5.4.1
Object selection
Electrons and muons are identified using identification, isolation, and tracking criteria described in Section 3.3. Three levels of object selection are used for electrons and muons, described in Table 5.3. Each level “baseline”, “Z”, and “W” leptons applies the selection of the previous levels along with additional criteria. The baseline leptons use the looser identification criteria and lower leptonpTin order to provide a higher efficiency of identifying and removing processes decaying to four prompt leptons (four-lepton veto requirement). Leptons associated with theW and Z boson must satisfy stricter criteria. The selection on the lepton associated with theW boson has the most stringent criteria to suppress the reducible backgrounds,Z+jets andZ+γ, which have a fake lepton associated with theW.
Baseline electrons must have pT>7 GeV and fall within the inner detector, |η|<2.5, as well as within the electromagnetic calorimeter, |ηcluster| < 2.47. The electrons must also satisfy the
LooseAndBLayerLLHquality criteria, which has an efficiency of 84-96% for electrons with 10< pT< 80 GeV . Electrons need to pass the impact parameter cuts of |z0sinθ| < 0.5 and |d0/σd0| < 5, designed to suppress fake electrons from pileup jets. Finally, the electrons must satisfy a track isolation requirement,LooseTrackOnly.
Electrons satisfying the Z electron criteria must fulfill the additional criteria of a tighter pT threshold, pT >10 GeV, a tighter identification criteria, MediumLH, which has an efficiency of 72- 93% for electrons with 10 < pT <80 GeV, and tighter isolation, GradientLoose. A calorimeter crack veto is also applied, where 1.37<|ηcluster|<2.47 is excluded, which allows for more efficient ZZ rejection.
Electrons passing theW electron criteria must pass thepTthreshold,pT>20, and even tighter ID and isolation criteria,TightLHandGradientrespectively, which has 68-88% efficiency for elec- trons with 10< pT<80 GeV.
Baseline muons must havepT>7 GeV and fall within the inner detector,|η|<2.5. They must also satisfy the Loose quality criteria and pass the impact parameter cuts of |z0sinθ| < 0.5 and
|d0/σd0|<3. Finally, the muons must satisfy a track isolation requirement,LooseTrackOnly, which has a 99% uniform efficiency.
Muons satisfying theZ electron criteria must fulfill the additional criteria of a tighterpTthresh- old, pT > 10 GeV, a tighter identification criteria, Medium and tighter isolation, GradientLoose, which has an efficiency of at least 95% for muons withpT>25 GeV. Muons passing theW electron criteria must pass thepTthreshold,pT>20 GeV.
Jets are reconstructed from topological clusters using the anti-ktalgorithm with distance param- eter ∆R= 0.4. Jets are required to havepT>25 GeV and fulfill the pseudorapidity requirement of|η|<4.5 . To suppress jets originating from pileup, jets are further required to pass aJ V T cut (J V T >0.59) if the jetpT is within 20< pT<50 GeV and it resides within|η|<2.4 [95].
Separate algorithms are run in parallel to reconstruct electrons, muons, and jets. A particle can be reconstructed as one or more objects. To resolve these ambiguities, a procedure called “overlap removal” is applied. For electrons, this overlap removal is applied in two steps. At the baseline selection, an electron that shares a track with a muon, and the sub-leading pTelectron from two overlapping electrons are removed. The second step removes W or Z electrons if they are within 0.2<∆R <0.4 of a jet. For muons, overlap removal is applied toW orZmuons to separate prompt muons from those originating from the decay of hadrons in a jet. AW orZ muon is removed if it is within ∆R <0.4 of a jet that at least 3 tracks.
The missing transverse momentum, with magnitude Emiss
Cut Value/description
Baseline Electron Baseline Muon
Acceptance pT>7 GeV,|ηcluster|<2.47,|η|<2.5 pT>7 GeV,|η|<2.5
Identification LooseAndBLayerLLH Loose
Isolation LooseTrackOnly LooseTrackOnly
Impact parameter |z0sinθ|<0.5 mm, |z0sinθ|<0.5 mm,
|d0/σd0|<5 |d0/σd0|<3
Z Electron Z Muon
Acceptance pT>15 GeV, exclude 1.37<|ηcluster|<2.47 pT>15 GeV
Identification MediumLH Medium
Isolation GradientLoose GradientLoose
W Electron W Muon
Acceptance pT>20 GeV pT>20 GeV
Identification TightLH
Isolation Gradient
Table 5.3: Summary of the three levels for electron and muon criteria. Each new level contains the selection of the previous level.
sum of the transverse momenta of the calibrated selected leptons and jets, and the sum of transverse momenta of additional soft objects in the event, which are reconstructed from tracks in the inner detector or calorimeter cell clusters.
5.4.2
Event selection
Table 5.4 summarizes the event selection. After passing the event cleaning cuts and the primary vertex requirement, having a reconstructed vertex with at least two tracks, events are required to pass the lowest unprescaled single lepton triggers. To minimize the loss of efficiency due to the turn-on of the triggers, the leading leptonpTis required to be greater than 25 GeV.
Event kinematics depend on the assignment of the leptons to the parent boson. Leptons asso- ciated with the Z boson have to satisfy the Z lepton criteria. They must be the same flavor and have opposite charges (SFOS) with an invariant mass within 10 GeV of the mass of theZ boson to suppress non-resonant backgrounds such ast¯t. If more than one pair can be formed, the pair whose invariant mass is closest to the mass of the Z boson is chosen as theZ lepton pair. The leptons associated with theZ boson are referred to as`leadZ and`subleadZ .
The remaining third lepton is associated to theW boson and must satisfy theW lepton criteria described in the section above. TheW lepton is referred to as`W. A requirement on the transverse mass of theW boson is applied to selectW bosons. The transverse mass of theW boson is calculated
Event selection
Event cleaning Reject LAr, Tile, and SCT corrupted events and incomplete events
Primary vertex Reconstructed vertex with at least two tracks
Trigger eeeevents must pass any of the electron triggers
µµµevents must pass any of the muon triggers µµe,eeµevents must pass any of electron or triggers
single electron triggers (MC) HLT e24 lhmedium L1EM18VH||HLT e60 lhmedium||HLT e120 lhloose
single electron triggers (data) HLT e24 lhmedium L1EM20VH||HLT e60 lhmedium||HLT e120 lhloose
single muon triggers (MC and data) HLT mu20 iloose L1MU15||HLT mu50
Lepton multiplicity Exactly 3 baseline leptons, at least 1 SFOS pair Zlepton criteria leptons associated withZ boson passZ lepton criteria W lepton criteria lepton associated withW boson passesW lepton criteria
Lead leptonpT p`T1>25 GeV
m``requirement |m``−mZ|<10
mT requirement mWT >30 GeV
Table 5.4: Signal event selection. Leptons are associated toZ boson if they form a SFOS pair that minimizes their invariant mass with respect to the mass of theZ boson.
with theW lepton and the missing transverse energy vector, and defined as:
mWT = q 2pW TE miss T (1−cos(∆φ)), (5.8)
where ∆φis the angle between the W lepton and the missing transverse energy vector.
The transverse mass is required to be above 30 GeV. This suppresses backgrounds that have low missing energy such asZ+jets,Z+γ, andZZ. No explicit missing transverse energy is required to have a selection identical to the 8 TeVW Z cross section measurement [83].