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CALOR 2012

June 4-8 2012

Santa Fe

Matteo Volpi

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Outline

Motivation. The tau lepton at hadron colliders.

Tau reconstruction at ATLAS. Identification.

Expected performance. Measurement in real data

(tag&probe Z, W samples). Energy calibration.

Tau energy scale factors and pileup offset correction Systematic errors determination Cross checks in real data (tag&probe Z sample)

Conclusion.

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Motivation

τ leptons are present in important signatures of new physics at the LHC.

SM Higgs favors decays to τ over other leptons.

Promising channel for SM Higgs searches in the mass range 115 < mH < 131 GeV. 

SUSY Higgs (charged and neutral) have substantial branching ratios to τ.

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The tau lepton at hadron colliders

Tau leptonic decay

Leptonic mode BR = 35% → very difficult to distinguish from prompt leptons at colliders. Not discussed today.

Hadronic decays (BR=65%) are a well-collimated collection of charged and neutral pions.

Most have 1 or 3 charged decays particles (prongs).

Tau hadronic decay

mτ = 1.8 GeV, heaviest lepton.cτ = 87 μm, short lifetime then decays inside the beam pipe. τ

Main challenge, distinguish these decays from QCD jets background.

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Jet or τ ?

Quark or gluon initiated jets are much more abundant than tau hadronic decays in pp collisions.

Hadronic τ decays have few basic features to distinguish them from gluon or quark initiated jets. Lower particle multiplcity. Narrower lateral spread. Different average composition of charged to neutral pions.

Calorimeter and tracking information are combined into hadronic tau decay identification variables capturing these distinct features.��

QCD jets Tau

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Tau leptons decaying hadronically are found via a two step procedure:

First step : reconstruction of tau candidates, seeded by anti-kt jets (ΔR=0.4) from topological clusters. This is robust against electronic and pileup

noise.

Jets have pT >10 GeV and |eta|<2.5.

Tracks associated to the tau candidate are counted within a cone ΔR<0.2.

Second step: identification variables are built, to discriminate against jets.

Tau reconstruction at ATLAS

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Tau identification variables

Rcal: shower width in the electromagnetic and hadronic calorimeter weighted by the transverse energy of each calo.

The number of tracks in the isolation

annulus.

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Tau identification variables

f3 lead clusters : the ratio of the energy of the first three leading clusters over the total energy of all clusters.

meff: invariant mass of the cluster system.

∆Rmax: maximal ∆R between a core track and the tau candidate axis.

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Electron rejection variables

Maximum transverse energy deposited in a cell in the EM pre-

sampler layer.

backgroundsignal

backgroundsignal

Fraction of transverse energy of the tau candidate deposited in the EM

calorimeter.

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Even after good electron overlap removal, still electrons faking taus are a non negligible background. A multivariate technique is employed using variables (fEM

track, ET,maxstrip ), to reject not

well identified electrons.

Dedicated muon veto to reduce background from muon fakes (additional to muon overlap removal).

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only Little discrimination power between

QCD jets and tau leptons in the reconstruction process a dedicated identification step is needed. Three options in ATLAS: Cuts based approach, Projective Likelihood and Boosted Decision Tree (BDT).

Tau identification at ATLAS

They use different sets of identification variables and are separately trained for single/multi prong and PT of the tau candidate.

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only Inverse background efficiency as a function of signal efficiency for

different estimators built from identification variables.

Three working points: loose, medium and tght.

For the tau vs QCD jets identfcaton they are optmized for each discriminant, which yield signal efciencies of approximately 60%, 45% and 30%, respectvely.

Identification performance

Jet rejection

Electron rejection

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Measuring tau identification in data Identification efficiencies can be measured in data.

Method: select W→ τν or Z→ ττ events using tag-and-probe methods without applying tau identification and count the events that pass identification.

Background is a major challenge: estmaton exploitng lepton-tau charge correlatons for Z→ ττ and ftng tau track multplicity for W→ τν.

The invariant mass of the visible decay products from a Z → ττ selection before (left) and after (right) tau identification.

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Measuring tau identification in data

Track multiplicity distributions from a W→ τν selection before tau identification (left) and after a tight boosted decision tree identification (right).

Summary:The measured efficiencies in both methods are in good agreement with Monte Carlo predictions within 5% (8 - 12%) for the W→ τν (Z → ττ )

method.

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Energy calibration Input clusters from Local Hadron

Calibration (LC): each cluster classified in its

hadronic/EM like shape calibration accounts for non-

compensation, dead material, out-of-cluster effects

for tau energy scale only, clusters in a cone ΔR< 0.2 are considered, for pileup robustness.

Advantage: distinct treatment of different decay products (improves resolution).

A final calibration brings the object to the tau energy scale (TES), it consist of two steps:

correcting for pile-up (PU) effects TES correction.

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Second step: TES correction accounting for object specific effects

out-of-cone, underlying event, etc. The mean energy response < Ereco /Etruth> is

divided in (η,n_p) bins .

First step: pileup offset correction For each (η, n_p) bin the PU contribution as a function of the number of

vertices is evaluated. Offset is subtracted from each τ candidate

Energy calibration

< E

reco

/Etr

uth >

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A fit is performed to obtain a calibration function R. The offset corrected energy is multiplied by 1/R.

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New TES uncertainties

2010 TES uncertainty fully MC based.

Large uncertainty from the calorimeter response mainly at low PT.

Large uncertainty on Higgs search.

2011 Reduce the large uncertainty on

calorimeter response with the deconvolution method employed for the jets based on the E/p measurements. Decompose tau into its decay

products. Take uncertainty from single

particles. Propagate this to the full tau visible

energy scale.

2010

2011

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2011 TES uncertainties contributions Data driven.

Calorimeter response. |η| < 0.8 : Single particle uncertainties for π± from in situ <E/P> at low

PT and combined test beam (CTB) data at high PT.

0.8<|η|< 2.5: Single particle uncertainties for π± from in situ <E/P> at low PT - no CTB available .

Monte Carlo driven. Response uncertainty for 0.8<|η|< 2.5 and high PT => Physics lists comparison.

Underlying event modeling => MC comparison with Perugia tune. Dead material modeling => Partially covered by the E/p (the remainder is

from MC).

TES calibration uncertainty. Non-closure. Pile-up dependence.

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Tau decomposition method

Pseudo-experiment is the energy of each tau after modifying the energy response

to propagate the single particle uncertainties to the τ.

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E/p in 2011 data Measure E with respect to P: Ecalo/PID.

Isolated tracks in MinBias events, mainlyπ± .

E in 0.2 cone around the extrapolated track.

Subtract remaining neutral em background.

Results show: Well modelled pion response within 5%Differences mainly coming from cluster noise suppression.

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Shower shape description Shower model effect on clustering negligible.

However we need to look at the shower model uncertainty on LC calibration : comparing effective LC weights and various cluster moments between FTFP_BERT

and QGSP_BERT physic lists effects of the order of 1% ( supported by LCW/EM comparisons in E/p analysis) applying 1% for multi prong decays, 0.5% for single prong decays.

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Pileup systematic

one-prong multi-prong

MC data driven The difference in the highest and lowest value is taken as the pileup uncertainty,

to avoid double counting the systematics deriving from the non-closure.

For one-prong candidates this uncertainty is found to be up to 2% at low PT and 0.5% at high.

For multi-prong decays, this uncertainty is 1% at low PT and 0.5% at high PT.

2%

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Decomposition method (data/MC)

MC based comparison with Perugia tune

E/p is sensitive to the amount of material in front of the calorimeter. Dead material contribution scaled by the fraction of the energy carried by particles outside the E/p scope

Response pTreco/pTtruth

Pile-up

Total TES systematic

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TES systematic summary

Systematic Type Contribution

Single particle response Data 2% to 3%

Underlying event MC 1%

Pileup MC 1% to 2%

Material modelling MC 1% to 2%

Non closure MC 1% one-prong; 2% multi-prong

Hadronic shower model for |η| >0.8 MC 1% one-prong; 2% multi-prong

Final 2011 tau energy scale uncertainty is about 3% at low PT a factor 2 smaller than 2010.

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TES is MC based, and part of the uncertainties come from MC-> important to cross check on real data the procedure.

2011: we don’t have enough statstc to provide the absolute energy scale with Z→ττ events.

BUT we use Ztautau data to cross-check the deconvoluton systematc extrapolaton for |η|>0.8, where we rely on MC for high pT taus.

We measure the shifts in the forward regions with respect to the central barrel region, and we hope to find no significant difference.

The visible mass of Z→ττ→μ-τhad decays is determined by the reconstructed energy of the visible decay products. As the lepton energy scale is determined independently, this visible mass is

proportional to PτT.

The value of the TES can be found by shifting PτT in simulation and comparing the resulting visible mass peak to that observed in data:

where α is a parameter that accounts for the TES variation.

In-situ TES systematic cross check

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The observed visible mass distribution is matched to a range of templates to derive the value of α which best describes data.

In-situ TES systematic cross check

Prefe

rred

Prefe

rred

Difference between central barrel and forward integrated region: 3.0 ± 2.6%. No significant shift seen.

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Conclusion

Identifying hadronic τ decays at LHC is important for several interesting new physics searches.

Tau reconstruction and identification in ATLAS has been performing well. Efficiencies and mis-identification

have been measured in data A lot of work to reduce the TES

systematics: data driven calorimeter

response uncertainty. data driven cross check for the uncertainty.

Final 2011 TES uncertainty about 3%, a factor 2 smaller than in 2010.

Future plans: for full 2012 data set we plan to use the Z to give TES and its uncertainty.

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Extra slides

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References

Determination of the tau energy scale and the associated systematic uncertainty in proton-proton collisions at sqrt{s}= = 7 TeV with the ATLAS detector at the LHC in 2011 : Tau energy scale. ATLAS-CONF-2012-054

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The ATLAS collaboration, Performance of the Reconstruction and Identification of Hadronic Tau Decays with ATLAS ATLAS-CONF-2011-152