Determination of the mis-reconstruction and mis-identification ...

Determination of the 

mis-reconstruction and mis-identification probability 

of hadronically decaying tau leptons 

using Z+jets events with ATLAS 

Felix Friedrich, Michael Stoebe 

supervised by Wolfgang Mader, Arno Straessner 

DPG Frühjahrstagung 2011, Karlsruhe – T 78 Experimentelle Methoden

CERN / ATLAS 

Felix Friedrich (TU Dresden) tau fake rate studies 

geometrical distance 

between two objects: 

with pseudorapidity: 

Width: 44 m 

Diameter: 25 m 

Weight: 7000 t 

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Motivation 

Many SM processes and searches for new physics have tau leptons in final state: 

SM processes with taus 

• Z � ττ 

• W � τν 

• Top-pair production 


Higgs boson / new physics search with taus 

• H � ZZ�4l • H ± � τ ± ν 

• H/A/h � ττ 

• Z’ � ττ • W’ � τν 

�for background estimation need to know number of misidentified tau leptons 

W� τν Z � τ(lep)τ(had) 

MSSM H � ττ 

Ref.: Observation of W->tau nu Decays 

with the ATLAS Experiment 

(ATLAS-CONF-2010-097) 

Ref.: Observation of Z->tau(had) tau(lep) 

Decays with the ATLAS detector 

(ATLAS-CONF-2011-010) 

Ref.: Data-driven background estimation for 

the H ->tau+tau- -> lh search at 7 TeV with the 

ATLAS detector (ATLAS-CONF-2010-096) 

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Tau reconstruction (hadronic decay mode) 

Calorimetry 

• seeded by calorimeter jets from topological 

clusters with E T> 10 GeV 

• collimated energy deposit in EM and Had 

calorimeters like QCD jets 

Tracking 

• well collimated collection of neutral and charged 

Pions and Kaons 

• mostly 1 or 3 charged tracks (prongs) 

leptonic decay mode is hard to distinguish from 

prompt electrons or muons 

� only hadronic tau decay mode considered 


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Tau identification 

τ identification tries to distinguish between 

tau jets and QCD jets 

various algorithms available: 

• cut-based 

• projective logarithmic likelihood (LLH) 

• Boosted Decision Tree (BDT) 

� 3 levels of efficiency available for each identification method: 

tight, medium, loose 

corresponding roughly to ~30%, ~50% and ~60% 

signal efficiency independent of E T 

� background is reduced by O(10-100) 

dedicated electron vetoes: 

• cut-based electron veto 

� available in tight, medium, loose 

• Boosted Decision Tree electron veto 


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Tau fake rates from Z+Jets 

• select clean Z->ee/μμ + jets sample 

• use both leptons to tag event 

• mass window of 71 - 111 GeV for both channels 

• additional jets are used to test tau ID 

ATLAS work in progress 

invariant mass of tag muons invariant mass of tag electrons 



l + l - 

QCD-Jet(s) 

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fake rate for 3 different tau identification algorithms (medium efficiency) 

and medium electron veto 


• lowest fake rate for 

Boosted Decision Tree algorithm 

• suffer from low statistics in high E T regions 

• flat in eta 



fake rate is about 

• 8% for cut-based method 

• 3-5% for BDT and Likelihood 

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ATLAS work in progress ATLAS work in progress 



• no dependence on number of vertices 

• fake rate from Z->ee+jets and 

Z->μμ+jets agrees within statistics 

• good agreement between simulation 

(MC) and data 

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Differences di-jets vs. Z+jets 

• tau fake rate not equal in Z+jets and 

QCD di-jets events 

• different fraction of quark and 

gluon jets in both samples 

• quark jets are more tau like 

� higher fake rate 


MC Simulation 

ATLAS work in progress ATLAS work in progress 

di-jet selection: 

• select a well-defined di-jet 

topology by using a 

tag-and-probe method 

• balanced in phi and p T 

• probe jet is used to test 

tau identification 

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Estimate tau fake rate in di-jet events 

idea: 

• use tau fake rates from Z+jets events to estimate tau fake rate in QCD di-jet events 

problem: 

• tau fake rates from Z+jets events and di-jet events don’t agree sufficiently 

possible solution: 

• find variables which are sensitive to quark-gluon fraction 

• parameterize tau fake rate from Z+jets events in those variables 

• in di-jet sample: apply on every jet the fake rate coming from Z+jets 

but: 

• which variables to use? 

• limited statistics in Z+jets � only coarse parameterization possible 

• which ranges/limits for those parameterization? 


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χ 2 - method 

� use simulated events 

� calculate fake rate for many variable combinations and ranges 

� separately for Z+jets and QCD di-jets 

� compare fake rates using a χ 2 - Test 

� use combination/ranges with best χ 2 



blue: fake rate in di-jet events 

black: fake rate in di-jet events, 

where jets are weighted by a 

fake rate coming from Z+jets 

� agreement using data not sufficient 

still room for improvements 

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Statistical method 

this method was developed and tested with tau reconstruction fake rate 

tau reconstruction fake rate is ≈ 1, due to tau reconstruction algorithm 

but in past two algorithms were used: 

� tau candidates only considered if reconstructed by both 

� tau reconstruction fake rate < 1 

• use simulated events 

• select probe jets in Z+jets and QCD di-jet events 

• choose combination of 3 variables 

• divide phase space of those variables, 

so that each region has the same statistics for Z+jets and di-jets 


Michael Stoebe 

� asymmetric regions possible 

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Statistical method 


yellow: 

reconstruction fake rate in di-jet events 

black: 

estimated fake rate in di-jet events 

by using a fake rate coming from Z+jets 


Michael Stoebe 

� good agreement (2010 data) 

within statistical uncertainty 

was also tested on different event 

topology, like full hadronic toppair 

decays (only simulation) 


MC Simulation 

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Summary 

• tau identification fake rate determined for QCD jets in Z+jet events 

• various tau identification algorithms studied 

• fake rate depends on event topology 

� different in Z+jets and di-jet events 

� sensitive to quark/gluon contribution of probe jets 

• two different methods studied to estimate fake rate in di-jet events 

by using Z+jet events 

• both looks promising, but still room for improvements 


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Backup 

Backup 


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Variables used in the different tau identification algorithms 


ATLAS-COM-CONF-2011-057 

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Variables used in the 

different 


algorithms 


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different 


algorithms 


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different 


algorithms 


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Electron Veto 

Variables used in the cut-based tau-electron veto algorithm 




ATL-COM-PHYS-2011-188 

(will be changed in CONF number 

as soon as available) 



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Electron Veto 

Variables used in the cut-based tau-electron veto algorithm 


ATL-COM-PHYS-2011-188 

(will be changed in CONF number 

as soon as available) 

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Determination of the mis-reconstruction and mis-identification ...

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