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

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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%