Tau Lepton Reconstruction and Identification at ATLAS

0.09 MB·English

by  Felix Friedrich

Tau Lepton Reconstruction and Identification at ATLAS
FelixFriedricha
onbehalfoftheATLASCollaboration
Institutfu¨rKern-undTeilchenphysik,TechnicalUniversityofDresden,Dresden,Germany
Abstract. TauleptonsplayanimportantroleinthephysicsprogramattheLHC.Theyareusedinsearchesfor
2
newphenomenaliketheHiggsbosonorSupersymmetryandinelectroweakmeasurements.Identifyinghadron-
1
0 icallydecayingtauleptonswithgoodperformanceisanessentialpartoftheseanalyses.Wepresentthecurrent
2 statusof thetaureconstruction and identificationat theLHC withtheATLASdetector. Thetauidentification
efficienciesandtheirsystematicuncertaintiesaremeasuredusingW → τνandZ → ττevents,andcompared
n
withthepredictionsfromMonteCarlosimulations.
a
J

6
1 Introduction
2 4
] Tau leptons are important signatures for Standard Model n / 0.00 0.1 2Z0→11ττ d+iWjet→ dτaνta ∫dt L = 130 pb-1 A3 TpLroAnSgs PprTe>li2m0in GareyV
x processes and searches for new physics. With a mass of ctio 0.08
a
p-e s1h.7o7rt7lGifeeVtim, tehoeft2a.u9i×s1th0e−1h3sea(vcτies=t 8le7pµtomn),atnhdetdauueletoptoitns mple Fr 0.06
e decaysinsidethebeampipeoftheLHC[1].Thetaulepton Sa 0.04
h
istheonlyleptonthathasahadronicdecaymode.Whileit
0.02
[
decaysin35%ofthetimeleptonically,thehadronicdecay
1 mode occurs 65% of the time. The majority of hadronic 00 0.020.040.060.08 0.1 0.120.140.160.18 0.2
v tau decays are characterized by one or three charged pi- ∆R
max
6 ons usually accompanied by neutral pions. The kinemat-
Fig.1.Maximaldistancebetweenatrackandthetauaxis,∆R .
6 ics of QCD jets are similar to that ofhadronicallydecay- max
4 Only tracks inside a cone of ∆R < 0.2 around the tau axis are
ing τ leptons, leading to a high potential probability for
5 considered[3].
misidentifyingthemastauleptons.Inaddition,thecross-
.
1 section of most of the Standard Model and new physics
0 processeswithtauleptonsinthefinalstatearesmallcom- energyiscalculatedusingallcalorimeterclusterswithina
2 pared to the overwhelming background from QCD pro- core of∆R < 0.2 aroundthe 4-vectorsum of clustersas-
1
cesses at LHC. Therefore well performing tau identifica- sociated with the jet seed. Calibration factors are derived
:
v tioniscrucial.InATLAS[2],taureconstructionandiden- from response functions using Monte Carlo simulations,
i tification[3]concentratesonthehadronicdecaymodesof which comefromthe ratioof reconstructedtau energyto
X
ataulepton.Theyareclassifiedaccordingtothenumberof true visible tau energy. Response functions are functions
ar reconstructedchargeddecayparticles(prongs).Thesede- dependentonthetautransversemomentumpT,andcalcu-
cayscanbedifferentiatedfromQCD jetsbytheircharac- latedseparatelyforsingle-andmulti-prongtauleptons,as
teristics,suchaslowtrackmultiplicity,collimatedenergy well as for different detector regions. The systematic un-
deposits, and in case of 3-prongtau leptonsthe displace- certainties on the tau energy scale are fully derived from
mentofthesecondaryvertex. MonteCarloandwerefoundtobe4%–7%[3].
3 Identification
2 Reconstruction
SincethereisnoattempttoseparateQCDjetsandtaulep-
Calorimeterjetswithatransverseenergylargerthan10GeV
tons in the reconstruction process a dedicated identifica-
and within the detector acceptanceare used as a seed for
tionstepisneeded.Itisbasedonvariableswhichprovide
thereconstructionoftaucandidates.Trackswithinacone
discrimination power between QCD jets and tau leptons.
of∆R = p(∆φ)2+(∆η)2 < 0.4aroundthetau axispass-
Whilethechargedtracksfromtheτleptondecayarecol-
ingcertainqualitycriteriaareassociatedtothetaucandi-
limated in a narrow cone, tracks from QCD jets are dis-
dateandusedtocalculatethediscriminatingvariables.The
tributedmorewidely(Figure1).Theenergydepositinthe
numberof tracks within ∆R < 0.2 is used to classify the
calorimeter is also collimated in a small area around the
taucandidateintosingle-ormulti-prongcategories.Vari-
tauaxis,whileforQCDjets,alargerareaisaffected(Fig-
ablesbasedoncalorimeterinformationarecalculatedfrom
ure2). Therearethreeindependentmethodsfortauiden-
calorimetercellsin∆R < 0.4aroundthetauaxis.Thetau
tification in ATLAS: a cut-based approach, placing rect-
a e-mail:[email protected] angular cuts on variables, a projective likelihood (LLH)
EPJWebofConferences
8 y
mple Fraction / 0.00 0000....00111.82461 Z20→11ττ d+iWjet→ dτaνta ∫dt L = 130 pb-1 A1 TpLroAnSg pPT>re2li0m iGnaerVy ackground Efficienc 110023 CBLiDukteTslihood ATLAtaSu pPerrefolirmmiannacrey
Sa 0.06 erse B 10
0.04 v
In 2011 dijet data ∫dt L = 130 pb-1
0.02
1-prong, 20 GeV < p ≤ 40 GeV
0 1 T
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
R Signal Efficiency
Cal
Fig. 2. Energy weighted shower width in the calorimeter, R , Fig.4.Signalefficiencyversusinversebackgroundefficiencyfor
Cal
for tau signal Monte Carlo (red) and compared to QCD di-jet the different tau identification methods shown for 1-prong tau
data(black)[3]. candidateswith20GeV< p <40GeV[3].
T
8
action / 0. 0.01.21 ATLAZ20→S11ττ Pd+irWjeetl→ imdτaiνtnaa ∫rydt L = 130 pb-1 3 prongs pT>20 GeV Efficiency 103 CBLiDukteTslihood ATLAtaSu pPerrefolirmmiannacrey
Fr d
mple 0.08 groun 102
Sa 0.06 ack
B
0.04 erse 10
v
0.02 In 2011 dijet data ∫dt L = 130 pb-1
-020 -15 -10 -5 0 5 10 15 20 1 3-prong, 40 GeV < pT ≤ 100 GeV
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Likelihood Score
Signal Efficiency
Fig.3.Outputscoreoftheprojectivelikelihoodtauidentification
Fig.5.Signalefficiencyversusinversebackgroundefficiencyfor
method[3].
the different tau identification methods shown for 3-prong tau
candidateswith40GeV< p <100GeV[3].
T
method,usingthelog-likelihood-ratioofsignalandback-
ground,andboosteddecisiontrees(BDT),tofindtheop-
timal separation in a multi-dimensionalphase space. The ency 104 BDT-based electron veto
ci Cut-based electron veto
amreetsheopdasrautseelydtirffaeinreendtfsoertssinogflied-eanntdifimcautilotin-pvroarnigabtaleuscaannd- nd Effi 103
u
o
didates.Inaddition,thelikelihoodandBDTaretrainedfor gr
differentnumbersofreconstructedverticesinordertotake ack 102
B
ewviethntspiiglne-aulpeiffintcoieancccioeusnot.fT∼hre6e0d%ed,i∼cat4e5d%woarnkding∼po3i0n%ts nverse 10 AtaTuL pAerSforPmrealnimcienary
I 1-prong, p > 20 GeV, | η| ≤ 1.37
(loose, medium, tight) are provided for all tau identifica- T
tionmethods.ThelikelihoodoutputscoreisshowninFig- 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
ure 3) for 3-prong tau candidates. For the training of the Signal Efficiency
identification algorithms, the QCD background was ob-
Fig.6.Signalefficiencyversusinversebackgroundefficiencyfor
tainedfromdata,whilethetaudecaysignalwassimulated
the different tau electron veto methods shown for 1-prong tau
inW →τνandZ →ττMonteCarlosamples.Theinverse
candidates with p > 20GeV inthecentral (barrel) part of the
T
background efficiency [3] versus signal efficiency for all
detector[3].
threemethodsisshownfor1-prong(Figure4)low-p and
T
3-prong(Figure5)high-p taucandidates.
T
Electronscanalsobemisidentifiedasataulepton.Due approachusingtheeventselectionfromtheATLASZ →
tothesignatureoftheelectroninthedetector,theywillbe
ττcross-sectionmeasurement[4].Eventsaretaggedwitha
reconstructed mostly as a 1-prong tau-candidate. To dis-
muonfromataudecay,andtheothertauleptonintheevent
tinguish between electrons and such tau leptons two ve-
isrequiredtodecayhadronically,formingtheprobethatis
toes–acut-basedandboosteddecisiontree(BDT)-based used to measurethe identificationefficiency.The electro-
– are available. The performanceof these electron vetoes weakbackgroundisdominatedbyW → µνandwasesti-
isshowninFigure6.
matedfromMonteCarlosimulation,whiletheQCDmulti-
jetbackgroundwasobtainedbyadata-drivenmethod.The
visible mass of the muon and the hadronic tau is shown
4 Identification Efficiency Measurements fordatabefore(Figure7)andafter(Figure8)applyingthe
tight BDT tau identification and agrees well with Monte
The performance and systematic uncertainties of the tau Carlo predictions. The tau identification efficiency was
identificationmethodsareevaluatedondatausingtwodif- alsomeasuredusingW →τνeventscollectedin1.37fb−1
ferentsignalchannels.ThefirstmethodusesZ →ττevents ofATLASdata.Variablesbasedonthemissingtransverse
in 800pb−1 ofATLASdata andrelieson a tag-and-probe energywereusedtoselecttheevents.Thenumberofhadronic
2011HadronColliderPhysicssymposium(HCP-2011)
nts / 5 GeV 111468000000 Before tau ID ∫Ads tT= L L 7=A T 8eS0V0 Pprbe-l1iminary Events3500 Dτ a(Wta→ 20 τ1ν1) (1.37 fb-1)
Eve 1200 DZ→ataττ 2 (0tr1u1th-matched) 3000 e (W→ eν)
1000 Z→ττ (non-truth-matched) Jet background
Multijet 2500
800 W → µ ν
W → τ ν ATLASPreliminary
600 Z → µ µ 2000
400 tt Before tau ID
200 1500
0
0 20 40 60 80 100 120 140 160 180 200 1000
m (µ,τ) [GeV]
vis h
500
Fig.7.Visiblemassoftheselectedmuonandtaucandidatefor
data and Monte Carlo simulation after full event selection, but 0
0 2 4 6 8 10 12 14 16
beforeapplyinganytauidentification.TheQCDmulti-jetback- Number of tracks
groundwasobtainedbyadata-drivenmethod[3].
Fig. 9. Number of charged tracks for tau candidates after full
GeV After tight BDT ID ∫dt L = 800 pb-1 event selection, but before applying any tau identification. The
5 250 s = 7 TeV threedifferenttemplatesareshown[3].
nts / ATLASPreliminary
Eve 200 DZ→ataττ 2 (0tr1u1th-matched)
150 MWZ→u →ltτiτj eµ (t nνon-truth-matched) ents1400 Data 2011 (1.37 fb-1)
100 Z → µ µ Ev1200 τ (W→ τν)
tWt → τ ν e (W→ eν)
50 1000 Jet background
00 20 40 60 80 100 120 140 160 180 200 800 ATLASPreliminary
m (µ,τ) [GeV]
vis h
600 After tau ID
Fig.8.Visiblemassoftheselectedmuonandtaucandidatefor (Tight BDT working point)
dataandMonteCarlosimulationafterfulleventselectionandap- 400
plyingthetightBDTtauidentification.TheQCDmulti-jetback-
groundwasobtainedbyadata-drivenmethod[3]. 200
0
0 2 4 6 8 10 12 14 16
tau candidates are derived by a template fit of the track Number of tracks
multiplicityofthetaucandidates.Threedifferenttemplates
were used: real hadronic tau decays, electrons misidenti- Fig. 10. Number of charged tracks for tau candidates after full
fiedastauleptons,andQCDmulti-jetsmisidentifiedastau event selection and applying tight BDT tau identification. The
leptons.WhilethefirsttwoareobtainedfromMonteCarlo threedifferenttemplatesareshown[3].
simulation,theQCDmulti-jettemplatewasestimatedfrom
a controlregionrich in QCD events.The track multiplic-
ity distributionisshownfor dataandMonte Carlobefore 2. ATLASCollaboration,JINST3,(2008)S08003
(Figure9)andafter(Figure10)applyingthetightBDTtau 3. ATLAS Collaboration, Conference
identification. Note, ATLAS-CONF-2011-152, 2011,
Themeasuredefficienciesinbothmethodsareingood https://cdsweb.cern.ch/record/1398195
agreementwithMonteCarlopredictionswithin5%(8%- 4. ATLASCollaboration,Phys.Rev.D84,(2011)112006,
12%)fortheW →τν(Z →ττ→µτ )method. http://arxiv.org/abs/1108.2016
had
5 Summary and Conclusion
ATLAS has a large physics program with tau lepton fi-
nal states, and a well performing tau identification is a
essential part of these analyses. Different techniques are
usedtoseparatetauleptonsfromthequarkandgluonini-
tiated jet background. The multivariate methods perform
betterthanasimplecut-basedapproach,especiallyfortau
leptonswith a transverse momentumlarger than 40 GeV.
The corresponding efficiencies and systematic uncertain-
tiesofthetauidentificationmethodshavebeenstudiedus-
ingStandardModelprocesses.
References
1. (ed.)Evansand(ed.)Bryant,JINST3,(2008)S08001