8.3.1 Object Definition
Events are required to have at least one reconstructed vertex with two or more associated tracks abovepT >0.4 GeV. If more than one vertex is found, the one with the highest sum of associated trackpT’s is considered.
Leptons are defined in two categories. Leptons fulfilling basic quality and identification requirements (“baseline” leptons) enter the ETmiss calculation as well as the overlap removal procedure (described below) and are object to the veto of more than one lepton that features in the analysis. Tighter criteria are applied to define “signal” leptons, the leptons that are selected in the final state.
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Figure 8.5: Comparison of the efficiency of the HLT_xe80_tc_lcw_L1XE50 trigger between 2015 data and MC with an inclusive preselection (top row), requiring zero b-jets (middle row) or at least one b-jet (bottom row). Electron channel (left) and muon channel (right) behave similarly.
Baseline electrons are required to have pT > 7 GeV, |η| < 2.47, and to fulfil ‘VeryLoose’
likelihood identification criteria, described in Ref. [231]. Signal electrons have to pass the baseline selection and satisfy pT > 25GeV, as well as the ‘Loose’ likelihood identification criteria. Their impact parameters with respect to the reconstructed primary vertex are
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Figure 8.6: Comparison of the efficiency of theHLT_xe100_mht_L1XE50trigger between 2016 data and MC with an inclusive preselection (top row), requiring zero b-jets (middle row) or at least one b-jet (bottom row). Electron channel (left) and muon channel (right) behave very similarly.
constrained to|z0sinθ|<0.5mm and|d0|/σd0 <5, where σd0 denotes the uncertainty of d0. Furthermore, signal electrons need to be isolated [83].
Baseline muons are selected via pT > 6 GeV, |η| < 2.6. They have to match the ‘Loose’
identification criteria described in Ref. [80]. Apart from passing the baseline requirements, signal muons are required to have pT >25 GeV. Requirements on their impact parameters
Chapter 8. Search for New Physics in Events with Missing Energy and Top Quarks 189 are given by |z0sinθ|<0.5mm and|d0|/σd0 <3. As signal electrons, signal muons need to be isolated.
An identified photon is required in the selection of the t¯t +γ sample that is used in the data-driven estimation of the tt¯+Z background. Photon candidates need to satisfy the
‘Tight’ identification criteria described in Ref. [232]. In addition, they are required to have pT > 145 GeV and |η| < 2.37. In order to ensure that the photon trigger is fully efficient for the selected events, the transition region between detector barrel and end-cap located between 1.37<|η|< 1.52 is excluded. Furthermore, photons must satisfy ‘Tight’ isolation criteria based on both track and calorimeter information.
As for the leptons, “baseline” jets fulfilling looser quality requirements are defined to enter the overlap removal and theETmiss calculation, where “signal” jets are considered in the selection.
Baseline jets are required to have pT > 20 GeV, signal jets must have pT > 25GeV and
|η| < 2.5. Signal jets with pT < 60GeV have to pass further cuts that aim at rejecting jets originating from pileup [151]. Events containing a jet that does not pass specific jet quality requirements are vetoed from the analysis in order to suppress detector noise and non-collision backgrounds [152, 233].
Jets resulting fromb-quarks (b-jets) are tagged using the MV2c10b-tagging algorithm, which is based on quantities like impact parameters of associated tracks and reconstructed sec-ondary vertices [89, 234]. A working point of 77% b-tagging efficiency is chosen.
Hadronically decaying tau leptons must fulfil the ‘Loose’ identification criteria described in Refs. [91, 92]. These tau candidates are required to have one or three associated tracks, with a total electric charge opposite to that of the selected electron or muon in the event.
Furthermore, they are required to have pT >20 GeV, and |η|<2.5.
Apart from the identified (baseline) objects in the events, the soft-term enters the ETmiss calculation. For this analysis, it is constructed from track information: tracks that are associated with the primary vertex but not with the baseline physics objects are taken into account [94, 235]. For the photon selection, the calibrated photon is directly included in the ETmiss calculation. Otherwise, photons and hadronically decaying tau leptons enter as jets, electrons, or via the soft-term.
Large-radius jets are clustered from all signal jets (small-radius R = 0.4) using the anti-kt
algorithm with R = 1.0 or 1.2. To reduce the impact of soft radiation and pileup, the large-radius jets are groomed using reclustered jet trimming, with apT fraction of 5% [236].
Electrons and muons are not included in the reclustering: the background acceptance would increase more than the signal efficiency. While large-radius jets are not directly considered in the overlap removal procedure, the signal jets from which they are reclustered, need to pass. The large-radius jet mass is used in the analysis. Its square is defined as the square of the four-vector sum of the momenta of the contained small-radius jets.
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Overlap Removal
Detector signals might be interpreted and reconstructed as different physical objects. To avoid this double-labelling – and double-counting – of such objects, a so-called overlap re-moval procedure (OR), which was optimised for this analysis, is applied. A potential overlap is considered depending on shared tracks, ghost-matching [237] or radial distance of the ob-jects,∆R.
Electron/Muon: Some “Loose muon” objects are reconstructed including calorimetric in-formation and can also be reconstructed as electrons. If an electron and a muon share a track, the electron is removed, except if the muon is based on calorimetric information.
Lepton/Jet: If an electron and a jet are closer than∆R < 0.2, the jet is removed, except if it isb-tagged. A jet is considered to overlap with a muon, if the muon can be ghost-matched to a nearby jet. The object is only reconstructed as a muon if the jet is not b-tagged and if it has less than three tracks abovepT = 500 MeV or ifpmuonT /pjetT >0.7.
Jet/Lepton: If a jet, after the above steps, overlaps with a lepton in a cone of radius R= 0.04 + 10/p`T, up to a maximum radius of 0.4, the lepton is removed.
Taus and photons: Taus are only used in the computation of themτT2 variable to define a veto on them. If the event passes the tau veto, the tau object is no longer used and instead the jet object is considered for the rest of the computations. Photons are only used in the t¯t+γ control region. Jets overlapping with a photon in a cone with radius R = 0.2 are removed. In the rest of the regions photons are not considered and overlapping photon/jets are always treated as jets. If an electron, after the previous steps, overlaps with a tau candidate or a photon in a cone with radiusR= 0.1, the electron is taken.