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ATLAS NOTE

Observation of W →

τν τ decays with the ATLAS Experiment

A. Andreazza4, S. Bedikian5, Y. Coadou3, Z. Czyczula5, S. M. Demers5,

L. Dell’Asta4, J. C. Dingfelder1, G. Nunes Hanninger1, J. Kraus1, J. Kroseberg1,

S. D. Protopopescu2, E. von Torne1

1Physikalisches Institut, University of Bonn, Germany2 Brookhaven National Laboratory, Upton, New York, USA

3Centre de Physique des Particules de Marseille (CPPM), CNRS/IN2P3, Aix-Marseille Universit e, France4 INFN & Universit a degli Studi, Milano, Italy

5Physics Department, Yale University, USA

Abstract

A search for W → τν τ decays, with the τ lepton decaying into hadrons, has been performed

with the ATLAS experiment at the LHC. The analysis is based on a data sample corre-

sponding to an integrated luminosity of 546 nb−1 which was recorded at a proton-proton

centre-of-mass energy of 7 TeV. A total of 78 data events are selected, with a background

of 11.1±2.3(stat.) ± 3.2(syst.) events from QCD processes, and of 11.8±0.4(stat.) ± 3.7(syst.)

events from other W and Z decays. The observed excess of data events over the total back-

ground is compatible with the Standard Model signal expectation, both in number of events

and in shapes of distributions of kinematical variables and variables used in the τ identifica-

tion. This is the first evidence of W → τν τ decays and of hadronically decaying τ leptons in

ATLAS.



December 10, 2010 – 15 : 48 DRAFT 2

Contents

1 Introduction 3

2 Data samples 42.1 Data Quality Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Data Samples and Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 Trigger matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Simulated Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Object reconstruction 12

4 Event selection 13

4.1 Background processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 QCD background estimation 21

5.1 Verification of the assumptions from the data-driven method . . . . . . . . . . . . . . . 30

5.1.1 Shape of S E missT

distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.2 Additional validation tests for the data-driven method . . . . . . . . . . . . . . . . . . . 31

5.2.1 Applying the data-driven method to a control sample . . . . . . . . . . . . . . . 31

5.2.2 Study of τ h candidates in different pT ranges . . . . . . . . . . . . . . . . . . . 32

5.2.3 Separation of 1-prong and 3-prongs τ h candidates . . . . . . . . . . . . . . . . 35

5.2.4 Redefining the signal and control regions . . . . . . . . . . . . . . . . . . . . . 38

5.2.5 Separation of events with one vertex and more than one vertex . . . . . . . . . . 39

5.3 Study of τ h

medium candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

6 Systematic Uncertainties 46

6.1 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

6.2 Cross section and luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

6.3 Energy scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

6.4 Electron Veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.5 Muon veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.6 Pile-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.7 Monte Carlo model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.8 Background estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

7 Conclusions 59



December 10, 2010 – 15 : 48 DRAFT 3

1 Introduction1

The τ lepton plays an important role in the LHC physics programme, for example in searches for a2

low-mass Higgs boson or supersymmetry [1–4]. Decays of Standard Model particles to τ leptons, in3

particular Z → ττ and W → τν τ , are important background processes in such searches, and their cross4

sections need to be measured beforehand. These decays will also be used to ensure that the reconstruction5

and identification of τ leptons are sufficiently well understood.6

At NNLO, the W → τν τ signal is predicted to be produced with a cross section times branching7

ratio of σ ×BR = 10.46× 103 pb [5], which is about ten times higher than for Z → ττ events. Since8

purely leptonic τ decays (τ ℓ in the following) cannot be easily distinguished from electrons and muons9

from W → eν e or W → µν µ decays, the analysis presented in this note uses only hadronically decaying τ 10

leptons (τ h in the following). The signal sample is dominated by events with low- pT W bosons producing11

τ leptons with typical visible transverse momenta between 10 and 40 GeV (Figure 1), where the visible12

transverse momentum is defined as the magnitude of the vectorial sum of the transverse momentum of 13

the τ h decay products except the neutrinos. In addition, the distribution of the missing transverse energy,14

associated with the neutrinos from the W and τ h decays, has a maximum around 20 GeV and a significant15

tail up to about 80 GeV.16

(GeV)miss

TTruth E

0 10 20 30 40 50 60 70 80 90 100

F r a c t i o n

o f E v e n t s

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

νhad τ→W

(a) True missing transverse energy

(GeV)τTruth

visT

E0 10 20 30 40 50 60 70 80 90 100

F r a c t i o n

o f E v e n t s

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035 νhad τ→W

(b) True visible transverse energy of the τ h

Figure 1: Generator-level missing transverse energy (left) and visible transverse energy of hadronically

decaying τ leptons (right) for W → τν τ events.

This note describes the first observation of hadronically decaying τ leptons from W → τ hν τ de-17

cays with the ATLAS experiment at the LHC. The analysis is based on the one developed in previous18

studies on simulated data [6, 7]. An improved separation of signal from background is achieved firstly19

through a selection based on the significance of the missing transverse energy, secondly through a new20

τ h identification algorithms [8, 9], and thirdly through a data-driven estimation of the QCD background.21

The layout of the note is the following: the sets of data and Monte Carlo samples used are listed22

in Section 2, the reconstruction and properties of the main objects used in this analysis: E missT , τ -jets23

and leptons is summarized in Section 3, the selection procedure is described in Section 4, a data-driven24

method for the estimation of QCD background is discussed in Section 5, followed by an analysis of the25

contribution of different sources of systematic uncertainties on the background and signal rate estimation26

in Section 6.27

The result presented here have been summarized in a CONF note on the first observation of W → τ hν τ 28

events and of hadronically decaying τ leptons in ATLAS [10].29



December 10, 2010 – 15 : 48 DRAFT 4

2 Data samples30

The analysis has been performed on data collected between March and August 2010 by the ATLAS31

experiment in proton-proton collisions at a centre-of-mass energy of 7 TeV. A summary of the runs used32

for this analysis is reported in Section 2.1. Only data taken during periods with stable beams and with33

a good data quality for all the tracking and calorimeter sub-detectors are used. With these basic data34

quality criteria, the total integrated luminosity available for the analysis amounts to 546 nb−1.35

Events are selected using all three ATLAS trigger levels: Level 1 (L1), Level 2 (L2) and Event Filter36

(EF), the latter two are referred to as High Level Trigger (HLT) [11]. The trigger requirements [12] are37

based on the presence of a τ h jet and E missT as main signatures of the W → τ hν τ decay. The L1 trigger38

selects narrow clusters of trigger towers with a pT threshold of 5 GeV [13]. With the L2 trigger, tracks are39

reconstructed around the L1 candidate. The event is accepted if there is at least one track with pT > 6 GeV40

and E missT above 5 GeV. A full event reconstruction is performed at the EF level and the events are41

required to have E missT > 15 GeV. This trigger requirement has an efficiency of (99.7±0.2%), computed42

from Monte Carlo simulation, to select W

→τ hν τ events satisfying the selection criteria described in43

Section 4.2. A full description of the trigger and of its efficiency is reported in Section 2.3.44

The results from data presented here are compared to expectations based on Monte Carlo simulations.45

All references about Monte Carlo samples are collected in Section 2.4.46

2.1 Data Quality Cuts47

The τ data quality selections are described in detail in a separate ATLAS note [14]. The selections48

require the cp tau offline flag to be set to green for the run and also that the calorimetric contribution of 49

the missing transverse energy is well computed. These selections were used to produce the Good Run50

List (GRL) shown in Tables 1 and 2.51

In those tables the luminosity refers to the luminosity delivered by the LHC while stable beams52

were declared. This luminosity is usually higher than the luminosity registered by ATLAS. The quoted53

number of events corresponds to the number of events collected by the L1Calo stream and passing the54

DESD MET skimming, as explained in the following.55

2.2 Data Samples and Formats56

The event samples from each run recorded by ATLAS were centrally skimmed into so-called “perfor-57

mance DPDs”. The samples considered in this note originate all from the large missing transverse energy58

performance DPDs (DESD MET) derived from the L1Calo stream. For this run period, the skim was59

defined through (see tag 00-06-00-03 of PrimaryDPDMaker):60

• At least one 15 GeV τ h candidate61

62

OR63

• At least 15 GeV of missing transverse energy (MET Topo)64

65

OR66

• An OR of several L1 trigger conditions: “L1 TAU5 XE10” OR “L1 TAU5 MU6” OR67

“L1 2TAU5 EM5” OR “L1 2TAU6I” OR “L1 2J5” OR “L1 2J10”68

As the events we are interested in, as described later, must have E missT higher than 30 GeV, we tight-69

ened the skimming to the request of having at least 15 GeV of missing transverse energy (MET Topo).70



December 10, 2010 – 15 : 48 DRAFT 5

Run Lumi (nb−1

) RecoTag Events in Skim152166 0.007719 r1297 p157 p159 1793

152214 0.004057 r1297 p157 p159 876

152221 0.02198 r1297 p157 p159 5342

152345 0.01871 r1297 p157 p159 4310

152409 0.08327 r1297 p157 p159 21422

152441 0.07193 r1297 p157 p159 18108

152508 0.01216 r1297 p157 p159 2898

152777 0.05413 r1297 p157 p159 13063

152844 0.008453 r1297 p157 p159 2229

152845 0.02933 r1297 p157 p159 7712

152878 0.0302 r1297 p157 p159 7280

152933 0.02246 r1297 p157 p159 6408

152994 0.006723 r1297 p157 p159 1570

153030 0.02731 r1297 p157 p159 6652

153134 0.03307 r1297 p157 p159 1735

153136 0.002106 r1297 p157 p159 560

153159 0.01209 r1297 p157 p159 3252

153200 0.008472 r1297 p157 p159 2257

153565 0.7547 r1297 p157 p159 204584

154810 0.1753 r1297 p157 p159 41893

154813 0.3258 r1297 p157 p159 61293154815 0.07559 r1297 p157 p159 17371

154817 0.5612 r1297 p157 p159 150220

155073 1.195 r1299 p161 p165 300441

155112 3.691 r1299 p161 p165 880131

155116 0.5645 r1299 p161 p165 130052

155160 1.361 r1299 p161 p165 347782

155228 0.04703 f259 m487 10004

155280 0.2882 f259 m492 7458

155569 1.033 f260 m492 238547

155634 1.126 f260 m497 260662

155669 0.5339 f260 m497 109595155678 1.209 f261 m497 287015

155697 4.329 f261 m497 822901

156682 1.407 f265 m512 72163

Table 1: Runs included in the GRL. The luminosity listed is only an approximate integrated luminosity

for the whole run, based on stable beams declaration of the LHC. The number of events corresponds to

those collected in the DESD MET skim of the L1Calo stream.



December 10, 2010 – 15 : 48 DRAFT 6

Run Lumi (nb−1) RecoTag Events in Skim

158045 1.004 f270 m534 44114

158116 16.27 f271 m534 849177

158269 3.569 f271 m534 191597

158299 1.389 f271 m534 79617

158392 8.554 f271 m544 441274

158443 1.448 f273 m544 48512158466 1.947 f273 m544 37669

158545 1.501 f273 m544 65453

158548 11.95 f273 m544 560242

158549 4.011 f273 m544 177917

158582 17.68 f273 m544 911644

158632 5.969 f274 m544 307145

158801 7.439 f274 m544 374712

158975 23.23 f275 m549 1000232

159041 29.68 f275 m549 188920

159086 60.04 f275 m549 2281898

159113 29.59 f275 m549 1291341159179 16.16 f275 m549 507302

159202 11.49 f275 m549 331955

159203 8.505 f275 m549 295757

159224 69.16 f275 m549 2391937

160387 60.71 f280 m569 265007

160472 83.31 f280 m569 311303

160479 6.507 f280 m569 18859

160530 92. f280 m574 389485

Total 18412648

Table 2: Runs included in the GRL. The luminosity listed is only an approximate integrated luminosity

for the whole run, based on stable beams declaration of the LHC. The number of events corresponds to

those collected in the DESD MET skim of the L1Calo stream.



December 10, 2010 – 15 : 48 DRAFT 7

The tau working group then processed the skimmed data to produce a common ntuple format (D3PD)71

using the TauD3PDMaker software (SVN tag 00-06-00-03). All analyses presented in this note use these72

common samples.73

2.3 Trigger74

The main characteristics of W → τ hν τ events are the hadronically decaying τ h lepton and sizable E missT .75

These signatures are used at the trigger level to select data samples enriched in signal events. At very76

low instantaneous luminosity, which comprises data-taking up to the middle of the period D1 [14] (run77

158269), events were selected by the L1 τ h trigger [12]. During later runs the HLT trigger was switched78

on selecting events using single and combined τ h triggers.79

Two single tau trigger signatures are of great importance for an efficient triggering of W → τ hν τ 80

events.81

tau12 loose The trigger starts from selecting narrow clusters of trigger towers with a pT threshold of 82

5 GeV at L1 which serve as seeds for the HLT. Subsequently several τ h identification requirements83

are imposed both at L2 and at EF [12].84

tauNoCut hasTrk6 The trigger starts from the same L1 selection as tau12 loose but the only require-85

ment it imposes at the HLT is the presence of a track at L2 with momentum greater than 6 GeV.86

The tau12 loose item was active and unprescaled until run 159179 while tauNoCut hasTrk6 was by87

design intended to be used in combination with an E missT requirement.88

Two combined τ h and E missT items were actively selecting W → τ hν τ events during the whole data-89

taking period: tau12 loose EFxe12 noMu and tauNoCut hasTrk6 EFxe15 noMu. The first two symbols90

represent the τ h signature described above while EFxeXX stands for the E missT requirement at the EF level91

where XX denotes the threshold. Furthermore both triggers require at least 5 GeV of E missT at the L2. The92

noMu suffix indicates that there were no muon corrections applied at the HLT for the E missT calculation.93

Note that the loose requirement on the τ h signature in the tauNoCut hasTrk6 EFxe15 noMu item is94

compensated by a higher energy threshold for the EF E missT in comparison to tau12 loose EFxe12 noMu95

to maintain a similar rate.96

At this early stage of ATLAS’s operation, a suitable event sample with τ h and E missT selected by a97

trigger that is independent from the one used in this analysis is not yet available. The trigger selection98

has therefore been evaluated based on Monte Carlo simulations. The efficiencies of selecting the signal99

events using the two triggers are summarized in Table 3. For the analysis presented in this note, the100

tauNoCut hasTrk6 EFxe15 noMu trigger is chosen for the event selection, since it is the most efficient101

one. In addition, it results in a smaller systematic uncertainty due to the fact that no selection based on102

the τ shower shape variables at the trigger level is performed. In order to simplify the offline analysis,103

the tauNoCut hasTrk6 EFxe15 noMu requirements were also applied to early data which were actually104

selected with the L1 trigger only.105

Figure 2 shows the fraction of events passing the L1 trigger of tauNoCut hasTrk6 EFxe15 noMu as a106

function of the momentum of the tight τ h candidate (see Section 3 for the definition). This requirement is107

fully efficient for tight τ h candidates with pT > 20 GeV and therefore also for the candidates considered108

in this analysis (see Section 4.2). The acceptance of the L2 tauNoCut hasTrk6 EFxe15 noMu trigger109

with respect to the proceeding trigger level is shown in Figure 3. The results obtained from Monte Carlo110

simulations are compared with data, showing a good agreement. For the efficiency curve, only data from111

period A-C [14], in which the HLT for τ h candidates was not active, are considered. Figure 4 shows the112

acceptance of the EF as a function of E missT . The trigger reaches its plateau at the threshold of E miss

T = 30113

GeV used for the offline event selection (Section 4.2).114



December 10, 2010 – 15 : 48 DRAFT 8

Signature L1 L2 EF Overall

tauNoCut hasTrk6 EFxe15 noMu 99.9±0.0 99.8±0.1 100±0.0 99.7±0.2

tau12 loose EFxe12 noMu 99.9±0.0 94.4±0.6 97.9±0.4 92.3±0.8

Table 3: Summary of trigger efficiency. The L1 efficiency is computed with respect to all events satisfy-

ing the offline selection for W → τ hν τ decays (see Section 4.2). The L2 and EF efficiencies are calculated

with respect to events passing the preceding trigger level. The overall efficiency refers to all three trigger

levels and is normalized to all events passing the offline selection. The quoted uncertainties are statistical

only.

[GeV]T

-tight Pτ

10 20 30 40 50 60 70 80

f r a c t i o n

o f E v t s

0

0.2

0.4

0.6

0.8

1

ντ→Pythia W

Figure 2: For tauNoCut hasTrk6 EFxe15 noMu: fraction of events passing the L1 trigger as a function

of pT of the tight τ h candidates. Only events satisfying the offline selection described in Section 4.2 are

considered.



December 10, 2010 – 15 : 48 DRAFT 9

[GeV]trk

T-tight Pτ

5 10 15 20 25 30 35 40 45 50

f r a c t i o n o f E v t s

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.90.95

1

ντ→Pythia W

= 7 TeVsData 2010

Figure 3: For tauNoCut hasTrk6 EFxe15 noMu: fraction of events accepted by the L2 trigger as a

function of the momentum of the leading track of the tight τ h candidate. Only events passing the L1

trigger are considered.

[GeV]missTE

15 20 25 30 35 40 45 50

f r a c t i o n o f E v t s

0

0.2

0.4

0.6

0.8

1

ντ→Pythia W

Figure 4: For tauNoCut hasTrk6 EFxe15 noMu: fraction of events passing the EF trigger as a function

of E missT . Only events satisfying the offline selection described in Sect. 4.2 and passing the L1 and L2

triggers are considered.



December 10, 2010 – 15 : 48 DRAFT 10

2.3.1 Trigger matching115

In the analysis it is not required that the selected τ h candidate is the object that actually fired the trigger.116

The impact of trigger matching has been checked on the Monte Carlo simulated W → τ hν τ sample117

and it was found to reduce the signal selection efficiency by about 0.2%. The effect of trigger matching118

is therefore negligible for the determination of the trigger efficiency.119

2.4 Simulated Samples120

The Monte Carlo samples of signal and background were generated at√

s = 7 TeV with PYTHIA [15]121

and passed through a GEANT4 [16] simulation of the ATLAS detector [17]. The main background122

processes considered are QCD di-jet production, the leptonic decays of W -bosons into eν and µν pairs123

and Z -boson decays into pairs of charged leptons. The QCD background samples were also produced124

using the DW [18] tuning which features a different description of the underlying events compared to125

the standard simulation. In addition, t t events, which may contain W → τν decays, can also contribute126

to background.127

Table 4 summarizes the Monte Carlo datasets used in this analysis. Also the Monte Carlo samples128

were processed to produce the common ntuple format (D3PD) using the TauD3PDMaker software (SVN129

tag 00-06-00-03).130

The Monte Carlo samples with MC09 tune and including effects of multiple interactions (pile-up)131

have been used for the QCD background estimation method (Section 5). The simulated events are re-132

weighted so that the distribution of the number of reconstructed primary vertex candidates per event133

matches the one measured in the ATLAS data. The Monte Carlo samples without pile-up have been used134

for the comparison plots in Section 4.135



MC DS ID Description Simulation tag Events Cross Section Filter Ef

107054 W → τν τ Pythia (incl) e514 s765 s767 r1430 r1429 149976 10.46 1

106043 W → eν e Pythia (no filter) e468 s765 s767 r1388 r1389 659931 10.46 1

106044 W → µν µ Pythia (no filter) e468 s765 s767 r1388 r1389 999885 10.46 1

106046 Z → ee Pythia (no filter) e468 s765 s767 r1388 r1389 999787 0.99 1

106047 Z → µµ Pythia (no filter) e468 s765 s767 r1388 r1389 998722 0.99 1

106052 Z → ττ Pythia e468 s765 s767 r1430 r1429 99980 0.99 1

106023 W → τ hν τ Pythia e468 s765 s767 r1302 100987 10.46×0.6479 1

105009 J0 e468 s766 s767 r1303 1399184 9.86 E+06 1

105010 J1 e468 s766 s767 r1303 1395383 6.78 E+05 1

105011 J2 e468 s766 s767 r1303 1397078 4.10 E+04 1

105012 J3 e468 s766 s767 r1303 1397430 2.20 E+03 1 105013 J4 e468 s766 s767 r1303 1397401 0.88 E+02 1

105014 J5 e468 s766 s767 r1303 1391612 2.35 E+00 1

105015 J6 e468 s766 s767 r1303 1347654 3.36 E-02 1

107414 W → τν τ Pythia (incl) e579 s766 s767 r1303 r1306 10.46 1

115859 J0 Pythia e570 s766 s767 r1303 361989 9.86 E+06 1

115860 J1 Pythia e570 s766 s767 r1303 332993 6.78 E+05 1

115861 J2 Pythia e570 s766 s767 r1303 370993 4.10 E+04 1

115862 J3 Pythia e570 s766 s767 r1303 392997 2.20 E+03 1

115863 J4 Pythia e570 s766 s767 r1303 397986 0.88 E+02 1

Table 4: Monte Carlo datasets used in the analysis.



December 10, 2010 – 15 : 48 DRAFT 12

3 Object reconstruction136

The identification of W → τ hν τ decays relies on the measurement of E missT , τ h identification and rejection137

of jets, electrons and muons.138

The transverse missing energy reconstruction is based on calorimeter information. This relies on139

a cell-based algorithm which sums the energy deposits of calorimeter cells inside three-dimensional140

topological clusters (topoclusters) [19]. These clusters are then corrected to take into account the dif-141

ferent responses to hadrons and to electrons or photons, dead material losses and out-of-cluster energy142

losses [20]. The x- and y-components of the calorimeter E missT term are calculated by summing over the143

transverse energies measured in these topological cluster cells i, calibrated according to the local cluster144

weighting scheme [21]:145

E miss x, y =−∑

i

E Caloi, x, y . (1)

The variable E missT is defined as:146

E missT =

( E miss

x )2 + ( E miss y )2. (2)

The resolution on E missT has been measured in minimum-bias events and depends on the scalar sum of 147

the transverse cell energies:148

∑ E T =∑i

( E Calo

i, x )2 + ( E Caloi, y )2. (3)

If E missT and ∑ E T are expressed in GeV, the E miss

T resolution is σ ( E missT ) = 0.49

√ ∑ E T [22].149

Hadronically decaying τ leptons are reconstructed starting from either calorimeter or track seeds [23].150

Track-seeded candidates have a seed track with pT > 6 GeV satisfying quality criteria on the number151

of associated hits in the silicon tracker ( N Sihit ≥ 7) and on the impact parameter with respect to the152

hit interaction vertex (|d 0| < 2 mm and | z0| × sinθ < 10 mm). Calorimeter-seeded candidates consist153

of calorimeter jets reconstructed with the anti-k t algorithm [24] (using a distance parameter D = 0.4)154

from topological clusters with calibrated E T > 10 GeV from the global cell energy-density weighting155

calibration scheme [21]. Candidates are labelled double-seeded when a seed track and a seed jet are156

within a distance ∆ R < 0.2 of each other. This analysis only considers τ h candidates that are double-157

seeded. The identification algorithm for τ h candidates (τ h-ID [9]) is based on the following quantities:158

• Track radius: Rtrack , the pT -weighted ∆ R width of tracks associated with the τ h candidate, mea-159

sured with respect to the calorimeter-seed axis.160

• Electromagnetic radius: REM, the E T -weighted ∆ R width of all cells in the first three layers of the161

EM calorimeter associated with the τ h candidate, measured with respect to the calorimeter-seed162

axis.163

• Leading track momentum fraction: f trk ,l, the ratio between the pT of the leading track and the164

total visible transverse momentum of the τ h candidate.165

Selection criteria on these variables are defined to provide a loose, medium and tight identification166

with average efficiencies for τ h of 60%, 50%, and 30%, respectively, and measured efficiencies for167

background jets of about 30%, 10% and 2%, respectively [9].168

The ATLAS standard electron reconstruction and identification algorithm [25] is designed to pro-169

vide various levels of background rejection optimised for high identification efficiencies, over the full170

acceptance of the inner-detector system. The ATLAS muon identification and reconstruction algorithms171

take advantage of the multiple sub-detector technologies which provide complementary approaches and172

cover pseudorapidities up to 2.7 over a wide pT range [26].173



December 10, 2010 – 15 : 48 DRAFT 13

In this analysis most electrons and muons are suppressed by discarding events containing a loose174

electron [25] or a combined muon [26].175

In addition, a specific electron and muon veto is applied to the selected τ h candidates provided by the176

τ h-ID algorithm [23], which rejects electrons and muons that are misidentified as τ h candidates, but are177

not identified by the ATLAS electron and muon identification:178

Electron Veto The baseline electron veto method relies on requirements applied to variables that provide179

good separation between electrons and hadronic τ candidates [27]: the ratio between the energy180

deposited in the first layer of the hadronic calorimeter and the leading track momentum (EHad/p),181

the ratio between the energy deposited in the electromagnetic calorimeter and the momentum of 182

the leading track (EEM/p), the maximum energy deposits in the second layer of the electromagnetic183

calorimeter not associated with the leading track and the ratio of high-threshold to low-threshold184

hits in the Transition Radiation Tracker (TRT).185

Based on these variables two flags are provided for the user: medium and tight, corresponding to186

different levels of electron suppression. The medium flag provides a factor of 50 rejection at the187

expense of losing about 5% of the reconstructed hadronic τ candidates while the tight criterion188

enables a suppression of electrons down to the per mill level with 15% loss of signal. In this189

analysis the tight electron veto was used as it matches best the requirements for the W → τ hν τ 190

signal extraction.191

Muon Veto One of the main characteristics of muons is the small amount of energy deposited in the192

Calorimeters. The baseline muon veto algorithm rejects events with total energy deposition in the193

Electromagnetic and Hadronic Calorimeters (at the electromagnetic scale) below 5 GeV. Since the194

energy threshold for the reconstruction of a τ h candidate is 10 GeV (at the jet scale) this veto is195

fully efficient for the signal.196

4 Event selection197

4.1 Background processes198

We consider the following background processes:199

• QCD multi-jet events200

Mis reconstructed QCD events where one jet is incorrectly identified as a hadronically decaying τ 201

lepton and a significant amount of missing transverse energy is also mis-reconstructed constitute202

the dominant background source. The cross section is several orders of magnitude larger than the203

signal cross section. Thus, a good understanding and effective suppression of these processes is204

critical for this analysis.205

• W → eν /µν 206

These processes contribute to the background if the lepton from the W -boson decay is identified207

as a single-prong hadronically decaying τ lepton or if a fake τ h candidate is reconstructed from208

initial-state QCD radiation. The first case is strongly suppressed by vetoing candidates tagged by209

an electron-veto algorithm and requiring that no muons are present in the event. The remaining210

small fraction of events for which the electron/muon is lost contributes with fake τ h candidates211

from initial-state radiation.212

• W → τν → eν /µν 213

Leptonic decay modes of τ leptons are difficult to distinguish from primary electrons and muons.214

Therefore, similarly to W → eν and W → µν , this process contributes to the background if the215



December 10, 2010 – 15 : 48 DRAFT 14

lepton is reconstructed as a single-prong hadronically decaying τ lepton. These events can be216

suppressed by vetoing electrons and muons in the event.217

• Z

→e+e−/µ +µ −218

Leptonic Z -boson decays contribute if one of the decay electrons/muons is incorrectly recon-219

structed as a hadronically decaying τ lepton and the other one is lost. As already discussed for220

the W → eν /µν processes, this background is strongly suppressed by explicitly vetoing single-221

prong electron-like or muon-like candidates in the τ had identification and by rejecting events with222

identified electrons and muons.223

• Z → τ +τ −224

The rate for this process is about ten times smaller than for the signal process. It contributes to225

the background if one of the τ leptons is identified as a hadronically decaying τ lepton while226

the second one is lost, i.e., neither reconstructed as second hadronically decaying τ lepton nor as227

electron or muon.228

• t t 229

This process has a much smaller cross section than the signal process and contributes to the back-230

ground if one of the W s produces a τ lepton in its decay and the other one decays into a pair231

of quarks, an electron, or a muon which are not reconstructed. Fully hadronic decays can also232

contribute to the fake τ h identification.233

4.2 Event selection234

In addition to the selection of good data quality and the trigger requirements described in Section 2,235

further preselection criteria are applied:236

• at least one primary vertex reconstructed with at least four tracks is required in the event;237

• events with “bad” jets [28] caused by out-of-time cosmic events or known noise effects in the238

calorimeters are rejected;239

• events are rejected if a jet with pT > 20 GeV is reconstructed in the pseudorapidity range 1.3 <240

|η|< 1.7, corresponding to a gap in the ATLAS calorimeter acceptance, in order to suppress fake241

E missT ;242

• events are rejected if ∆φ ( jet, E missT ) < 0.5 rad, for jets with pT >20 GeV, to suppress events with243

mis-reconstructed jet energy.244

After this preselection, the events are further required to have the typical W → τ hν τ signature, i.e.,245

a τ h jet accompanied by missing energy due to the neutrinos that are not detected. A missing transverse246

energy of E missT > 30 GeV is required1). Then, the τ h candidates are selected: candidates reconstructed247

by both τ h reconstruction algorithms, the track-seeded and the calorimeter-seeded, and identified as tight248

τ h candidates (as described in Section 3) are considered. The highest- pT candidate of these is required249

to have a visible transverse momentum between 20 and 60 GeV. The event is rejected if the selected250

τ h candidate is reconstructed in the pseudorapidity range 1.3 < |η| < 1.7 which corresponds to the gap251

in the calorimeter systems. The distribution of the basic kinematic properties and the τ h-ID variables of 252

τ h candidates is shown in Figure 5, where data is compared with a Monte Carlo QCD di-jet sample with253

DW tune. The τ h candidates are required to have a minimum pT of 20 GeV and to pass the first two254

preselection criteria - the requirement on the number of vertices and the rejection of “bad“ jets.255

1)Here, the MET LocHadTopo implementation of the missing transverse energy reconstruction is used.



December 10, 2010 – 15 : 48 DRAFT 15

[MeV]T

phadτ

0 10 20 30 40 50 60 70 80 90 10010×

F r a c t i o n o f E v e n t s

0

0.05

0.1

0.15

0.2

0.25

0.3

= 7 TeV)sData 2010 (

Pythia QCD Jets (DW tune)



(a)

ηhadτ

-3 -2 -1 0 1 2 3


0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1





(b)

φhadτ

-3 -2 -1 0 1 2 3


0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

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(c)

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0

0.02

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0.06

0.08

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νhadτ→W



νhadτ→W



νhadτ→W


(d)

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0

0.02

0.04

0.06

0.08

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0.12

0.14

0.16

0.18

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νhadτ→W


(e)

trk-1F0 0.2 0.4 0.6 0.8 1 1.2 1.4


0

0.05

0.1

0.15

0.2

0.25


νhadτ→W



νhadτ→W



νhadτ→W


(f)

Figure 5: Distributions for all reconstructed τ h candidates with a minimum pT of 20 GeV which pass

the trigger and the first two preselection criteria defined in Section 4 for data and simulated Monte Carlo

QCD di-jet samples with DW tune. In the last three histograms also the expected distribution for the

W → τ hν τ signal from Monte Carlo is shown. (a) τ h pT (b) τ h η (c) τ h φ (d) EM radius (e) Track radius

(f) Leading track momentum fraction of the τ h candidate.



December 10, 2010 – 15 : 48 DRAFT 16

A good agreement of all distributions between data and the QCD Monte Carlo simulations can be256

seen for the τ h candidates which are at this early stage of the event selection dominated by QCD pro-257

cesses.258

Electron and muon vetoes are applied to suppress the background from W

→eν e, W

→µν µ , W

→259

τ ℓν τ , Z → ee, Z → µµ and Z → ττ , referred to as electroweak (EW) background. Events with loose elec-260

trons or combined muons identified by the electron/muon reconstruction algorithms with pT > 5 GeV261

are rejected. Additional electron and muon vetoes provided by the τ h-ID algorithm which reject elec-262

trons and muons misidentified as τ h candidates and have not been reconstructed by the electron and muon263

algorithms are applied as explained in Section 3.264

Finally, the event selection includes a requirement on the significance of the missing transverse en-265

ergy, defined as:266

S E missT

= E miss

T [GeV]

0.5[GeV1/2] · ∑ E T [GeV]

. (4)

Events are rejected if S E missT

<6. This requirement is essential for the rejection of QCD background,267

for which lower S E missT values are expected than for W → τ hν τ events. Figure 6 (left) shows the two-268

dimensional distribution of E missT and

√ ∑ E T for signal, QCD background and data, together with the269

S E missT

requirement. The discriminating power of S E missT

is clearly visible in Figure 6 (right) showing the270

two-dimensional distribution of S E missT

and the transverse mass mT2) of the τ h and E miss

T system.271

The selection results in 78 events in data for an integrated luminosity of 546 nb−1. For Monte Carlo272

simulation, the estimated number of signal events that pass the selection is 55.3±1.4 events. The back-273

ground from other W and Z decays is 11.8±0.4 events. The QCD di-jet background must be estimated274

from data, as described in Section 5. In fact, the simulated samples are too small (see Table 7) and the275

uncertainties on the cross section too big to be able to rely on Monte Carlo for a good estimate of this276

background. The contribution from t t events is found to be negligible. The number of events passing277

each selection criterium for all data runs and Monte Carlo samples is shown in Table 5. An overview of 278

the full event selection, for data and simulated electroweak background, is given in Table 6.279

The Monte Carlo samples with pile up were weighted for the vertex multiplicity found in data, which280

is shown in Table 8. The vertex multiplicity was obtained after the trigger requirement and the vertex and281

bad jet cleaning. The trigger applied at this stage has different requirements for E missT at L2 for different282

runs, but the effect of this discrepancy was found to be negligible.283

2)The transverse mass in a W → τ hν τ decay is defined as mT =

2 · E τ hT · E missT · 1−cos∆φ

τ h, E miss

T

.



Run May Repro D1 D2 D3 D4 D5 D6

Events 4049579 1605779 1801437 1682089 3470818 1291341 3526951

Skimming 223201 473711 435833 366195 682871 257377 643115

GRL 220981 473506 432596 365464 681123 257377 610408

Trigger 28541 54375 65718 68556 169707 59265 210401

CollCand 28540 54373 65717 68555 169704 59264 210394

JetClean 27610 52085 63184 65910 163272 57042 202241

JetVeto 21046 40205 49010 51133 126817 44219 156997

DeltaPhi jet 11731 23882 28963 30164 74451 25767 92393

METcut 532 1500 1757 1781 4476 1483 5662

τ h-ID 94 169 178 182 477 171 551

τ h-ID Et 74 135 133 134 366 129 412

τ h-ID eta 72 135 132 131 365 126 409

τ h-ID lep 21 46 41 41 112 45 131

LeptVeto 20 41 37 33 93 35 111

METSign 2 2 4 2 13 3 18

Table 5: Cut flow for data.



Data W → τ hν τ W → eν e W → µν µ W → τ ℓν τ Z → ee Z

Events 18412648 3700.3 5711.2 5711.2 1756.0 540.5 5

Skimming 3645658 1838.7±6.0 4868.5±2.5 1298.5±2.4 581.7±3.6 92.0±0.2 135

GRL 3586463 1838.7±6.0 4868.5±2.5 1298.5±2.4 581.7±3.6 92.0±0.2 135

Trigger 986439 954.5±5.2 3560.7±3.4 521.4±1.6 296.5±2.8 75.3±0.2 59

CollCand 986422 954.5±5.2 3560.7±3.4 521.4±1.6 296.5±2.8 75.3±0.2 59

JetClean 948247 942.7±5.2 3519.6±3.4 511.3±1.6 292.2±2.8 74.1±0.2 58

JetVeto 729822 767.2±4.8 2836.7±3.5 415.7±1.5 240.6±2.6 48.8±0.2 47

DeltaPhi jet 415951 728.3±4.7 2735.3±3.5 400.7±1.5 229.4±2.6 24.5±0.1 45

METcut 29686 411.5±3.8 1828.3±3.3 317.1±1.3 121.9±1.9 1.13±0.03 34τ h-ID 3190 135.3±2.2 1564.1±3.1 44.9±0.5 50.6±1.3 0.88±0.02 5.

τ h-ID Et 2428 119.1±2.1 1491.8±3.1 26.8±0.4 34.7±1.1 0.59±0.02 3.2

τ h-ID eta 2408 118.0±2.1 1482.0±3.1 26.6±0.4 34.4±1.0 0.59±0.02 3.2

τ h-ID lep 811 102.0±2.0 16.5±0.4 22.3±0.4 6.1±0.4 <0.01 2.5

LeptVeto 685 94.8±1.9 6.7±0.2 4.9±0.2 2.3±0.3 <0.005 0.1

METSign 78 55.3±1.4 4.2±0.2 3.7±0.1 1.8±0.2 0.0

Table 6: Number of events passing the selection criteria for data and expected values for Monte Carlo signal and E

integrated luminosity of 546 nb−1.



J0 J1 J2 J3 J4 J5

Events 1399184 1395383 1397078 1397430 1397401 1391612 13

Skimming 1053 9412 77978 316343 676161 980236 11

GRL 1053 9412 77978 316343 676161 980236 11

Trigger 16 1418 41340 273176 627235 917572 10

CollCand 16 1418 41340 273176 627233 917568 10

JetClean 15 1383 40624 268933 615844 898534 10

JetVeto 12 1156 29716 179205 382279 547382 73

DeltaPhi jet 12 937 13112 50031 96314 116397 11

METcut 0 18 364 1353 3859 9246 1

τ h-ID 0 0 39 209 962 2603 4

τ h-ID Et 0 0 22 58 91 184

τ h-ID eta 0 0 22 57 91 184

τ h-ID lep 0 0 20 49 74 163

LeptVeto 0 0 13 37 57 117

METSign 0 0 1 2 1 4

Table 7: Number of events passing the selection criteria for Monte Carlo QCD di-jets backgrounds. Number



December 10, 2010 – 15 : 48 DRAFT 20

[GeV]T

E∑0 5 10 15 20 25 30

[ G e V ]

T m i s s

E

0

20

40

60

80

100


Pythia QCD Jets

τ νhτ→W

-1Integrated Luminosity 546 nb

[GeV]Tm0 20 40 60 80 100 120 140

m i s s

T E

S

0

2

4

6

8

10

12

14 = 7 TeV )sData 2010 (

Pythia QCD Jets

τ νhτ→W


Figure 6: Distribution of events in the E missT –

√ ∑ E T plane after the trigger requirement (left) and S E miss

T–

mT plane after the lepton veto requirement (right) for data, simulated signal events and QCD background.

The applied E missT and S E miss

Tcriteria are indicated as solid lines.

Data

1 vtx 473774

2 vtx 324715

3 vtx 118554

4 vtx 29787

5 vtx 6775

Table 8: Number of well reconstructed vertexes, with at least four associated tracks, in data.



December 10, 2010 – 15 : 48 DRAFT 21

5 QCD background estimation284

Given the small number of available simulated QCD background events after the full event selection,285

which is due to the small size of produced Monte Carlo samples in conjunction with the large rejection286

factors of the selection criteria and identification algorithms, it is clear that we cannot rely on simulated287

event samples alone to accurately predict the rate of QCD processes. Therefore, a data-driven method is288

used to estimate the normalization and shape of kinematic and τ h-ID variable distributions for the QCD289

background.290

The method used to estimate the QCD background from data is based on the selection of four in-291

dependent data samples, three in QCD background-dominated regions (control regions) and one in a292

signal-dominated region (signal region). The samples are selected with criteria on S E missT

and on τ h-ID,293

which are assumed to be uncorrelated, after applying the event selection described in Section 4. In fact,294

S E missT

depends on global event properties and the τ h candidate contributes to its value only through its295

total pT , while the τ h-ID is based on shower shape and tracks of the τ h candidate. An indirect correlation296

may arise anyhow due to the dependence of the τ h-ID rejection on the pT of the τ h candidate [9]. This297

effect has been estimated in Section 6.8. The following four regions are used in this analysis:298

• Region A: events with S E missT

> 6 and τ h candidates satisfying the tight τ h-ID;299

• Region B: events with S E missT

< 6 and τ h candidates satisfying the tight τ h-ID;300

• Region C: events with S E missT

> 6 and τ h candidates satisfying the loose τ h-ID but failing the tight301

τ h-ID;302

• Region D: events with S E missT

< 6 and τ h candidates satisfying the loose τ h-ID but failing the tight303

τ h-ID.304

Region A is referred to as the signal region and regions B, C and D as control regions. Figure 7 illustrates305

the four regions.306

-IDτLoose and fail Tight Tight

T m i s s

E S

0

2

4

6

8

10

12

14

16

18

20

AC

BD

= 7 TeV )sData 2010 (


(a)

-IDτLoose and fail Tight Tight

T m i s s

E S

0

2

4

6

8

10

12

14

16

18

20

AC

BD

τ νhτ→W


(b)

Figure 7: S E missT

distribution for events in which the selected τ h candidate passes the loose but fails the

tight τ h-ID and for events in which the selected τ h candidate passes the tight τ h-ID. Distributions are

shown for data (a) and W → τ hν τ simulation (b) after applying the event selection described in Section 4,

except for the last requirement on S E missT

. The area of the boxes are proportional to the event yield.

The S E

miss

T

distribution for QCD background events in the signal region is estimated as follows:307



December 10, 2010 – 15 : 48 DRAFT 22

Region A B C D

Data 78 607 254 7107

W → τ hν τ 55.3±1.4 39.5±1.2 71.0±1.6 54.2±1.4

EW 11.8

±0.4 6.5

±0.2 44.5

±0.7 22.1

±0.5

ci 0.69±0.02 1.72±0.05 1.14±0.03

Table 9: Number of observed events in the four regions for the data-driven estimation of QCD back-

ground. Monte Carlo estimates of the number of W → τ hν τ signal and EW background events and the

correction coefficients ci are also shown.

• the shape is determined from the observed events in regions C and D;308

• the distribution in region CD is then normalized to the ratio of the numbers of events in regions B309

and D.310

This prediction is based on two assumptions, namely that the shape of the S E missT

distribution for311

QCD background is the same in the combined regions AB and CD and that the signal and electroweak 312

background contribution in the three control regions is negligible. Provided that the two assumptions are313

satisfied, the method does not rely on any other inputs. In fact, the latter condition is not fully satisfied314

and corrections to account for this are applied at a later stage. The estimate for QCD background in the315

signal region A is then obtained by:316

NAQCD = NBNC/ND (5)

where Ni represents the number of observed events in region i.317

The assumption that the shape of the S E missT

distribution for QCD background in regions AB and CD318

is the same has been verified with a data control sample, as described in Section 5.1.319

The second assumption, requiring the signal contamination in the control regions to be small is320

checked with a W → τ hν τ Monte Carlo sample. The fraction of signal events in the control regions is321

found to be non-negligible, in particular for the control region C. This can also be seen in Figure 7. In322

addition, the contribution of EW backgrounds in the signal region and control regions is significant and323

needs to be taken into account. Table 9 shows the number of data events and the expected signal and EW324

background events (Nisig and Ni

EW, respectively) in regions A, B, C and D. The ratios of simulated signal325

and EW background events in the control regions and the signal region are denoted by the coefficients326

ci =Ni

sig + NiEW

NAsig + NA

EW

, i = B,C,D (6)

and are summarized in Table 9.327

The QCD background determination in the signal region needs to take into account the signal leakage328

into the background control regions as well as the EW background contamination. Defining N Anon−QCD329

as the number of non-QCD data events (signal and EW background events) in region A, the corrected330

number of data events in the three control regions (N Bcorr, NC

corr and NDcorr) is obtained by subtracting the331

number of signal and EW background events ciNAnon−QCD from the observed number of data events in332

each of the three control regions:333

NBcorr = NB− cBNA

non−QCD, NCcorr = NC− cCNA

non−QCD and NDcorr = ND− cDNA

non−QCD. (7)

Using the relation NA =NAnon−QCD + NA

QCD and applying this correction to Equation 5 yields334

NA

QCD = (NB

−cB(NA

−NA

QCD))

NC

−cC(NA

−NA

QCD)

ND− cD(NA−NAQCD) . (8)



December 10, 2010 – 15 : 48 DRAFT 23

After solving the resulting second order polynomial equation for NAQCD, Equation 8, the estimated335

QCD background in region A is 11.1±2.3 events and non-QCD events in region A is 66.9±10.5.336

The results of the QCD background estimation can be seen in Figure 8 for the S E missT

distribution.337

The data distribution corresponds to the combined region AB and the QCD background to the combined338

region CD after subtraction of EW and signal contributions based on Monte Carlo simulation. The QCD339

background is normalized by a factor (N B − cBNAnon−QCD)/(N D − cDNA

non−QCD). A good agreement is340

observed, with an excess of data that is compatible with the simulated distribution of W → τ hν τ signal341

events.342

TmissES

0 2 4 6 8 10 12 14

N u m b e r o f E v e n t s / 0 . 5

1

10

210

310

TmissES

0 2 4 6 8 10 12 14


1

10

210

310-ID)

hτ= 7 TeV ) (TightsData 2010 (

-ID)hτQCD background (Loose

EW background

τ νhτ→W


Figure 8: S E missT

distribution for the combined region AB (tight τ h-ID region) and the combined control

region CD (loose τ h-ID region). Also shown are the expected signal and EW backgrounds in region AB

from simulated samples added to the histogram for the control region. The normalization of the QCD

background distribution is explained in the text.

In order to cross-check the validity of the method, an independent estimation of the number of QCD343

background events in region A is performed. For this, a data sample was selected as described in section344

5 without applying the τ h-ID requirement for the τ h candidates. Using the tight τ h-ID misidentification345

rate for QCD jets, parametrized with data as a function of pT [9] (Figure 9, right), the estimated number346

of events with misidentified τ h candiates can be extracted. The contribution from signal and EW back-347

ground in the selected sample is subtracted. But it was not verified if the pT spectrum in Figure 9 (left)348

represents the correct distribution of the QCD background when the tight τ h ID is required. This estima-349

tion, however, can be used as a simple test to verify if the number of QCD background events obtained350

with the data-driven method is sensible. Indeed, the estimated number of misidentified τ h candidates in351

this sample is 6.6±1.2(stat.)±1.1(syst.) events, which is in agreement with the number of expected QCD352

background events in signal region A obtained from the data-driven method (the systematic uncertainty353

considered the 9.6% due to energy calibration and 14.5% due to pile-up effects [9]).354

To confirm the signal observation, several characteristic distributions for W → τ hν τ decays and355

τ h candidates are shown in Figures 10 to 14. Figure 10 shows the ∆φ (τ h, E missT ) and mT distributions356

for data in the signal region and in the different enriched QCD background regions. In both distributions357



December 10, 2010 – 15 : 48 DRAFT 24

[GeV]T

p

20 25 30 35 40 45 50 55 60

N u m b e r o f E v e n t s

0

20

40

60

80

100

120

140

160

Candidateshτ


(a)

[GeV]T

p

0 10 20 30 40 50 60 70 80 90 100

b k g d

ε

-210

-110

1

-1Integrated Luminosity 244 nb / Loose Cuts (Data/MC) / Medium Cuts (Data/MC) / Tight Cuts Data(MC)

(b)

Figure 9: (a) Distribution of the transverse momentum of τ h candidates, without applying the τ h-ID

requirement, for events in the signal region A. (b) τ h misidentification efficiency for QCD as a functionof pT . The number of τ h candidates in (a) is 442 events and the expected number of signal and EW

background from Monte Carlo simulation is 147±2 and 76±1 events, respectively.

the expected characteristic signature of W → τ hν τ decays can be observed: E missT and the τ h are most358

likely to lie in opposite direction in the transverse plane and the transverse mass distribution reaches its359

maximum between 60 and 80 GeV. Figure 11 shows the distribution of the number of tracks and of the360

electric charge. Displayed are the data distribution in signal region A and the distribution of QCD events361

in the control region B or C with the additional expectation from signal and EW background. The QCD362

samples are corrected for the estimated contribution of signal and EW background in the respective re-363

gion and normalized to the number of expected QCD events in region A. These distributions show strong364

evidence of hadronic τ decays. In particular the track multiplicity distribution with peaks at one and365

three tracks, as expected for τ h decays, which mostly result in one or three charged particles. The electric366

charge distribution for the selected τ h candidates shows a slight, but statistically not yet significant ex-367

cess of events with a positive electric charge. Kinematic observables for E missT and the τ h candidates are368

shown in Figure 12 and Figure 13. The variables used in the τ h-ID were also studied. Figure 14 shows369

the distributions for f trk ,l, Rtrack and REM.370

The agreement between data in the signal region and the control regions, based on the estimated371

values from the data-driven method, combined with the signal and EW expectation from Monte Carlo372

further supports the observation of hadronically decaying τ leptons from W → τ hν τ decays in ATLAS.373



December 10, 2010 – 15 : 48 DRAFT 25

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5

)

π

N u m b e r o f E v e n t s / (

0

50

100

150

200

250

300

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5

)

π


0

50

100

150

200

250

300

Signal Region (AB)

Bkgd Control Region (CD)

EW background

τ νhτ→W


(a)

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0 20 40 60 80 100 120

N u m b e r o f E v e n t s / 1 0 G e V

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350

[GeV]Tm

0 20 40 60 80 100 120


0

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100

150

200

250

300

350

Signal Region (AB)

Bkgd Control Region (CD)

EW background

τ νhτ→W


(b)

) [rad]T

miss

,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

10

20

30

40

50

) [rad]T

miss

,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

10

20

30

40

50

Signal Region (A)

Bkgd Control Region (B)

EW background

τ νhτ→W


(c)

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

35

40

45

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

35

40

45

Signal Region (A)


EW background

τ νhτ→W


(d)

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

10

20

30

40

50

60

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

10

20

30

40

50

60

Signal Region (A)

Bkgd Control Region (C)

EW background

τ νhτ→W


(e)

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

35

40

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(f)

Figure 10: Distribution of ∆φ (τ h, E missT ) in (a),(c) and (e) and transverse mass mT in (b),(d),(f) for data

in the signal region and the QCD background control region, combined with MC signal and EW back-

ground. The QCD background distribution is normalized to the estimated number of QCD background

events: (N B− cBNAnon−QCD)/(N D− cDNA

non−QCD) in (a) and (b), NAQCD in (c), (d) and (f).



December 10, 2010 – 15 : 48 DRAFT 26

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

35

40

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(a)

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

35

40

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(b)

Electric charge

-3 -2 -1 0 1 2 3

N u

m b e r o f E v e n t s

0

10

20

30

40

50

60

Electric charge

-3 -2 -1 0 1 2 3

N u


0

10

20

30

40

50

60

Signal Region (A)


EW background

τ νhτ→W


(c)

Electric charge

-3 -2 -1 0 1 2 3

N u


0

10

20

30

40

50

60

Electric charge

-3 -2 -1 0 1 2 3

N u


0

10

20

30

40

50

60

Signal Region (A)


EW background

τ νhτ→W


(d)

Figure 11: Distribution of the number of tracks (a) and (b) and of the electric charge (c) and (d) of the τ hcandidates. Displayed is the data distribution in the signal region compared to different control samples

(region B, S E missT

<6, left; region C, loose τ h ID,right) and the additional contribution of EW and signal

expected from Monte Carlo.



December 10, 2010 – 15 : 48 DRAFT 27

[GeV]Tmiss

E

30 40 50 60 70 80 90 100


0

10

20

30

40

50

[GeV]Tmiss

E

30 40 50 60 70 80 90 100


0

10

20

30

40

50

= 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(a)

φ missTE

-3 -2 -1 0 1 2 3

/ 6 )

π

N u m b e

r o f E v e n t s / (

0

5

10

15

20

25

30

φ missTE

-3 -2 -1 0 1 2 3

/ 6 )

π

N u m b e

r o f E v e n t s / (

0

5

10

15

20

25

30Signal Region (A)


EW background

τ νhτ→W


(b)

[GeV]T

E∑0 100 200 300 400 500 600


0

5

10

15

20

25

30

35

[GeV]T

E∑0 100 200 300 400 500 600


0

5

10

15

20

25

30

35

Signal Region (A)


EW background

τ νhτ→W


(c)

Figure 12: (a) distribution of the missing transverse energy, (b) the φ distribution of E miss

T and (c) the

∑ E T distribution compared for loose τ h ID (region C) and tight τ h ID (region A).



December 10, 2010 – 15 : 48 DRAFT 28

[GeV]T

p

20 25 30 35 40 45 50 55 60

N u m b e r o f E v e n t s / 5 G e V

0

5

10

15

20

25

[GeV]T

p

20 25 30 35 40 45 50 55 60


0

5

10

15

20

25

Signal Region (A)


EW background

τ νhτ→W


(a)

[GeV]T

p

20 25 30 35 40 45 50 55 60


0

5

10

15

20

25

[GeV]T

p

20 25 30 35 40 45 50 55 60


0

5

10

15

20

25

Signal Region (A)


EW background

τ νhτ→W


(b)

η

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

N u m b e r o f E v e n t s / 5

0

5

10

15

20

25

30

35

η

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5


0

5

10

15

20

25

30

35

Signal Region (A)


EW background

τ νhτ→W


(c)

η

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5


0

5

10

15

20

25

30

35

η

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5


0

5

10

15

20

25

30

35

Signal Region (A)


EW background

τ νhτ→W


(d)

φ

-3 -2 -1 0 1 2 3

/ 3 )

π


0

5

10

15

20

25

30

35

40

φ

-3 -2 -1 0 1 2 3

/ 3 )

π


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(e)

φ

-3 -2 -1 0 1 2 3

/ 3 )

π


0

5

10

15

20

25

30

35

40

φ

-3 -2 -1 0 1 2 3

/ 3 )

π


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(f)

Figure 13: Distribution of (a) and (b) the pT spectrum, (c) and (d)η and (e) and (f) φ of the τ h candidates.

Displayed is the data distribution in the signal region compared to different control samples (region B,

S E missT

<6, left; region C, loose τ h ID, right) and the additional contribution of EW and signal expected

from Monte Carlo.



December 10, 2010 – 15 : 48 DRAFT 29

trk,l f

0 0.2 0.4 0.6 0.8 1 1.2 1.4

N u m b e

r o f E v e n t s / 0 . 0

5

0

10

20

30

40

50

60

trk,l f

0 0.2 0.4 0.6 0.8 1 1.2 1.4

N u m b e

r o f E v e n t s / 0 . 0

5

0

10

20

30

40

50

60

>6)miss

TE = 7 TeV ) (SsData 2010 (

<6)miss

TEQCD background (S

EW background

τ νhτ→W


(a)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b e r

o f E v e n t s / 0 . 0

0 5

0

20

40

60

80

100

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b e r

o f E v e n t s / 0 . 0

0 5

0

20

40

60

80

100

Signal Region (AC)

Bkgd Control Region (BD)

EW background

τ νhτ→W


(b)

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m b e r o f E v

e n t s / 0 . 0

2

0

20

40

60

80

100

120

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m b e r o f E v

e n t s / 0 . 0

2

0

20

40

60

80

100

120

Signal Region (AC)


EW background

τ νhτ→W


(c)

Figure 14: Distribution of the τ h-ID variables for the regions with S E

miss

T

>6 and S E

miss

T

<6 (regions AC

and BD) combined with the expected signal and EW background contributions from simulation.



December 10, 2010 – 15 : 48 DRAFT 30

5.1 Verification of the assumptions from the data-driven method374

As a test of the validity of the data-driven method, the assumption that the S E missT

and τ h-ID variables375

are not correlated are verified in this section. This study, however, could not be performed using QCD376

Monte Carlo simulation due to the small size of the event samples combined with the large rejection377

factors of the event selection criteria. Even by loosening some of the criteria, e.g. replacing the tight τ h-378

ID criterium by the medium τ h-ID, it was not possible to test the data-driven method with Monte Carlo379

samples. Therefore, the tests needed to be done in data, using a control sample produced by selecting380

τ h candidates with more than three tracks (Ntrack >3)3)381

5.1.1 Shape of S E missT

distribution382

For the successful prediction of the number of QCD background events in the signal region with the383

data-driven method it has to be verified first that the S E missT

distribution for the QCD background in the384

combined regions AB and CD is the same, i.e., that the S E missT

distribution is independent of the τ h-ID385

selection. This has been done with a data control sample of τ h candidates with more than three tracks:386

Figure 15(a) compares the S E missT

distribution for events that pass the loose τ h-ID but fail the tight τ h-ID387

with events that pass the tight τ h-ID, with the additional requirement that the selected τ h candidates have388

Ntrack >3. Both distributions agree within the statistical uncertainties. To check if these distributions also389

represent events with selected τ h candidates with any number of tracks, Figure 15(b) compares S E missT

for390

τ h candidates that pass the loose but fail the tight τ h ID. A similar level of agreement is observed.391

T

missE

S

0 2 4 6 8 10 12 14


-410

-310

-210

-110

1

10

Tight

Loose and fail Tight


(a)

T

missE

S

0 2 4 6 8 10 12 14


-410

-310

-210

-110

1

10

>0trackN

>3trackN


(b)

Figure 15: S E missT

distributions. (a) Distribution for a data control sample of τ h candidates with Ntrack >3,

for τ h candidates that pass the loose τ h-ID but fail the tight τ h-ID and for τ h candidates that pass the

tight τ h

-ID. (b) Distribution for selected τ h

candidates that pass the loose τ h

-ID for Ntrack

>3 and for any

number of tracks.

3)Reconstructed τ h candidates with large track multiplicities are dominated by misidentified jets.



December 10, 2010 – 15 : 48 DRAFT 31

5.2 Additional validation tests for the data-driven method392

In order to confirm that the data-driven method yields consistent results and can be used to estimate the393

number of QCD background, several cross checks are performed. The method is applied to different394

subsamples of the selected events and the results are compared with the expectations.395

5.2.1 Applying the data-driven method to a control sample396

In order to verify if the data-driven method correctly predicts the number of QCD background events397

in the signal region we apply the method to a QCD background enriched sample produced by selecting398

τ h candidates with more than three tracks. The numbers are listed in Table 10.399

A B C D

Data 5 95 92 2355

W → τ hν τ 2.7±0.3 0.7±0.2 11.5±0.7 6.9±0.5

EW 1.8±

0.1 0.6±

0.1 18.1±

0.5 6.9±

0.3

ci 0.29±0.08 6.58±0.56 3.07±0.29

Table 10: Number of observed events in the four regions for the data-driven estimation of QCD back-

ground for a data control sample of τ h candidates with Ntrack >3. The Monte Carlo estimates of the

number of W → τ hν τ signal and EW background events for this sample and the correction coefficients ci

are also shown.

As can be observed in Table 10, the data samples with selected τ h candidates with Ntrack >3 are quite400

small and, according to Monte Carlo simulations, still contain a significant contribution of signal and EW401

background events. Nonetheless, the estimated number of QCD background events of 3.2±1.1 in signal402

region A is in agreement with the observed number of data events which remain when the signal and EW403

background expectations are subtracted. This indicates the consistency of the data-driven method.404



December 10, 2010 – 15 : 48 DRAFT 32

5.2.2 Study of τ h candidates in different pT ranges405

A different approach to confirm the validity of the data-driven method for a QCD background estimation406

is to consider τ h candidates in different pT regions separately. To obtain data samples of approximately407

the same size for this study, the following pT regions have been defined:408

• τ h candidates with a transverse momentum between 20GeV < pT <30GeV;409

• τ h candidates with a transverse momentum between 30GeV < pT <60GeV.410

The number of events for the first sample (20GeV < pT <30GeV) in data and the signal and EW411

Monte Carlo samples in the four different regions are listed in Table 11.412

A B C D

Data 23 201 58 2487

W → τ hν τ 21.3±0.9 13.7±0.7 27.9±1.0 24.1±1.0

EW 2.5±0.2 1.9±0.1 10.4±0.3 7.8±0.3ci 0.66±0.04 1.61±0.08 1.34±0.07

Table 11: Number of observed events and Monte Carlo expectations in the four regions for a data sample

with τ h candidates within a transverse momentum range of 20GeV < pT <30GeV.

The number of expected QCD-background events, based on the numbers in Table 11, is 1.9 ± 0.9.413

Figure 16 shows the distributions of kinematic and τ h-ID variables for these τ h candidates.414

The number of events in the four regions, for τ h candidates of the second sample within 30 GeV < pT 415

<60GeV, are given in Table 12.416

A B C DData 55 406 196 4620

W → τ hν τ 34.0±1.1 25.9±1.0 43.1±1.3 30.1±1.1

EW 9.2±0.3 4.5±0.2 34.1±0.6 14.3±0.4

ci 0.70±0.03 1.79±0.06 1.03±0.04

Table 12: Number of observed events and Monte Carlo expectations for a data sample with τ h candidates

within a transverse momentum range of 30GeV < pT <60GeV.

The resulting number of expected QCD background events in signal region A is 9.4 ± 2.1. The417

distribution of kinematic and τ h

-ID variables for these τ h

candidates is shown in Figure 17.418

For both subsamples a very good agreement of data and control sample with the additional con-419

tribution from MC signal and EW background can be observed. Also the resulting number of QCD420

background events are consistent with the estimation for the whole sample of 11.1±2.3 events, which421

again confirms the validity of the data-driven method for an estimation of QCD background events.422



December 10, 2010 – 15 : 48 DRAFT 33

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

-ID)hτ= 7 TeV ) (TightsData 2010 (


EW background

τ νhτ→W


(a)

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π

N u m b e r o f E v e n t s /

(

0

2

4

6

8

10

12

1416

18

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


(

0

2

4

6

8

10

12

1416

18

Signal Region (A)


EW background

τ νhτ→W


(b)

Number of Tracks

0 1 2 3 4 5 6 7 8


0

2

4

6

8

10

12

14

16

Number of Tracks

0 1 2 3 4 5 6 7 8


0

2

4

6

8

10

12

14

16

Signal Region (A)


EW background

τ νhτ→W


(c)

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m b e r o f E v e n t s / 0 . 0 2

0

5

10

15

20

25

30

35

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m b e r o f E v e n t s / 0 . 0 2

0

5

10

15

20

25

30

35

Signal Region (AC)


EW background

τ νhτ→W


(d)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

5

10

15

20

25

30

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

5

10

15

20

25

30

Signal Region (AC)


EW background

τ νhτ→W


(e)

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

2

4

6

8

10

12

14

16

18

20

22

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

2

4

6

8

10

12

14

16

18

20

22

Signal Region (AC)


EW background

τ νhτ→W


(f)

Figure 16: Comparison of data and different control regions with the additional expected contribution

of signal and EW backgrounds for τ h candidates within a transverse momentum range of 20GeV < pT

<30GeV. (a) S E missT

distribution for tight τ h candidates (region AB) and loose candidates failing the tight

τ h-ID (region CD). (b) ∆φ (τ h, E missT ) distribution for events in the signal region A and for loose candidates

failing the tight τ h-ID (region C). (c) track multiplicity distribution for τ h candidates. (d) - (f) Distribution

of the τ h-ID variables for events with S E missT

>6 (region AC) and S E missT

<6 (region BD).



December 10, 2010 – 15 : 48 DRAFT 34

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310-ID)

hτ= 7 TeV ) (TightsData 2010 (


EW background

τ νhτ→W


(a)

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

30

35

40

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

30

35

40

Signal Region (A)


EW background

τ νhτ→W


(b)

Number of Tracks

0 1 2 3 4 5 6 7 8


0

5

10

15

20

25

Number of Tracks

0 1 2 3 4 5 6 7 8


0

5

10

15

20

25

Signal Region (A)


EW background

τ νhτ→W


(c)

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

10

20

30

40

50

60

70

80

90

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

10

20

30

40

50

60

70

80

90

Signal Region (AC)


EW background

τ νhτ→W


(d)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

10

20

30

40

50

60

70

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

10

20

30

40

50

60

70

Signal Region (AC)


EW background

τ νhτ→W


(e)

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

5

10

15

20

25

30

35

40

45

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

5

10

15

20

25

30

35

40

45

Signal Region (AC)


EW background

τ νhτ→W


(f)

Figure 17: Comparison of data and different control regions with the additional contribution of signal

and EW background for τ h candidates within a transverse momentum range of 30GeV < pT <60GeV.

(a) S E missT

distribution (b) ∆φ (τ h, E missT ) (c) track multiplicity distribution for τ h candidates and (d) - (f)

distribution of the τ h-ID variables.



December 10, 2010 – 15 : 48 DRAFT 35

5.2.3 Separation of 1-prong and 3-prongs τ h candidates423

For a further confirmation of the validity of the data-driven method to extract the QCD background,424

the selected events have been divided into subsamples according to their number of tracks, and the425

performance of the method has been studied separately. The following subsamples have been defined:426

• τ h candidates with exactly one track (”1-prong”).427

• τ h candidates with more than one track (”3-prongs”).428

For the 1-prong candidates, the number of data events in the four defined regions as well as the Monte429

Carlo expectations for signal and EW backgrounds are listed in Table 13.430

A B C D

Data 26 71 45 289

W → τ hν τ 27.6±1.0 19.1±0.9 25.9±1.0 23.6±1.0

EW 3.2

±0.2 2.5

±0.1 3.9

±0.2 2.3

±0.1

ci 0.70±0.04 0.96±0.05 0.84±0.04

Table 13: Number of observed events and Monte Carlo expectations in the four regions for a data sample

with 1-prong τ h candidates.

The number of expected QCD background events obtained from the method is 5.5 ± 2.8. The431

distribution of kinematic and τ h-ID variables for 1-prong τ h candidates is shown in Figure 18.432

Also in this case a good agreement can be observed between data and the combination of the data433

control samples and the signal and electroweak Monte Carlo expectations. Although the statistic preci-434

sion for 1-prong τ h candidates is smaller than for 3-prong τ h, due to the smaller τ h misidentification rate,435

one can clearly see an excess of signal events in data.436

For the 3-prong candidates, the number of data events in the four defined regions as well as the Monte437

Carlo expectations for signal and EW backgrounds are listed in Table 14.438

A B C D

Data 52 536 209 6818

W → τ hν τ 27.7±1.0 20.4±0.9 45.1±1.3 30.6±1.1

EW 8.5±0.3 4.0±0.2 40.6±0.7 19.7±0.5

ci 0.68±0.03 2.38±0.08 1.41±0.05

Table 14: Number of observed events and Monte Carlo expectations for a data sample with 3-prong

τ h candidates.

Based on the numbers in Table 14 the expected QCD background in the signal region A is 7.8 ± 2.2.439

For this sample of 3-prong candidates, the distribution of several important variables is shown in440

Figure 19.441

Also in the case of 3-prong candidates an excellent agreement of all distributions in data and com-442

pared to the QCD control sample and the additional Monte Carlo expectation for signal and EW back-443

ground can be observed.444

In addition, the extracted numbers for the QCD background in the signal region A, obtained sepa-445

rately for the subsamples of 1-prong and 3-prongs τ h candidates are consistent with the total number of 446

QCD background estimated with this method for the whole data sample of 11.1

±2.3 events, which again447

corroborates the reliability of this method.448



December 10, 2010 – 15 : 48 DRAFT 36

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

T

missE

S

0 2 4 6 8 10 12 14


1

10

210 -ID (Region AB)hτTight

-ID (Region CD)hτLoose

EW background

τ νhτ→W


(a)

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

Signal Region (A)


EW background

τ νhτ→W


(b)

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

10

20

30

40

50

60

70

80

90

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

10

20

30

40

50

60

70

80

90

Signal Region (AC)


EW background

τ νhτ→W


(c)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

5

10

15

20

25

30

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

5

10

15

20

25

30

Signal Region (AC)


EW background

τ νhτ→W


(d)

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

5

10

15

20

25

30

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

5

10

15

20

25

30

Signal Region (AC)


EW background

τ νhτ→W


(e)


and EW backgrounds estimated from Monte Carlo for 1-prong τ h candidates. (a) S E missT

distribution, (b)

∆φ (τ h, E missT ) distribution and (c) - (e) distribution of the τ h-ID variables.



December 10, 2010 – 15 : 48 DRAFT 37

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

-ID (Region AB)hτTight


EW background

τ νhτ→W


(a)

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

30

35

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

5

10

15

20

25

30

35

Signal Region (A)


EW background

τ νhτ→W


(b)

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

20

40

60

80

100

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16


2

0

20

40

60

80

100 Signal Region (AC)


EW background

τ νhτ→W


(c)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

10

20

30

40

50

60

70

80

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 5

0

10

20

30

40

50

60

70

80

Signal Region (AC)


EW background

τ νhτ→W


(d)

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

10

20

30

40

50

trk,l f

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


5

0

10

20

30

40

50Signal Region (AC)


EW background

τ νhτ→W


(e)


and EW backgrounds estimated from Monte Carlo for 3-prong τ h candidates. (a) S E missT

distribution (b)

∆φ (τ h, E missT ) distribution (c) - (e) Distribution of the τ h-ID variables.



December 10, 2010 – 15 : 48 DRAFT 38

5.2.4 Redefining the signal and control regions449

Another test consisted in redefining the signal and control regions in the following way:450

• Region A: events with S E missT > 8 and τ h candidates satisfying the tight τ h-ID (signal region);

451

• Region B: events with S E missT

< 6 and τ h candidates satisfying the tight τ h-ID (control region);452

• Region C: events with S E missT

> 8 and τ h candidates satisfying the loose τ h-ID but failing the tight453

τ h-ID (control region);454

• Region D: events with S E missT

< 6 and τ h candidates satisfying the loose τ h-ID but failing the tight455

τ h-ID (control region).456

The region 6 < S E missT

< 8 is not used and the new signal region should contain less QCD background457

events. The number of events in the new regions are shown in Figure 15.458

A B C DData 25 608 80 7126

W → τ hν τ 18.5±0.8 39.5±1.2 27.2±1.0 54.2±1.4

EW 5.3±0.2 6.5±0.2 22.7±0.5 22.1±0.5

ci 1.93±0.09 2.10±0.09 3.21±0.13

Table 15: Number of observed events and Monte Carlo expectations in the four regions, excluding the 6

< S E missT

< 8 region.




December 10, 2010 – 15 : 48 DRAFT 39

5.2.5 Separation of events with one vertex and more than one vertex460

The data-driven method has also been tested using subsamples of events with one vertex and with more461

than one vertex to validate its performance with events with pile-up effect. All vertexes with more than462

three tracks are counted and used to classify the event. The cut flows are shown in Table 16.463

One vertex More than one vertex

Data W → τ hν τ Data W → τ hν τ

Trigger 986444 954.9±5.2 986444 954.9±5.2

QCD jets rejection 188974 366.1±3.6 226979 362.2±3.6

E missT > 30 GeV 9820 204.6±2.7 19867 206.9±2.8

τ selection 1274 64.6±1.6 1134 53.4±1.4

Lepton rejection 288 52.3±1.4 397 42.6±1.3

S E missT

> 6 58 37.9±1.2 20 17.4±0.8

Table 16: Number of events passing the selection criteria for data and Monte Carlo signal, normalized

to the integrated luminosity of 546 nb−1. The samples are separeted in events with one reconstructed

vertex with more than three tracks and events with more than one reconstructed vertex with more than

three tracks.

Based on the numbers in Table 17 the expected QCD background in the signal region A is 10.5 ±464

2.5.

A B C D

Data 58 230 181 2171

W

→τ hν τ 37.9

±1.2 14.3

±0.7 44.7

±1.3 14.4

±0.7

EW 7.5±0.3 2.3±0.1 26.9±0.6 7.0±0.3ci 0.365±0.021 1.576±0.054 0.472±0.024

Table 17: Number of observed events and Monte Carlo expectations in the four regions for events with

one reconstructed vertex with more than three tracks.465


A B C D

Data 20 377 73 4936

W

→τ hν τ 17.4

±0.8 25.2

±1.0 26.3

±1.0 39.8

±1.2

EW 4.2±0.2 4.2±0.2 17.6±0.5 15.1±0.4

ci 1.36±0.07 2.03±0.10 2.54±0.12

Table 18: Number of observed events and Monte Carlo expectations in the four regions for events with

more than one reconstructed vertex with more than three tracks.

The resulting number of QCD background events is consistent with the estimation for the whole data467

sample of 11.1±2.3 QCD background events. Figures 20 and 21 show the S E missT

and the variable Rtrack 468

for events with one and more than one reconstructed vertex, respectively.469

Figures 22 and 23 show the number of tracks and mT distributions for events with one and more than470

one reconstructed vertex, respectively.471



December 10, 2010 – 15 : 48 DRAFT 40

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

T

missE

S

0 2 4 6 8 10 12 14


1

10

210

310

-ID)hτ= 7 TeV ) (TightsData 2010 (-ID)hτQCD background (Loose

EW background

τ νhτ→W


(a)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 . 0 0 5

0

10

20

30

40

50

60

70

80

track R

0 0.02 0.04 0.06 0.08 0.1 0.12


0 . 0 0 5

0

10

20

30

40

50

60

70

80

>6)miss

TE = 7 TeV ) (SsData 2010 (<6)

missT

EQCD background (S

EW background

τ νhτ→W


(b)

Figure 20: (a) Distribution of S E missT

distribution for data in the combined region AB (tight τ h-ID region)

and the combined control region CD (loose τ h-ID region). Also shown are the expected signal and EW

backgrounds in region AB from simulated samples. (b) Distribution of Rtrack for events in the combined

region AC (S E missT

> 6) and control region BD (S E missT

< 6) together with the expectations from MC for

signal and EW background. Only events with one reconstructed vertex with more than three tracks are

considered.

TmissES

0 2 4 6 8 10 12 14


1

10

210

310

TmissES

0 2 4 6 8 10 12 14


1

10

210

310

-ID)hτ= 7 TeV ) (TightsData 2010 (

-ID)hτQCD background (LooseEW background

τ νhτ→W


(a)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b e r o f E v e n t s / 0 . 0 0 5

0

5

10

15

20

25

30

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b e r o f E v e n t s / 0 . 0 0 5

0

5

10

15

20

25

30

>6)missTE

= 7 TeV ) (SsData 2010 (

<6)miss

TE

QCD background (S

EW background

τ νhτ→W


(b)

Figure 21: (a) Distribution of S E missT

distribution for data in the combined region AB (tight τ h-ID region)

and the combined control region CD (loose τ h-ID region). Also shown are the expected signal and EW

backgrounds in region AB from simulated samples. (b) Distribution of Rtrack for events in the combined

region AC (S E missT

> 6) and control region BD (S E missT

< 6) together with the expectations from MC for

signal and EW background. Only events with more than one reconstructed vertex with more than three

tracks are considered.



December 10, 2010 – 15 : 48 DRAFT 41

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

Number of Tracks

0 1 2 3 4 5 6 7


0

5

10

15

20

25

30

= 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(a)

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

[GeV]Tm

0 20 40 60 80 100 120


0

5

10

15

20

25

30

= 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(b)

Figure 22: (a) Distribution of the number of tracks in tau candidates. (b) Distribution of the transverse

mass. Only events with one reconstructed vertex with more than three tracks are considered.

Number of Tracks

0 1 2 3 4 5 6 7


0

2

4

6

8

10

12

Number of Tracks

0 1 2 3 4 5 6 7


0

2

4

6

8

10

12

= 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(a)

[GeV]Tm

0 20 40 60 80 100 120


0

2

4

6

8

10

12

[GeV]Tm

0 20 40 60 80 100 120


0

2

4

6

8

10

12

= 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(b)

Figure 23: (a) Distribution of the number of tracks in tau candidates. (b) Distribution of the transverse

mass. Only events with more than one reconstructed vertex with more than three tracks are considered.



December 10, 2010 – 15 : 48 DRAFT 42

5.3 Study of τ h medium candidates472

As a final consistency test for the data-driven method, the number of QCD background was also estimated473

for a looser τ h-ID selection, replacing the tight by the medium τ h ID requirement. The number of data474

events in the four defined regions as well as the Monte Carlo expectations for signal and EW backgrounds475

are listed in Table 19. The number of estimated QCD background in region A is 50.4±11.9.476

Also for these events several characteristic variables have been investigated. Figures 24, 25 and 26477

show the S E missT

distribution and the distribution of kinematic quantities repectively, for data in the signal478

region A and the QCD control region combined with the expected signal and EW contributions, from479

Monte Carlo simulation. Figure 27 shows the distribution of the τ h-ID variables.

A B C D

Data 197 3109 134 4583

W → τ hν τ 98.7±1.9 75.8±1.7 27.4±1.0 18.0±0.8

EW 32.4±0.6 17.5±0.4 24.2±0.5 11.1±0.4

ci 0.712±0.018 0.394±0.011 0.222±0.009

Table 19: Number of observed events and Monte Carlo expectations in the four regions for events satis-

fying the medium τ h-ID instead of the tight τ h-ID.

480

T

missE

S

0 2 4 6 8 10 12 14


10

210

310

T

missE

S

0 2 4 6 8 10 12 14


10

210

310 -ID (Region AB)hτTight


EW background

τ νhτ→W


Figure 24: S E missT

distribution for data in the combined region AB (medium τ h-ID region) and the com-

bined control region CD (loose τ h-ID region). The signal and EW background contributions in the control

region is considered and subtracted from it. Also shown are the expected signal and EW backgrounds in

region AB from simulated samples added to the histogram for the control region.

The results show again a good agreement among the distributions, indicating a very good reliability481

of the data-driven method.482



December 10, 2010 – 15 : 48 DRAFT 43

[GeV]TmissE

30 40 50 60 70 80 90 100 110


0

20

40

60

80

100

[GeV]TmissE

30 40 50 60 70 80 90 100 110


0

20

40

60

80

100 = 7 TeV )sData 2010 (

QCD background

EW background

τ νhτ→W


(a)

[GeV]T

E∑0 100 200 300 400 500 600


0

20

40

60

80

100

[GeV]T

E∑0 100 200 300 400 500 600


0

20

40

60

80

100

Signal Region (A)


EW background

τ νhτ→W


(b)

[GeV]T

p

20 25 30 35 40 45 50 55 60


0

10

20

30

40

50

[GeV]T

p

20 25 30 35 40 45 50 55 60


0

10

20

30

40

50Signal Region (A)


EW background

τ νhτ→W


(c)

Figure 25: Distribution of different kinematic variables shown for data and control regions for candidates

passing the medium τ h ID. (a) E missT and (b) ∑ E T , (c) τ h pT .



December 10, 2010 – 15 : 48 DRAFT 44

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

20

40

60

80

100

120

140

) [rad]T

miss,Ehτ(φ∆

0 0.5 1 1.5 2 2.5 3

/ 1 5 )

π


0

20

40

60

80

100

120

140

Signal Region (A)


EW background

τ νh

τ→W


(a)

[GeV]Tm

0 20 40 60 80 100 120

N u m b e r

o f E v e n t s / 1 0 G e V

0

10

20

30

40

50

60

70

80

[GeV]Tm

0 20 40 60 80 100 120

N u m b e r

o f E v e n t s / 1 0 G e V

0

10

20

30

40

50

60

70

80

Signal Region (A)


EW background

τ νh

τ→W


(b)

Number of Tracks

0 1 2 3 4 5 6 7


0

10

20

30

40

50

60

70

80

Number of Tracks

0 1 2 3 4 5 6 7


0

10

20

30

40

50

60

70

80

Signal Region (A)


EW background

τ νhτ→W


(c)

Electric charge

-3 -2 -1 0 1 2 3


0

20

40

60

80

100

120

140

Electric charge

-3 -2 -1 0 1 2 3


0

20

40

60

80

100

120

140

Signal Region (A)


EW background

τ νhτ→W


(d)

Figure 26: Distribution of different kinematic variables shown for data and different control regions for

candidates passing the medium τ h ID. (a) ∆φ (τ h, E missT ) and (b) transverse mass, (c) number of tracks and(d) electric charge of the τ h candidates.



December 10, 2010 – 15 : 48 DRAFT 45

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m

b e r o f E v e n t s / 0 . 0

2

0

20

40

60

80

100

120

EM R

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

N u m

b e r o f E v e n t s / 0 . 0

2

0

20

40

60

80

100

120

Signal Region (AC)


EW background

τ νh

τ→W


(a)

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b

e r o f E v e n t s / 0 . 0

0 5

0

20

40

60

80

100

track R

0 0.02 0.04 0.06 0.08 0.1 0.12

N u m b

e r o f E v e n t s / 0 . 0

0 5

0

20

40

60

80

100

Signal Region (AC)


EW background

τ νh

τ→W


(b)

trk,l f

0 0.2 0.4 0.6 0.8 1 1.2 1.4


5

0

10

20

30

40

50

60

70

trk,l f

0 0.2 0.4 0.6 0.8 1 1.2 1.4


5

0

10

20

30

40

50

60

70

>6)miss

TE = 7 TeV ) (SsData 2010 (

<6)miss

TEQCD background (S

EW background

τ νhτ→

W


(c)

Figure 27: Distribution of the τ h-ID variables for candidates passing the medium τ h ID for the combined

signal region AC and the combined control region BD and the expected contribution from signal and EWbackground.



December 10, 2010 – 15 : 48 DRAFT 46

6 Systematic Uncertainties483

The systematic uncertainties on the number of QCD background events, estimated with the data-driven484

method, and on the number of EW background and W

→τ hν τ events, based on Monte Carlo simulated485

samples, have been evaluated for various sources of systematic effects. Table 20 summarizes the resulting486

systematic uncertainties.487

signal EW background QCD background

Central values [events] 55.3 11.8 11.1

Statistical error [events] ±1.4 ±0.4 ±2.3

Systematic uncertainties

Theoretical cross section ±5% ± 5% –

Luminosity ±11 ± 11% –

Energy scale ±21% ±14% –

Electron veto –

±11% –

Muon veto – ±16% –Pile-up ±1 ±0.2% –

Monte Carlo model ±16% ±17% –

QCD background estimation – – ±29%

Total systematic uncertainty [events] ±16.1 ±3.7 ±3.2

Table 20: Summary of the systematic uncertainties for the data-driven estimation of the QCD background

and for the expectations for EW background and signal based on simulation. The single systematic

uncertainties are quoted as relative values, while the resulting total uncertainties are quoted as absolute

values.

6.1 Trigger488

At this early stage of operation of the ATLAS experiment, a suitable event sample with τ h candidates489

and E missT selected by a trigger that is independent from the one used in this analysis is not yet available.490

The trigger selection has therefore been evaluated based on Monte Carlo simulations for events passing491

events selection described in Section 4.492

Given that the tauNoCut hasTrk6 EFxe15 noMu requirements on E missT and τ h pT are much softer493

than those applied in the event selection, expected systematic uncertainty is low. Figure 28 shows the494

EF turn-on curve for two different E missT algorithms: LocHadTopo, used in this analysis and Topo. In495

both cases the trigger is fully efficient for events with E missT above 30 GeV. The systematic uncertainty is496

therefore negligible.497

The chosen trigger, tauNoCut hasTrk6 EFxe15 noMu , was running in various periods using differ-498

ent track reconstruction algorithms at L2 (SiTrack, IDScan [29]). This results in an efficiency variation499

of 0.4% across the whole sample which is estimated from the Monte Carlo signal sample. This source of 500

systematic error is negligible compared to many others and therefore is not considered in the analysis.501

6.2 Cross section and luminosity502

The expected number of signal and EW background events is computed from Monte Carlo samples and503

scaled according to their NNLO cross sections and the integrated luminosity of the data sample. These504

values have a systematic uncertainty of 5% from the cross section calculation [30] and 11% from the505

luminosity measurement [31].506



December 10, 2010 – 15 : 48 DRAFT 47

[GeV]missTE

15 20 25 30 35 40 45 50

f r a c t i o

n o f E v t s

0.5

0.6

0.7

0.8

0.9

1

LocHadTopo

Topo

Figure 28: For tauNoCut hasTrk6 EFxe15 noMu: fraction of events passing the EF trigger as a function

of E missT for two different algorithms. Only events satisfying the offline selection described in Sect. 4.2

and passing the L1 and L2 triggers are considered.

6.3 Energy scale507

The signal acceptance depends on the energy scale of the topological clusters used in the computation of 508

E missT and S E miss

T. At the current level of detector calibration in the region |η|< 3.2, the uncertainty on the509

energy scale is better than 7% for energetic clusters. In the forward region |η|>3.2, it is estimated to be510

10%. In addition, the yield of signal and EW background events is is sensitive to the resolution of E missT .511

To evaluate the energy scale uncertainties, the checks have been performed following the same pro-512

cedures as in [32]. The E missT and ∑ E T have been recomputed accordingly. In the test on the energy scale513

for |η| < 3.2, also the tau energy has been rescaled, using the rescaled topocluster. The description of 514

tests is given below. Test results are summarized in Table 21 for signal and the main EW background515

contributions. As systematic uncertainty the maximum deviations for each of the four tests are summed516

in quadrature. Since the upper and lower variation are very similar, it is preferred to quote symmetric517

uncertainties on the acceptances for signal and EW background of

±21% and

±14%, respectively.518

The effect of the variation of the energy scale for |η|< 3.2 on E missT , S E missT

and the selected tau energy519

are shown in Figure 29 for the signal, and in Figures 30 and 31 for the W → τ ℓν τ and Z → ττ samples.520

The E missT distribution is shown for events just before and just after the requirement on E miss

T , as well as521

the distribution of S E missT

. The distribution of the selected tau energy is shown for events just before and522

after the cut on the tau energy.523

Topological cluster energy scale524

The transverse energy originating in W and Z events is mainly deposited in the central region of the525

calorimeter (|η|< 3.2). The uncerainty on the cluster energy scale is derived from E / p studies on single526

hadrons [33, 34] as the difference between data and Monte Carlo simulation. It is at most 20% for pT of 527

500 MeV and within 5% at high pT . Clusters in this angular region have been scaled according to a factor528



December 10, 2010 – 15 : 48 DRAFT 51

W → τ hν τ W → eν e W → µν µ W → τ ℓν τ Z → ττ Total EW background

Expected events 55.3 4.2 3.7 1.8 2.0 11.8

Systematics uncertainties

relative +19% +14% +3% +19% +34% +15%

-22% -11% -5% -15% -30% -13%

events +10.6 +0.6 +0.1 +0.3 +0.7 +1.7

-12.1 -0.5 -0.2 -0.3 -0.6 -1.5

Energy scale |η|< 3.2a N −1

0.07 0.93 +15% +8% -5% +13% +20%

0.06 1.17 +12% +6% -5% +15% +17%

0.05 1.50 +10% +6% -4% +16% +14%

-0.05 1.50 -11% +1% +3% -7% -6%

-0.06 1.17 -14% 0% 0% -7% -8%

-0.07 0.93 -18% -1% -4% -7% -12%

Energy scale

|η

|> 3.2

a

0.10 -1% -1% -2% -8% -2%

-0.10 +3% +4% 0% 0% +3%

E missT resolution

α [GeV 1/2]

0.50 +1% -1% +1% +4% +8%

0.55 +4% +2% 0% +10% +8%

0.60 +6% +5% 0% +7% +16%

0.65 +10% +7% 0% +7% +26%

Excluding FCAL inner ring +7% +9% +2% +1% +8%

Table 21: Relative variation of acceptances for the systematics tests on the topological cluster energyscale, for signal and EW backgrounds



December 10, 2010 – 15 : 48 DRAFT 52

1 + a

1 + N −1 pT

for different values of a and N covering conservatively the above uncertainties. The τ h529

E T is also scaled according to the E T variation of the topological clusters associated to the reconstructed530

τ h candidate.531

The variations are larger than the 3% systematic mismatch between the reconstructed and the true532

visible energy of the tau, as shown in figure 32. The mismatch has been evaluated also on Monte Carlo533

signal sample with different tuning of the underlying event (DW sample) and was found to be compatible534

with the mismatch found in MC09 samples.535

vis /ErecE

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

20

40

60

80

100

120

140

160

η-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

v i s

/ E

r e c

E

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

[GeV]T

p20 25 30 35 40 45 50 55 60

v i s

/ E

r e c

E

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

1.06

Figure 32: Mismatch between the reconstructed and the true visible energy of the tau candidate. Thedistribution has been fitted with a gaussian distribution, whose mean is 0.97 and sigma is 0.08. Mismatch

as a function of η (middle plot) and of the tau pT (bottom plot). The mismatch dependence on pT is

0.98-0.0003 pT .

In the forward region (|η| > 3.2) the energy scale uncertainty is estimated from data to be a =536

±10% [34] and therefore FCal clusters have been scaled by that amount.537

E missT resolution538

The resolution on E missT is measured to be 0.49

√ ∑ E T in minimum bias events, but it is slightly degraded539

when requiring the presence of high- pT jets [32]. The sensitivity to the E

miss

T resolution has been checked540

adding a gaussian smearing on the x and y components of E missT , in order to reproduce in the simulation a541

E missT resolution of α

√ ∑ E T , with a range of values for α which cover the uncertainty due to te presence542

of high- pT jets.543

Energy reconsctruction in FCAL inner ring544

The energy reconstruction in the FCal inner ring cells, |η| > 4.5 is poorly understood in Monte Carlo.545

The impact of cutting this region when computing E missT and ∑ E T is mainly a reduction of ∑ E T and546

therefore an increase in the acceptance for the S E missT

selection.547



December 10, 2010 – 15 : 48 DRAFT 53

6.4 Electron Veto548

The electron veto algorithm, described in Section 3, relies on variables which are not perfectly modeled549

in simulation [35]. The mis-modeling of the distributions by Monte Carlo compared to data can affect550

both the efficiency of selecting τ h candidates as well as the efficiency of electron suppression.551

In the absence of an independent sample of τ h candidates the potential systematic uncertainty on the552

number of hadronic τ decays passing the electron veto flag has been computed based on a sample of 553

jets mis-identified as tight τ h candidates in the following way. The efficiency of the electron veto has554

been defined as a fraction of τ h candidates passing the tight τ h-ID requirements described in Section 3 and555

electron veto (using the tight flag) with respect to all tau candidates passing the τ h-ID tight selection. The556

selected τ h candidates were also required to be identified as neither loose electrons nor combined muons.557

This efficiency has been measured in data and in simulations, yielding (87.7±0.8) % and (87.6±1.0) %558

respectively. Given the good agreement between these numbers, no systematic uncertainty is assigned559

for the effect of the electron veto on the QCD background.560

The uncertainty on the effect of the electron veto in rejecting high momentum electrons has been561

accessed using a ”tag-and-probe” method on a sample of Z → ee events. One electron is selected as562

the tag object and a the second one is selected as a probe object when it has been misidentified as a563

τ h candidate. In this method, the electron to τ fake rate is defined as the ratio of the number of events564

where the probe passes τ h identification and the electron veto and the number of events passing the565

τ h identification. The tag electron is required to be selected by the tight electron identification with566

E T > 20 GeV lying centrally in the detector and outside the gap in the calorimeter systems (|η|< 1.37 or567

1.52 < |η|< 2.47). As a probe candidate a tight τ h candidate seeded by both reconstruction algorithms is568

selected, with the same E T and pseudorapidity requirements as the tag electron and with a distance∆ R >569

0.4 from it. In addition, an opposite sign of the electric charge of tag electron and probe τ h candidate570

is required. This selection provides a pure sample of Z → ee events, as shown in Figure 33. Only pairs571

which have an invariant mass within the range 80 to 100 GeV are considered for the estimation of the fake572

rate. Since the number of events found that is found by this selection in a data sample with an integrated573

luminosity of 546 nb−1, the cut on the τ h-ID is relaxed to selecting candidates that satisfy the medium574

instead of the tight τ h ID. With this selection there are 111 events passing the tag and probe selection cuts575

in data, and 11 events passing the tight electron veto. The measured fake rate in data is therefore (0.06 ±576

0.02)% compared to the Monte Carlo estimation of (0.07 ± 0.04)%. Due to the limited number of events577

in data a method to estimate the remaining background of QCD and other EW processes could not yet578

be applied. The ratio of the fake rate between data and simulation is therefore 0.7±0.4. This difference579

of 30% between data and simulation is used as a systematic uncertainty on the W → eν e background rate580

and results in an overall systematic uncertainty of 11% on the EW background.581

6.5 Muon veto582

The background from the W → µν µ and Z → µµ processes is suppressed by rejecting events if there583

is a combined muon reconstructed by the STACO algorithm with pT > 5 GeV. The efficiency of this584

suppression cannot be cross checked with the same tag and probe method as for the electron veto because585

in most cases (83%) the τ h-candidate is a QCD jet from the underlying event and not a muon from the586

W and Z decay (Figure 34).587

In this case rate of backgrond events passing the muon veto is proportional to the muon reconstruc-588

tion ineffiency, which has been assessed with the standard tag-and-probe techniques on the Z → µ µ 589

samples for the measurement of the W cross section. From a recent review of the results from differ-590

ent groups [36], the Monte Carlo estimation of the muon reconstruction efficiency for STACO is ≈92%.591

Comparison with data show a dependence of the efficiency from the running conditions and for the period592

corresponding to the data sample used in this analysis it is 88%.593



December 10, 2010 – 15 : 48 DRAFT 54

Invariant Mass0 20 40 60 80 100 120 1

N u m b e r o f E v e

-310

-210

-110

1

316 (nb)-1

Zee

We

W h

JF35

Z

Data

Figure 33: The invariant mass of opposite-sign tag electron and probe τ h pairs passing the selection

described in the text. The distribution is shown for a subsample of data (316 nb−1) and the normalized

Monte Carlo expectation for the Z → ee signal sample and the most important background samples. The

peak near the Z mass can clearly be seen.

Taking the difference between data and Monte Carlo estimation as the systematic error, the muon594

reconstruction inefficiency is evaluated to 8%±4%. This translate in a systematic error of 1.9 events over595

the combined W → µν µ and Z → µµ background of 3.8 events and in an overall systematic uncertainty596

on the EW background of 16%.597

6.6 Pile-up598

The variation of the beam conditions at the LHC provided a variable number of pile-up events in the data599

sample considered in this analysis. This has been accounted for by reweighting the simulated events so600

that the distribution of the number of reconstructed primary vertex candidates per event matches the one601

measured in the ATLAS data. The systematic uncertainty associated with this procedure is evaluated by602

varying the event weights within their statistical uncertainties and found to be 1% for Monte Carlo signal603

events and 0.2% for EW background.604

6.7 Monte Carlo model605

Recently it has been shown that the DW tune [18] of Pythia, which was derived to describe the CDF606

II underlying event and Drell-Yan data, models the forward activity of the underlying event better than607

the MC09 tune [37], which is the defaut for Pythia. As Monte Carlo sample with DW tune and pile-up608

events are not available at the moment, a systematic uncertainty from the differe11.0814nt modeling of 609

the underlying event was evaluated for signal to be 16%. Concerning the EW background, the samples610

with DW tune were available only for Z → ττ and W → τ ℓν τ channels. So the effect of the tuning has611

been evaluated only on these backgrounds and the highest of the two systematics (17% for Z → ττ and612

14% for W

→τ ℓν τ ) has been considered for all the EW backgrounds.613



December 10, 2010 – 15 : 48 DRAFT 55

µ-τR∆0 1 2 3 4 5 6 7

E v e n t s / 0 . 1 b i n

0

50

100

150

200

250

300

350

400

450

Figure 34: Distribution of ∆ R between the τ h-candidate and the muon from the W decay in simulated

W → µν µ events. The colored region corresponds to the muon correctly rejected by the muon veto.

In Tables 22, 23 it is shown the cut flow for different signal Monte Carlo samples: the first column614

refers to the default Pythia tune, the second column refers to the DW tune and the last column is the cut615

flow for simulated events with MC09 tune and pile-up. From these tables it can be seen that the main616

difference between MC09 and DW concerns the cut on S E missT

. The effect on trigger is about 2.6% and617

the one on tau identification is approximately 1%.618

In Tables 25 and 24, the cut flows Z → ττ and W → τ ℓν τ are reported.619



December 10, 2010 – 15 : 48 DRAFT 56

W → τ hν τ MC09 W → τ hν τ DW W → τ hν τ PileUp

Events 3700.3 3700.3 3700.3

Skimming 1796.6±5.8 1787.1±3.6 1838.7±6.0

GRL 1796.6±

5.8 1787.1±

3.6 1838.7±

6.0

Trigger 921.1±5.0 894.0±3.1 954.5±5.2

CollCand 918.2±5.0 893.3±3.1 954.5±5.2

JetClean 909.8±5.0 883.6±3.0 942.7±5.2

JetVeto 744.8±4.7 721.6±2.8 767.2±4.8

DeltaPhi jet 713.5±4.6 689.6±2.8 728.3±4.7

METcut 392.8±3.6 378.6±2.2 411.5±3.8

TauID 141.6±2.2 135.1±1.3 135.3±2.2

TauID Et 123.4±2.1 119.0±1.3 119.1±2.1

TauID eta 122.2±2.1 117.7±1.3 118.0±2.1

TauID lep 106.8±1.9 102.6±1.2 102.0±2.0

LeptVeto 100.8±

1.9 97.2±

1.1 94.8±

1.9

METSign 76.2±1.7 63.7±0.9 55.3±1.4

Table 22: Number of events passing the selection criteria for signal Monte Carlo with MC09 and DW

tunes.

6.8 Background estimation620

The following sources of systematic uncertainty have been considered for the estimation of QCD back-621

ground from data:622

• Correction for signal and EW background: The systematic effect due to the correction for the623

signal and EW background contamination in the three control regions on the estimation of the624

QCD background has been studied. This systematic effect is evaluated by varying the fraction of 625

EW background events within the combined statistical and systematic uncertainties of the Monte626

Carlo prediction presented in Table 20. It amounts to ±6%.627

• S E missT

and τ h-ID correlation: An assumption of the method used for the estimation of QCD back-628

ground is that these two variables are not correlated. As can be seen in Figures 15(a) and 8, the629

S E missT

distribution for region AB is slightly shifted towards higher S E missT

values compared to region630

CD. This may be an effect of a non-negligible correlation between S E missT

and the τ h-ID. As a check,631

the regions A and C have been enlarged by changing the value used in the S E missT

requirement from632

6 to 4, in order to obtain QCD-background-dominated samples in all four regions. The observed633

value of N A, after subtraction of EW and signal contributions based on Monte Carlo, has been634

compared with the estimate from Equation 5. The same check is performed for the medium τ h-ID.635

The largest disagreement is observed for the first cross-check and amounts to 28%.636

• Other cross-checks: As shown in Section 5, the application of the computed misidentification637

efficiency to the selected data sample results in an estimated QCD background of 6.6±1.2(stat.)±638

1.1(syst.) events. In W → τ hν τ events, the E missT , and therefore also S E miss

T, is correlated with the pT 639

of the τ h candidate. The fact that the τ h identification efficiency is not uniform as a function of pT 640

can lead to a potential systematic uncertainty. In order to verify this, the analysis is repeated for two641

different pT ranges: 20–30 GeV and 30–60 GeV. The stability of the background estimation is also642

checked separately for τ h candidates with 1 or more tracks and for events with single or multiple643



December 10, 2010 – 15 : 48 DRAFT 57

W → τ hν τ MC09 W → τ hν τ DW W → τ hν τ PileUp

Events 1 1 1Skimming 0.486 0.483 0.497

GRL 1.000 1.000 1.000

Trigger 0.513 0.500 0.519

CollCand 0.997 0.999 1.000

JetClean 0.991 0.989 0.988

JetVeto 0.819 0.817 0.814

DeltaPhi jet 0.958 0.956 0.949

METcut 0.550 0.549 0.565

TauID 0.360 0.357 0.329

TauID Et 0.872 0.881 0.880

TauID eta 0.990 0.989 0.991TauID lep 0.875 0.872 0.865

LeptVeto 0.943 0.947 0.929

METSign 0.756 0.656 0.583

Table 23: Relative efficiency of the cuts for signal Monte Carlo with MC09 and DW tunes.

W → τ ℓν τ MC09 W → τ ℓν τ DW W → τ ℓν τ PileUp

Events 1756.0 1756.0 1756.0

Skimming 573.1±2.6 536.0±2.1 581.7±3.6

GRL 573.1±2.6 536.0±2.1 581.7±3.6

Trigger 320.3±2.1 275.8±1.7 296.5±2.8

CollCand 319.6±2.1 275.7±1.7 296.5±2.8

JetClean 317.1±2.1 273.0±1.6 292.2±2.8

JetVeto 261.8±2.0 225.6±1.5 240.6±2.6

DeltaPhi jet 251.6±1.9 216.7±1.5 229.4±2.6

METcut 125.7

±1.4 108.2

±1.1 121.9

±1.9

TauID 56.6±1.0 46.0±0.7 50.6±1.3TauID Et 38.0±0.8 32.0±0.6 34.7±1.1

TauID eta 37.8±0.8 31.9±0.6 34.4±1.0

TauID lep 6.5±0.3 4.9±0.2 6.1±0.4

LeptVeto 2.4±0.2 2.3±0.2 2.3±0.3

METSign 2.1±0.2 1.8±0.1 1.8±0.2

Table 24: Number of events passing the selection criteria for Monte Carlo W → τ ℓν τ with MC09 and

DW tunes..



December 10, 2010 – 15 : 48 DRAFT 58

MET_et [GeV]0 10 20 30 40 50 60 70 80 90 100


-210

-110

MC09 νhτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(a)

MET_et [GeV]0 10 20 30 40 50 60 70 80 90 100


-310

-210

-110

MC09 νhτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(b)

MET_sumet [GeV]

0 100 200 300 400 500 600 700 800


-410

-310

-210

-110 MC09 ν

hτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(c)

MET_sumet [GeV]

0 100 200 300 400 500 600 700 800


-310

-210

-110 MC09 ν

hτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(d)

MET_significance0 2 4 6 8 10 12 14 16 18 20


-4

10

-310

-210

-110 MC09 νhτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(e)

MET_significance0 2 4 6 8 10 12 14 16 18 20


-310

-210

-110 MC09 νhτ→W

DW νhτ→W

MC09 νhτ→W

DW νhτ→W

(f)

Figure 35: E missT , S E miss

Tand ∑ E T distributions for signal events, for Monte Carlo with MC09 and DW

tunings. The plots on the left refer to events passing the cut on E missT and the plots on the right show

events just before the cut on S E missT

.



December 10, 2010 – 15 : 48 DRAFT 59

Z → ττ MC09 Z → ττ DW Z → ττ PileUp

Events 540.5 540.5 540.5

Skimming 181.9±0.6 182.5±0.4 194.6±0.8

GRL 181.9±

0.6 182.5±

0.4 194.6±

0.8

Trigger 106.5±0.5 104.9±0.3 115.1±0.7

CollCand 106.4±0.5 104.8±0.3 115.1±0.7

JetClean 105.3±0.5 103.5±0.3 113.6±0.7

JetVeto 83.1±0.4 81.6±0.3 87.9±0.6

DeltaPhi jet 67.5±0.4 66.0±0.3 71.4±0.6

METcut 32.7±0.3 31.9±0.2 35.4±0.4

TauID 15.0±0.2 14.4±0.1 14.8±0.3

TauID Et 12.1±0.2 11.9±0.1 12.1±0.3

TauID eta 12.0±0.2 11.8±0.1 11.9±0.3

TauID lep 9.1±0.2 8.8±0.1 8.9±0.2

LeptVeto 4.4±

0.1 4.3±

0.1 4.2±

0.2

METSign 3.0±0.1 2.5±0.1 2.0±0.1

Table 25: Number of events passing the selection criteria for Monte Carlo Z → ττ with MC09 and DW

tunes..

reconstructed primary vertices. All variations on the expected number of QCD background events644

in the signal region are statistically compatible with the estimation on the full sample.645

The total systematic uncertainty associated with the QCD background estimation is determined to be646

29%. Since this systematic uncertainty is computed comparing samples with different τ h identification647

efficiencies, it also includes the uncertainties due to the τ h identification algorithm.648

7 Conclusions649

A search for W → τν τ decays, with the τ lepton decaying into hadrons, has been presented. A total of 650

78 data events have been selected from a data sample that corresponds to an integrated luminosity of 651

546 nb−1, recorded with the ATLAS experiment at the LHC in proton-proton collisions at√

s = 7 TeV652

from March to August 2010. The background contribution from QCD processes has been estimated from653

data and amounts to 11.1±2.3(stat.)±3.2(syst.) events. The remaining background from W and Z decays654

is 11.8±0.4(stat.)±3.7(syst.) events, estimated from Monte Carlo simulation. The observed excess of data655

events over the total background amounts to 55.1±

10.5(stat.) ±

5.2(syst.)

events. It is compatible with656

the expected number of signal events of 55.3±1.4(stat.) ±16.1(syst.). Also the shapes of distributions of 657

kinematical variables and variables used in the τ h-ID are compatible with those obtained from simulated658

signal events. The probability for the signal to be due to background is 8.7× 10−10, using a bayesian659

approach, and it corresponds to 6.2σ , using a one-sided normal distribution. This is the first evidence of 660

W → τν τ decays and of hadronically decaying τ leptons in ATLAS.661

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