are shown in Table¯ 6.3, separately for the electron and muon channel. The pre-selected sample is divided into two sub-samples based on the proximity of ab-tagged track-jet and the selected large-Rjet:
• The top-quark-enriched sample is selected by requiring∆R(b-jet,large-Rjet) < 1.0, enhancing the likelihood that the jet contains top-quark decay products. To further increase the fraction of fully-contained(4)top-quark jets, the large-Rjet must satisfypT >350 GeV requirement.
• The W-boson-enriched sample is selected by requiring ∆R(b-jet,large-Rjet) > 1.0. This selection enhances the selection oftt¯events, where theB-hadron from top-quark decay is not clustered in the large-R jet, thus the large-Rjet contains only the W-boson decay products,
(3)MTW =q
2pT`ETmiss(1−cos∆φ), where∆φis the opening angle between the lepton andETmissdirection in the plane perpendicular to the beam pipe.
(4)Fully-contained here refers to having all the top-quark decay products clustered within the large-Rjet.
6.3. Event selection
and can be used as a probe to studyW-tagging algorithms. TheW-boson-enriched sample is kinematically limited to large-Rjets withpTup to approximately 600 GeV. This is due to the selection acceptance decreasing with large-RjetpTdue to the decrease in the∆Rseparation between theB-hadron and the large-Rjet.
The total yield oftt, the individual backgrounds and the data, are shown in Table¯ 6.4for the two sub-samples, with the electron and muon channel yields combined. Finally, thet¯tand single top-quark MC samples are split according to thecontainedlabelling definition of the selected large-Rjet. Thett¯ MC sample is split into three categories,top,W andother, depending on whetherqq¯0bpartons are contained within the jet, or onlyqq¯0, or neither of the two options, respectively. Similarly, the single top-quark MC sample is split intoW andothercategories based on the same labelling criteria. The W category contains events from the single top-quarktW production, where the top-quark decays leptonically and theW-boson decays hadronically and is reconstructed as a large-Rjet.
Table 6.3: Yields for the individual predicted processes and data after the pre-selection, shown separately for the electron channel (e+jets column) and the muon channel (µ+jets column). Only the statistical uncertainties on the predicted yield corresponding toL=36.1 fb−1are shown(5).
Process e+jets µ+jets
t¯t 112350±220 121800±220 Single top-quark 11150± 70 11380± 70 W+jets 19040±110 21960±130 Z +jets 2552± 24 2310± 23
Diboson 765± 14 817± 14
Fake/non-prompt lep. 17400±400 7600±600 Total prediction 163200±500 165800±700
Data 130000 140000
A normalisation discrepancy is observed when comparing data with prediction, attributed to the known mismodelling of thepT of the top quark, where the MC simulation predicts harderpTspectrum than that observed in data, as documented in various comparisons oftt¯generator predictions with unfolded differential cross-section measurements [172,179,180]. This mismodelling translates for example into thepTof the leading large-Rjet, hence the selection on the minimum leading jetpTleads to a normalisation acceptance effect. To assess the possible impact of the large-RjetpT mismodelling on the shape of substructure observables, a data-driven reweighting test to compensate the leading-pT large-Rjet mismodelling intt¯is performed and documented in App.B. The impact of the reweighting on the shape of the investigated sub-structure observables is found to be small in the relevant regions of phase space. The normalisation mismodelling in different intervals of large-RjetpTis mitigated by the design of the template fit performed in the signal efficiency measurement, described in Sec.6.6.
(5)Unless, otherwise stated, it is assumed that the MC-predicted yields are always normalised to the integrated luminosity of the corresponding dataset. Therefore, for weighted predictions, i.e. the MC-simulated samples and the matrix-method-estimated fake/non-prompt leptons background, the statistical uncertainty of the prediction is calculated as the
sEvents
P
i
w2i, wherewiis the total weight of theithevent. The statistical uncertainty in particular for MC samples is typically smaller than the statistical uncertainty of data, since a larger number of events are generated than the number of data events.
6. Measurement of signal efficiency of boosted top-quark andW-boson taggers
Basic control distributions showing the level of agreement between data and prediction after the pre-selection and for the top-quark andW-boson-enriched sub-sample selections are shown in App.A.
Table 6.4: Yields for the individual predicted processes and data for the sub-samples selected for top-quark tagging (Top-quark selection) andW-boson tagging (W-boson selection). The yields in the electron and muon channel are combined together.
Process W-boson Top-quark
selection selection tt¯ 112680±220 30610±120 Single top-quark 12380± 70 2562± 31
W+jets 13140± 90 4180± 40
Z+jets 2011± 19 557± 8
Diboson 812± 14 297± 9
Fake/non-prompt lep. 14800±600 2690±180 Total prediction 155900±800 40900±290
Data 146400 35250