The presence of tau leptons in the final state is an important signature in
many Higgs boson and SUSY searches. With the ATLAS detector, hadronically
decaying tau leptons can be reconstructed in a wide range of transverse
energies. The reconstruction algorithm for hadronically decaying tau leptons is
explained and the performance of tau lepton identification is shown.
Particularly important is the discrimination of hadronically decaying tau
leptons against overwhelming backgrounds from QCD jets

Kaczmarska, Anna

Lai, Stan

English

arXiv

The presence of tau leptons in the final state are an important signature in many Higgs boson and SUSY searches. With the ATLAS detector, hadronically decaying tau leptons can be reconstructed in a wide range of transverse energies. The reconstruction algorithm for hadronically decaying tau leptons is explained and the performance of tau lepton identification is shown. Particularly important is the discrimination of hadronically decaying tau leptons against overwhelming backgrounds from QCD jets.The presence of tau leptons in the final state is an important signature in many Higgs boson and SUSY searches. With the ATLAS detector, hadronically decaying tau leptons can be reconstructed in a wide range of transverse energies. The reconstruction algorithm for hadronically decaying tau leptons is explained and the performance of tau lepton identification is shown. Particularly important is the discrimination of hadronically decaying tau leptons against overwhelming backgrounds from QCD jets

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ATL-PHYS-PROC-2008-02129September2008Reconstruction and Identification of Hadronic Tau Decays with ATLASS. LaiAlbert-Ludwigs Universita¨t Freiburg, 79104, Freiburg, GermanyA. KaczmarskaInstitut of Nuclear Physics PAN, 31342, Krako´w, Poland(on behalf of the ATLAS Collaboration)The presence of τ leptons in the final state is an important signature in many Higgs boson and SUSY searches. Withthe ATLAS detector, hadronically decaying τ leptons can be reconstructed in a wide range of transverse energies. Thereconstruction algorithm for hadronically decaying τ leptons is explained and the performance of τ lepton identificationis shown. Particularly important is the discrimination of hadronically decaying τ leptons against overwhelmingbackgrounds from QCD jets.1. INTRODUCTIONThe τ lepton, with a mass of 1776.84 ± 0.17 MeV [1], is the only lepton heavy enough to decay both leptoni-cally and hadronically. It decays approximately 65% of the time to one or more hadrons. The reconstruction andidentification of τ leptons are important in many searches for new phenomena, and they can appear in final statesduring the production of Higgs bosons or supersymmetric particles [2]. This paper will discuss the reconstructionand identification of hadronically decaying τ leptons with the ATLAS detector [3].2. RECONSTRUCTION OF HADRONICALLY DECAYING τ LEPTONSHadronically decaying τ candidates are reconstructed using at least one of two possible seed types. The first seedtype is a track with pT > 6 GeV. This leading track must satisfy further quality criteria to be considered a valid seedfor a τ candidate.The second type of seed consists of jets reconstructed using topological clusters (topoclusters) [2] with ET > 10GeV. Topoclusters are formed using cells that exceed calorimeter noise by 4σ, and neighbouring cells that exceedenergy thresholds above calorimeter noise by 2σ and 0σ are associated to the cluster in a second and third step,respectively. These topoclusters are then grouped into a topojet using a seeded cone algorithm [4] with a cone radiusof ∆R =√(∆η)2 + (∆φ)2 = 0.4 which form the aforementioned seeds for τ candidates. These topojets are thenmatched to seed tracks in a cone of radius ∆R = 0.2 around the topojet. If such a match is found, the τ candidateis considered as having two valid seeds. For reconstructed τ leptons in Z → ττ events, 70% of τ candidates have twovalid seeds, 25% have only a topojet seed, and 5% have only a track seed.The energy of the τ candidate is calculated in two ways. For τ candidates with a topojet seed, the cells in a cone of∆R = 0.4 are summed and weighted using a calibration similar to that used for the Liquid Argon calorimeter of theH1 experiment [5]. Scale factors, determined from Monte Carlo and depending on the pT and η of the τ candidate,are used to further correct the energy scale.For τ candidates with a track seed, an energy flow approach is used, where energy deposits in cells matched tocharged tracks are subtracted and replaced by the momenta of such tracks. This energy of the τ candidate is alsocorrected for energy leakage coming from charged particles outside the narrow cone.Tracks associated to the τ candidate in a cone of ∆R = 0.2 are required to pass track quality criteria, albeit lessstringent than those for the leading track. Figure 1 (left) shows the track multiplicity distribution for τ candidatesmatched to true hadronically decaying τ leptons. Topoclusters found in the electromagnetic (EM) calorimeter withET > 1 GeV that are isolated from tracks are interpreted as energy deposits from pi0 mesons in the τ lepton decay.Insert PSN Here Track multiplicity0 1 2 3 4 5 6 7 8 9 Probability 00.10.20.30.40.50.60.70.80.91ATLAS PreliminaryReconstructionIdentification with cutsIdentification with NNττ→ Z ATLAS Preliminary Invariant mass (GeV)0 0.5 1 1.5 2 2.5 Arbitrary units020406080100120140160νρ→τν) pi0pi 2  →(1 a→τνpi→τFigure 1: Left: Track multiplicity for τ candidates in Z → ττ events after reconstruction and identification steps. Right:Reconstructed invariant mass distribution for τ → piν, τ → ρν, τ → a1ν decays in W → τν events.We find that approximately 66% of τ → piν decays are reconstructed with zero pi0 subclusters, while more than 50%of τ → ρν (τ → a1ν) decays are reconstructed with one (two) pi0 subcluster(s).Based on the calorimeter information, the associated tracks and reconstructed pi0 clusters, a variety of othervariables are calculated to be used for the identification of τ leptons. These variables include the radius of theτ candidate in the EM calorimeter, isolation variables for the calorimeter energy and tracks, reconstructed charge(based on the associated tracks), and the invariant mass of the τ candidate. The invariant mass distribution forselected τ candidates based on the above mentioned variables is shown in Figure 1 (right).3. IDENTIFICATION METHODS FOR τ LEPTONSBecause of the large production cross section for QCD jets, identification methods are needed to discriminate τcandidates arising from true τ lepton decay and those from QCD processes. Identification methods are also used todistinguish hadronically decaying τ leptons from electrons and muons. At ATLAS, various identification methodshave been studied, ranging from simple cut-based criteria, to multivariate techniques such as: projective likelihoods(LH), boosted decision trees, neural networks, and probabilty range searches. Here, only cut-based vetoes againstmuons and electrons and the projective likelihood discriminant against QCD jets will be discussed.Muons are vetoed by requiring that the calorimetric energy deposited by the τ candidate has ET > 5 GeV. Forelectrons, cuts are placed upon 1-prong τ candidates on the following two quantities: the ratio of the transverseenergy deposited in the EM calorimeter to the track transverse momentum which tends to be higher for electronsthan for charged hadrons, and the ratio of high threshold hits to low threshold hits in the Transition RadiationTracker for the track, which also tends to be higher for electrons. This veto suppresses electrons fromW → eν eventsby a factor of 60, while retaining a 95% efficiency for τ leptons in W → τhadν.Discriminating variables used to distinguish τ leptons from QCD jets include: radius and profile of EM calorimeterenergy deposits, track width distributions, isolation variables calculated from calorimetric energy deposits and tracks,impact parameter significance of the leading track, invariant mass of the associated tracks, ratios of energy depositsto the sum of track transverse momenta, and the transverse flight path significance of the τ candidate vertex. Sampledistributions of a subset of these variables are shown in Figure 2.The LH takes these variables and forms probability distribution functions (PDFs) for τ leptons (signal) and jets(background), which are separated into the following categories: the pT of the τ candidate, the number of associatedtracks, valid seed types, and the presence of pi0 subclusters. A likelihood ratio is formed using the product of individuallikelihood ratios for the signal and background hypothesis in each variable. The performance of the LH is shown inFigure 3 for 1-prong and 3-prong τ candidates. For τ candidates with 30 < ET < 60 GeV, a 40% τ identificationefficiency corresponds to a QCD jet rejection of 1500± 100 (1480± 100) for 1-prong (3-prong) candidates.Insert PSN HereEM radius0 0.05 0.1 0.15 0.2 0.25 0.3normalized to unit area00.020.040.060.080.10.120.140.160.18-lepton (1 prong)τ-lepton (3 prong)τQCD jetsATLAS PreliminaryisoltrkN0 1 2 3 4 5 6normalized to unit area00.20.40.60.81-lepton (1 prong)τ-lepton (3 prong)τQCD jetsATLAS Preliminary]2 [GeV/ctrkM0 0.5 1 1.5 2 2.5 3 3.5normalized to unit area00.020.040.060.080.10.120.14-lepton (3 prong)τQCD jetsATLAS PreliminaryFigure 2: Distributions of selected identification variables for 1-prong τ leptons, 3-prong τ leptons, and QCD jets.Efficiency0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Rejection210310410Rejection 10-30 GeV30-60 GeV60-100 GeV>100 GeVATLAS PreliminaryEfficiency0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Rejection210310410Rejection10-30 GeV30-60 GeV60-100 GeV>100 GeVATLAS PreliminaryFigure 3: Left: QCD jet rejection as a function of 1-prong τ efficiency for the projective likelihood discriminant. Right: QCDjet rejection as a function of 3-prong τ efficiency for the projective likleihood discriminant.4. CONCLUSIONS AND OUTLOOKATLAS is planning an extensive program to investigate physics channels with hadronically decaying τ leptons in thefinal state. Sensitivity to such channels requires robust reconstruction algorithms and well performing identificationmethods to distinguish true τ leptons from QCD jets, electrons, and muons. Studies for extracting cross sectionsfor Standard Model processes such as W , Z, and tt¯ production with hadronic τ leptons in the final state have beenperformed [2]. An understanding of these Standard Model processes will aid the understanding of searches for newphysical phenomena with final state τ leptons.References[1] C. Amsler et al. (Particle Data Group), Physics Letters B667, 1 (2008).[2] The ATLAS Collaboration, “Expected Performance of the ATLAS Experiment, Detector, Trigger, and Physics”,CERN-OPEN-2008-020, Geneva, 2008, to appear.[3] The ATLAS Collaboration, G. Aad et al., “The ATLAS Experiment at the CERN Large Hadron Collider”, JINST3 (2008) S08003.[4] S.D. Ellis et al., Prog.Part.Nucl.Phys. 60:484-551, (2008).[5] C. Schwanenberger, The Jet Calibration in the H1 Liquid Argon Calorimeter, arXiv:physics/0209026v1[physics.ins-det] (2002).Insert PSN Here

Reconstruction and Identification of Hadronic Tau Decays with ATLAS

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