Feb. 29th, 2016, DPG, University of Hamburg
Study of Z boson events in the decay to a tau lepton pair in the e + 𝜇 channel in Run 2 with the CMS experiment
Yiwen Wen
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DESY
Y.Wen
Outlines
1. Introduction-Theoretical motivation: why Z → 𝜏 𝜏 → 𝑒 + 𝜇 matters ?
2. Event selections3. Background Estimates
-Misidentified lepton-Top Pair
4. Results5. Summary
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✤ in LHC run 1, CMS and ATLAS combined their search for 𝐻 → 𝜏𝜏 results, 5.4 sigma reached(ATLAS-CONF-2015-044)
✤ the Z → 𝜏𝜏 as standard candle for commissioning of 𝐻 → 𝜏𝜏 analysis
✤ e + 𝜇 make up 6% of 𝜏𝜏 decays✤ small branching faction counter-balanced by higher lepton reco.
efficiency and lower fake rate in comparison to the reco. of hadronic 𝜏 decay
✤ also benefits from absence of large Z → ee or Z → 𝜇𝜇 background which dominate 𝜏𝜏 → ee and 𝜏𝜏 → 𝜇𝜇
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1.Introductionmotivations
Y.Wen
1.Introduction
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a typical 𝑒 + 𝜇 event in CMS
𝜇
e
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2.Event selection
✤ 13 TeV p-p collision data collected by CMS in LHC run 2 corresponding to integrated luminosity of 2.2 fb−1
✤ major backgrounds:top pair production modeling in MC simulationmis-identified lepton contribution modeling in data-driven method
✤ dilepton selection:pileup data-MC correction appliedtrigger
e + 𝜇 cross trigger: Muon17 Electron 12 OR Muon8 Electron17leptons and trigger object matched in cone ∆R<0.3data-MC trigger efficiency correction applied
electron and muon electron p
T > 13 GeV, muon p
T > 10 GeV
relative isolation < 0.15 prompt electron and muon requirementdata-MC isolation efficiency correction applied
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3. Background estimations
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mis-identified lepton
✤ mis-identified lepton come from QCD multi-jet and W+Jets events
✤ ABCD method:A:signal region B:same sign(SS) dilepton selection C:opposite sign(OS) selection with inverted isolation(Iso>0.4) D:SS selection with inverted isolation(Iso>0.4)
✤ NC/ND is taken as median of the three ratios: inverting the isolation of the electron only, the muon only, or both
✤ NA =NB (NC/ND)✤ in this case, NC/ND=1.9+/-0.6
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3. Background estimations
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D ζ discriminant between Z → 𝜏 𝜏 and top pair
Dζ = ζ̂ ⋅!ETmis −αζ̂ ⋅( !pT ,e +
!pT ,µ ) = Pζ −αPζ vis
top pair decay Z → 𝜏 𝜏 decay✤ D ζ - built from momenta
of
leptons and missing transverse energy
✤ for Z → 𝜏 𝜏, decay products contained inside a narrow cone around the 𝜏 trajectory, missing transverse energy highly correlated with the two leptons
✤ for top pair, the leptons from a top can have trajectories far from associated neutrinos, missing transverse energy less correlated with the two leptons
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3. Background estimations
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define signal region
✤ after dilepton selection✤ define topological cuts as
missing transverse energy < 80 GeVD
ζ > -60 GeV
Dζ = ζ̂ ⋅!ETmis −αζ̂ ⋅( !pT ,e +
!pT ,µ )
optimized value:0.85
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3. Background estimations
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top-pair background modeling
✤ to obtain high purity top-pair enriched events
✤ define Control region as:Missing transverse energy > 80 GeVD ζ < -60 GeV
✤ normalization Scale Factor(SF)SF = (Ndata-Nother)/NttIn this case SF = 0.93+/-0.02
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4.Result
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e𝜇 invariant mass distribution
✤ applied topological cuts:missing transverse energy < 80 GeVD ζ > -60 GeV
✤ good agreement between data and MC simulation
✤ cross-section could be estimated in the 30 - 90 GeV mass range
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Summary
✤ perform studies on inclusive Z boson production followed by Z → 𝜏 𝜏 decay in final state 𝑒 + 𝜇
✤ data correspond to 2.2 fb−1 collected by the CMS detector in LHC 13 TeV run 2
✤ my studies focused on background estimations
✤ data and MC simulation in good agreement
✤ works on improvement of background modeling, uncertainty and cross-section measurement still on-going
✤ stay tuned for more data to come11
Back up
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Background modeling
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before and after apply TT SF
D ζ Missing ET
before: before:after: after:
χ2: 127 χ2: 222 χ2: 98.6
χ 2 = (Ndata (i)− NMC (i))2
NMC (i)i=1
bins
∑
χ2: 240
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related physics object selections
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✤ primary vertexgood vertex criteria
✤ pile-up reweighting applied
✤ electronp
T > 13 GeV η < 2.5
prompt electron requirement∆β-corrected relative isolation < 0.15 isolation efficiency correction applied
✤ muonp
T > 10 GeV η < 2.4
prompt muon requirement∆β-corrected relative isolation < 0.15 isolation efficiency correction applied
Isol =pT ,h±
+max(0, pT ,γ∑ + pT ,h0
− 0.5 ⋅ pT ,pu∑∑ )∑pT ,l
✤ triggere + 𝜇 cross trigger: Muon17 Electron 12 OR Muon8 Electron17leptons and trigger object matched in cone ∆R<0.3trigger efficiency correction applied
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Tag and probe method
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✤ The Tag and Probe method is to measure defined object efficiency from data at CMS by exploiting di-object resonances like Z
✤ Resonances are reconstructed as pairs with one leg passing a tight identification (tag) and one passing a loose identification (probe)
✤ the (tag + passing probe) and (tag + failing probe) lineshapes are fit separately with a signal + background model
✤ the efficiency is computed from the ratio of the signal yields in the two lineshapes above
✤ the procedure is repeated in bins of the probe variables (e.g. pT, η ...) to compute efficiency histograms as a function of those variables
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Uncertainty table
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sources uncertaintyluminosity 5%
top pair 10%VV 20%
same sign(ABCD ) 30%Drell-Yan 10%
𝜇 ID 5%electron ID 6%