The presence of tau leptons in the final state is an important signature in
searches for physics beyond the Standard Model. Hadronically decaying tau
leptons can be reconstructed over a wide kinematic range at ATLAS. The
reconstruction algorithm for hadronically decaying tau leptons and the
performance of tau lepton identification is described. A review of physics
processes with tau lepton final states is given, ranging from Standard Model
processes in early data, such as W and Z boson production, to searches for new
phenomena beyond the Standard Model.Comment: 6 pages, 5 figures to be published in HEP Madagascar 2009 Conference
  Proceeding

Lai, Stan

English

arXiv

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 over a wide range of transverse energies. This talk will describe the tau reconstruction algorithm for hadronically decaying tau leptons at ATLAS. The performance of tau lepton identification is also discussed. A review of physics processes with tau leptons is given, ranging from Standard Model processes in early data, such as W and Z boson production, to searches for the Higgs boson and supersymmetry once Standard Model processes are well understood

Lai, S

CERN Document Server

ATL-PHYS-SLIDE-2009-24708September20091Tau Physics at the LHCwith ATLASStan LaiUniversität Freiburgon behalf  of:The ATLAS CollaborationHigh Energy Physics Conference in Madagascar26. August 2009Stan Lai HEP Conference in Madagascar26 August 20092Outline• Basic tau lepton properties •The tau reconstruction algorithm at ATLAS• Standard Model channels with tau lepton final states in early data• Searches for new physics with tau leptons• Summaryplots and prospects shown for ECM = 14 TeV(many studies for ECM = 10 TeV in progress)Stan Lai HEP Conference in Madagascar26 August 20093Basic Tau PropertiesBecause leptonic tau decays cannot really be separated from electrons and muons,we say „tau reconstruction and ID“ to refer to hadronically decaying tau leptonsWe try to distinguish between 1-prong and 3-prong tau decaysobserved first in 1977 byMartin Perl et al.(SLAC-LBL)}47%} 15%Stan Lai HEP Conference in Madagascar26 August 20094Taus and JetsBoth hadronically decaying taus and jets deposit energy in the EM and Had calorimetersChallenge: separate out a clean sample of taus from the overwhelming QCD jet rate!• narrow, collimated• 1 or 3 tracks• higher fractional   EM deposit• can define isolation  regions with   low activity• leading track carries   much of tau momentum• wide• can have many tracks• higher fractional   HAD deposit• isolation regions busy• jet momenta spread   over tracksStan Lai HEP Conference in Madagascar26 August 20095The ATLAS Tau Reconstruction AlgorithmThe ATLAS Tau Reconstruction Algorithm can be organized in four steps1) Seed Building• track seed finding• calorimeter seed finding2) Base Reconstruction• track association• calculation of ID variables• energy/energy flow calculation3) Tau Substructure Reconstruction• pi0 cluster reconstruction (not discussed)4) Identification Algorithms• electron and muon vetoes• cut-based ID• multi-variate ID: projective likelihood, NN,   boosted decision trees, probability density range searchesATLAS properties:ATLAS has pixel/SCT/TRT  with  B = 2 Ttracker provides coverage to |η| < 2.5ECAL (LAr calorimeter) covers to |η| < 3.2 with high granularityHCAL is coarser than the ECAL:proper coverage to |η| < 3.2forward coverage 3.2 < |η| < 4.9tracker X/X0 = 0.5 - 1.8 lengthsStan Lai HEP Conference in Madagascar26 August 20096Tau Seeds at ATLASThree categories of tau candidates based on the seed composition:• track-only seeded candidates (quality track with pT > 6 GeV)• calo-only seeded candidates (TopoJet with ET > 10 GeV)• candidates seeded by both (track and TopoJet matched within ΔR < 0.2)TopoJet - cone algorithm run over TopoClustersTopoCluster - iterative clustering procedure:• start with seed cells (exceed 4σnoise)  add neighbour cells (exceed 2σnoise and 0σnoise)• split cluster if multi local maximatrack-onlycalo-onlyboth calo &trackseed composition for truth-matchedtau candidates in Z→ττtopocluster schematicStan Lai HEP Conference in Madagascar26 August 20097Base Reconstruction at ATLASDirection (η,ϕ) of tau determined by leading track(unless no track seed - then determined by jet barycentre)Quality tracks associated in ΔR<0.2 of seed direction Energy calculated by:• cell weighting (default): Ecalib = w(η,ϕ,i) x Eraw • energy flow:  neutral energy + charged track   momenta terms (corrected for energy leakage)ID variables calculated: isolation, width, ratios of calo energy and track momenta, etc.ET = 20 - 60 GeVEM Radius # isolation tracks MtracksStan Lai HEP Conference in Madagascar26 August 20098Electron Veto at ATLASTau candidates are divided into categories based on:• EHCAL (energy deposited in HCAL)• Estripmax (energy in strip layer that is                   not associated to leading track)Then (category-dependent) cuts to veto electrons are placed on:• EECALT/pTlead trk (ratio of transverse energy in ECAL to  transverse momentum of leading track)• NTRTHT/NTRTLT(ratio of high threshold to low threshold hits in the TRT)Stan Lai HEP Conference in Madagascar26 August 20099Tau ID algorithms at ATLASThe ID at ATLAS is needed to separate τ-leptons from jetsID algorithms are based on the variables such asisolation fraction, EM radius calculated in thebase reconstructionATLAS has studied the following:• Cut-Based • Bayesian Projective Likelihood • Probability Density Range Searches• Neural Networks• Boosted Decision Treesτ-identification is robust over a wide pT  range cut-based approaches may have preference in the early data taking periodNN performanceat low pTlikelihoodperformanceat high pTATLASATLASStan Lai HEP Conference in Madagascar26 August 200910The ATLAS Tau TriggerAt Level 1, hardware trigger uses calorimeter towers of ΔηxΔϕ=0.1x0.1 in 2x1 group in ECAL and 2x2 group in HCALEM isolation also used in 0.1 wide ring around the seed tower at L1HLT L2 uses ID variables such as: track multiplicity, isolation, narrowness of candidateHLT EventFilter uses „pseudo-offline“ algorithms to reconstruct candidate & filter out fakestau20i triggerStan Lai HEP Conference in Madagascar26 August 200911Standard Model Processes with Tau Final States Standard Model processes with τ-leptons in the final state will be key to understanding τ reconstruction and identification in early data (10-100 pb-1)Key channels:Z→ττ • the golden channel to understand τ-leptons at ATLAS• the lepton-hadron channel allows us to trigger on electron/muon, providing  an unbiased sample of hadronically decaying τ-leptons on the other legW→τν • challenging channel to separate out from multijet background• use of ETMISS is necessary for both trigger and offline• requires the τ-trigger which biases the sample of τ-leptonstop pair production  tt→WbWb→τνbqq'b  or  tt→WbWb→τνblνb  • gives a higher pT spectrum than that for Z→ττ• event environment is different due to extra jets in the eventStan Lai HEP Conference in Madagascar26 August 200912Reconstructing the ττ invariant massZ/H/AττStan Lai HEP Conference in Madagascar26 August 200913The Z→ττ ChannelThe Z→ττ channel (one τlep decays leptonically, one τhad decays hadronically) avoids the use of τ-triggers, and yields an unbiased sample of τhad-leptons.Main Backgrounds: W+jets, Z+jets, multi-jet production, top pair productionAnalysis cuts:• isolated lepton  (pT > 15 GeV)• ETMISS > 20 GeV• MT(lepton, ETMISS) < 30 GeV• ∑ET  < 400 GeV• b-jet veto•  τhad identification (pT > 15 GeV)• Δφ(lepton, τhad) cuts• QleptonxQт = -1control regions defined to evaluate multi-jet and W+jet contributions from data:non-isolation, high MT(lepton, ETMISS), QleptonxQт =1Stan Lai HEP Conference in Madagascar26 August 200914Understanding τ-ID with Z→ττ The Z→ττ sample will be a key control sample for understanding τ-ID with dataTag (electron/muon) and probe (τ-lepton) methods can be used to understand:• the τ-ID efficiency (inclusive / 1-prong / 3-prong)• the τ-trigger efficiency (for different thresholds/trigger streams)Using the known σ(pp→Z)xBR(Z→ττ), efficiencies can be measured in data The visible mass distribution of the Z→ττ products can be used to evaluate the energy scale for the τ-leptonsThe invariant mass of Z→ττ products can be reconstructed using collinear approximation• should peak at well measured   Z mass (91.2 GeV)• peak of distribution gives us a handle on the   ETMISS scale τ energy scale determination with visible mass from Z→ττuncertainties arestatistical onlyStan Lai HEP Conference in Madagascar26 August 200915The W→τυ ChannelApproximately a factor of 10 greater signal yield for W→τυ compared to Z→ττbut more challenging due to lack of additional τ-lepton legTrigger stream for the process is τ + ETMISS trigger (70% efficient wrt offline)Main Backgrounds: W→eυ, W→μυ, Z→ττ, Z→ee, top pairs, multi-jetsAnalysis Cuts:• trigger τ-lepton (pT > 20 GeV) and trigger ETMISS > 30 GeV• offline τ-lepton (pT = 20-60 GeV)• ETMISS > 50 GeV (60 GeV)• additional jet (pT > 15 GeV)ETMISS>50 GeV ETMISS>60 GeVsensitivity to the ETMISS cutcontrol regions for multi-jets:• non-isolated leptons• Ntrkτ > 3 regionStan Lai HEP Conference in Madagascar26 August 200916Searches for New Physics with Tau Final States τ-leptons play an important role in many searches for new phenomenaVBF Higgs Boson (H →τ τ)  (not discussed here – see talk by P. Fleischmann)• most sensitive channel for SM Higgs searches at low massCharged Higgs Boson searches (H± → τ ± ν)• occurs if mH± < mt, and produced in decay t → H+b• even if mH± > mt, associated tbH+→bqqbτν production is a clean search channel                                                                                                  (at high tan β)Neutral MSSM Higgs Boson searches• much of SUSY parameter space predicts abundance of H/A→τ τ decayInclusive SUSY searches with τ-lepton final states• enhanced for high tan β regions  • can differentiate SUSY models based on polarization informationSearches for heavy gauge bosons (Wʹ→τν, Zʹ→ττ) • heavy bosons couple only to third generation?  ● polarization information also availableStan Lai HEP Conference in Madagascar26 August 200917Supersymmetric PhenomenologySUSY – all standard modelparticles have supersymmetricpartners with differing spin by ½Two Higgs doublets are necessarytan β: ratio of vacuum expectation values of Higgs doubletsother SUSY parameters that define the phenomenology (mSUGRA):• m1/2 : common gaugino mass at GUT scale• m0: common scalar mass at GUT scale• A0: trilinear coupling at GUT scale• μ: sign of Higgsino mixing parameterIn SUSY phenomenology, cascade decays are a prominent feature For high values of tan β, the NLSP is thestau, so that the production of SUSY              particles leads to:                 stau decays to τ-lepton + LSPStan Lai HEP Conference in Madagascar26 August 200918SUSY Searches with TausHigh tan β regions favour light 3rd generationsuperpartners – τ-leptons in SUSY final statesEffective mass tail dominated bytop pair and W/Z+jets productionAt tan β = 50, the τ-lepton mode has comparablesensitivity to the 1-lepton modeSelection:• 4 jets (pT thresholds 100,50,50,50 GeV) • 1 τ-lepton  pT > 40 GeV• veto isolated electrons/muons• ETMISS > 100 GeV and ETMISS > 0.2xMEff•  transverse sphericity > 0.2• Δφ(jet1,2,3, ETMISS) > 0.2• MT(τ, ETMISS) > 100 GeVsuppresses top pairs and W+jets1 fb-1Stan Lai HEP Conference in Madagascar26 August 200919Higgs Bosons in the MSSMThe Higgs sector of the MSSM consists of 5 different Higgs bosons:• two CP even neutral Higgs bosons (h0, H0)• one CP odd neutral Higgs boson (A0)• two charged Higgs bosons (H+, H-)At tree level, the Higgs sector isparametrized by tan β and mAbranching ratios for charged Higgs at different tan β Much of the MSSM parameter spacepredicts decays of the MSSMHiggs bosons to τ-leptonsAt large tan β, associated neutralHiggs production with bottompairs is dominantBR(h0/H0/A0→ττ) ~ 10%Charged Higgs decays to τυ arealso predicted for mH+ < mt usingt → H+b→τνb and for mH+ > mtusing tbH+→bqqbτν (at high tan β)Stan Lai HEP Conference in Madagascar26 August 200920Charged Higgs Boson SearchesFor mH+ < mt, the most sensitive channel is tt→bH+bW→bτhadνbqqτ + ETMISS or τ + 3 jets + ETMISS trigger stream is usedBasic cuts:• identified τ-lepton (pT > 35 GeV)• two b-tagged and two non-b-tagged jets                                         (pT > 15 GeV)• veto on isolated leptons• ETMISS > 40 GeV (50 GeV if no jets in trigger)• mW  and mtop within 30 (40) GeV of PDG value• |Δφ(top quarks)| > 2.5, pT balance (top quarks)Likelihood discriminant is built based on further kinematics of the eventmh-max scenarioStan Lai HEP Conference in Madagascar26 August 200921Z’ SearchesExist models where heavy resonances couple preferentially to 3rd generationProspects for  Z´→τhadτlep with mZ´ = 600 GeVSelection:• τhad-lepton pT > 40 GeV• isolated electron/muon pT > 27/22 GeV • opposite charge• ETMISS > 30 GeV• MT(lepton, ETMISS) < 35 GeV• pTtotal < 70 GeV• Mvisible(τhadτlep) > 300 GeV• cos[Δφ(τhad, lepton)] > -0.99suppress W+jetssuppress top pairseliminate back-to-back τhadτlep for collinear approximationSM couplingsassumedStan Lai HEP Conference in Madagascar26 August 200922Summary & Outlook• Tau reconstruction and identification is an active field at ATLAS• ID methods have been refined, optimized and tuned• Well developed studies of tau physics with first data (~100 pb-1)• Standard Model channels (W, Z, top pairs) will provide invaluable feedback for the existing algorithms• Efficiencies, fake rates, energy scale of tau-ID will be measured in data• Many exciting physics possibilities beyond the Standard Model that involve tau lepton final states• The ATLAS physics program with taus is very extensive, and is looking forward to collision data – the possibilities are endless!  Stan Lai HEP Conference in Madagascar26 August 200923Reconstruction of π0 showers in tau decaysJust as 1-prong and 3-prong taus have different caloproperties, so do τ→πν and τ→ρν→ππ0ν If possible, it makes sense to require different cuts/pdfsto identify τ→πν and τ→ρν decay if we can separate themATLAS reconstructs π0 subshowers by reconstructing EMtopoclusters - but need to eliminate clusters from chargedparticles:parametrized shower subtraction for clusters matched to tracksdecay 0 π0 1 π0 ≥ 2 π0 τ→πν 61% 23% 16%τ→ρν 18% 50% 32%τ→a1ν 9% 36% 55%Barrel regionStan Lai HEP Conference in Madagascar26 August 200924Conversion Track Tagging in tau decaysIn approximately 40.5% of τ-decays a π0 is presentThe process π0→γγ and γ→e+e- distorts the track spectrumof τ-leptons - can we tag such tracks as conversion tracks?There are studies at ATLAS to identify two-track vertex candidates and thencalculate electron probability that track originated from an electronMain property: high threshold TRT (Transition Radiation Tracker) hits• electrons radiate photons which also can produce hits in the TRT gas  hence providing more hits at „high“ thresholdIf we tag tracks from conversions, we can reduce the bias introduced on theτ track multiplicity spectrumStan Lai HEP Conference in Madagascar26 August 200925Tau ID in early dataOptimizing the rejection/efficiency of τ-ID in Monte Carlo is interesting but data will be a much different story!• proper MC modeling of variables is going to be inaccurate• many variables require precision alignment/calibration before they are effective  (secondary vertexing, track impact parameters, hadronic calibration)• many variables will have different behaviour due to underlying event and pile-up  (track and calorimeter isolation with wide cones)For early data - can try to use only variables suspected to be  under control with tens/hundreds of pb-1Try to avoid using multi-variate techniques, and use rectangular cuts insteadATLAS looking at optimization of rectangular cuts on a set of „safe variables“i.e. EM radius, calorimeter isolation with narrow cone, width of track system,      ratio of ETEM to ETHAD at the EM scale, ratio of EM/HAD energy to ΣpTtracksOptimization for different pT bins, different efficiencies is being studiedStan Lai HEP Conference in Madagascar26 August 200926Measuring τ fake rates in jet samplesThis can be done with very early data Fake Rate = (# probe jets passing τ-ID) / (# probe jets)Real   -production is ~negligible in comparison with jet production• can be corrected by MC subtraction of real   -lepton contribution)  (MC more reliable for W/Z and top events than for QCD jet events)Can further ensure a „jet pure“ sample by:• requiring back-to-back jets• pT  balance between jets:   pT(jets) < pTlead jet / 2• very „jet-like“ tag jet: Ntracktag-jet > 4   (or increase track cut for higher pT)No other requirements on probe jet - need to keep unbiased sample of jetsStatistics will not be a problem (so will do better than MC studies!)Can generate background PDFs from dataStan Lai HEP Conference in Madagascar26 August 200927Taus with Topsτ-leptons coming from top pair production give a τ-sample with higher pT than Z→ττTypical topology is tt→WbWb→τνbqq'b  or  tt→WbWb→τνblνb                                      Trigger paths: τ trigger, τ + ETMISS trigger, multi-jet trigger, lepton trigger Can use b-tagging to help select top events since W+jets background is largeFor lepton-τ channel, same sign events can be used as background control sampleNice cross check to  Z→ττ for τ-ID efficiency and τ-trigger efficiency when using events selected with unbiased triggerCould be the best study  sample of τ-leptons with higher pT100 pb-1Stan Lai HEP Conference in Madagascar26 August 200928Standard Model Higgs BosonThe decay H→ττoccurs at about 10%BR at low Higgs massesThe qqH (Vector Boson Fusion)channel gives forward tagging jetsin addition to the H→ττ signatureElectroweak measurements of the top mass and the W mass seem to hint at a low mass Higgs Boson:mH = 90-27+36 GeV or mH<163 GeV (95% CL)In this region the qqH→qqττ has the most sensitivityto the SM Higgs BosonStan Lai HEP Conference in Madagascar26 August 200929Tau Polarization in SUSYStan Lai HEP Conference in Madagascar26 August 200930Z' PolarizationNew Heavy Gauge Bosons such as W′ and Z′ bosons are predicted for some modelsPerhaps such bosons have preferential couplings(or maybe even exclusive couplings) to third generation  leptons/quarksCould look for a Z′→ττ or   W′→τν excess abovebackgroundEven if no preferential couplings to third generation can infer other properties using τ-channels: Z′ polarizationWe would want to separate out left-handed and right-handed τ-leptons (hadronically decaying)fraction of energy carried by τ in lab frame 

Tau Physics at the LHC with ATLAS

The presence of tau leptons in the final state is an important signature in searches for physics beyond the Standard Model. Hadronically decaying tau leptons can be reconstructed over a wide kinematic range at ATLAS. The reconstruction algorithm for hadronically decaying tau leptons and the performance of tau lepton identification is described. A review of physics processes with tau lepton final states is given, ranging from Standard Model processes in early data, such as W and Z boson production, to searches for new phenomena beyond the Standard Model.The presence of tau leptons in the final state is an important signature in searches for physics beyond the Standard Model. Hadronically decaying tau leptons can be reconstructed over a wide kinematic range at ATLAS. The reconstruction algorithm for hadronically decaying tau leptons and the performance of tau lepton identification is described. A review of physics processes with tau lepton final states is given, ranging from Standard Model processes in early data, such as W and Z boson production, to searches for new phenomena beyond the Standard Model

ATL-PHYS-PROC-2009-12608October2009Tau physics at the LHC with ATLASStan Laion behalf of the ATLAS CollaborationAlbert-Ludwigs Universität Freiburg, 79104, Freiburg, GermanyAbstract. The presence of τ leptons in the final state is an important signature in searches forphysics beyond the Standard Model. Hadronically decaying τ leptons can be reconstructed over awide kinematic range at ATLAS. The reconstruction algorithm for hadronically decaying τ leptonsand the performance of τ lepton identification is described. A review of physics processes with τlepton final states is given, ranging from Standard Model processes in early data, such as W and Zboson production, to searches for new phenomena beyond the Standard Model.Keywords: Tau lepton, ATLASPACS: 14.60.FgIntroductionThe τ lepton, with a mass of mτ = 1776.84±0.17 MeV [1], is the only lepton heavyenough to decay both leptonically and hadronically. It decays approximately 65% of thetime to one or more hadrons and 35% of the time leptonically. The reconstruction andidentification of τ leptons are important in many searches for new phenomena, and theycan appear in final states in the production of Higgs bosons, supersymmetric (SUSY)particles, and other particles not described by the Standard Model [2, 3]. Standard Modelprocesses, such as the W , Z boson and t ¯t production can also result in signatures with τleptons, and events from these processes can be used to measure key quantities such asthe τ lepton identification efficiency for the ATLAS reconstruction algorithm.A challenge in identifying hadronically decaying τ leptons (τhad) is to distinguishthem from hadronic jets which are produced in processes with very large cross-sections.However, τhad leptons possess certain properties that can be used to differentiate themfrom jets. They usually decay into one (1-prong) or three (3-prong) charged particlesand their decay products are well collimated with an invariant mass less than mτ . Theτ lepton proper lifetime is 87 µm, leading to decay vertices that can be resolved inthe silicon tracker from the primary interaction vertex. In addition, τhad leptons deposita considerable fraction of their visible energy in the electromagnetic calorimeter inconstrast with jets which deposit their energy primarily in the hadronic calorimeter.The reconstruction and identification of τ leptonsHadronically decaying τ candidates are reconstructed at ATLAS using two seedtypes. The first seed type is a track with pT > 6 GeV that satisfies quality criteriaon the number of associated silicon hits and the impact parameter with respect tothe interaction vertex. The second type of seed consists of jets reconstructed usingtopological clusters (topoclusters) [2] with ET > 10 GeV. Topoclusters are formedusing cells that exceed calorimeter noise by 4σ . Neighbouring cells that exceed energythresholds above calorimeter noise by 2σ and 0σ are associated to the cluster in a secondand third step, respectively. These topoclusters are then grouped into a topojet usinga seeded cone algorithm [4] with a cone radius of ∆R = 0.4 which forms seeds forτhad candidates. These topojets are then matched to the seed tracks in a cone of radius∆R = 0.2 around the topojet. If such a match is found, the τhad candidate is consideredas having two valid seeds. For reconstructed τhad leptons in Z → ττ events with pT > 10GeV and |η| < 2.5, 70% of τhad candidates have two valid seeds, 25% have only atopojet seed, and 5% have only a track seed.The energy of the τhad candidate is calculated in two ways. For τhad candidates witha topojet seed, the cells in a cone of ∆R = 0.4 are summed and weighted according to afunction dependent on the η , φ and calorimeter layer of the cell, similar to the methodused for the Liquid Argon calorimeter of the H1 experiment [5]. For τhad candidates witha 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 energyof the τhad candidate is also corrected for energy leakage coming from charged particlesoutside the narrow cone.Tracks associated to the τhad candidate in a cone of ∆R = 0.2 are also required topass track quality criteria on the number of associated hits in the silicon tracker and theimpact parameter to the interaction vertex. Topoclusters found in the electromagnetic(EM) calorimeter with ET > 1 GeV that are isolated from tracks are interpreted as energydeposits from pi0 mesons in the τ lepton decay. This procedure finds that approximately66% 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 pi0clusters, a variety of other variables are calculated to be used for the identification ofτhad leptons. These variables include the radius of energy deposits of the τhad candidatein the EM calorimeter (shown in Fig. 1 [left]), isolation variables for the calorimeterenergy and tracks, the reconstructed charge (based on the associated tracks), the invariantmass of the τhad candidate (with and without pi0 subclusters), the impact parametersignificance of the leading track, ratios of energy deposits to the sum of track transversemomenta, and the transverse flight path significance of the τhad candidate vertex (for τhadcandidates with more than one track).Muons are vetoed by requiring that the calorimetric energy deposited by the τhadcandidate has ET > 5 GeV. For electrons, cuts are placed upon τhad candidates onthe following two quantities: the ratio of the transverse energy deposited in the EMcalorimeter 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 theTransition Radiation Tracker for the track, which also tends to be higher for electrons.This veto suppresses electrons by a factor of 60, while retaining 95% of τhad leptons.Many identification methods that use the reconstructed variables to suppress fakeτhad lepton candidates from hadronic jets are studied at ATLAS, including cut-basedidentification, a projective likelihood, neural networks, and boosted decision trees. Therejection against jets as a function of identification efficiency for τhad leptons is shownemR0 0.05 0.1 0.15 0.2 0.25 0.3ATLASNormalized to unit area00.020.040.060.080.10.120.140.16 signalτjet backgroundEfficiency0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Rejection210310410510Rejection = 60 - 100 GeVT1-prong, E > 100 GeVT1-prong, E = 60 - 100 GeVT3-prong, E > 100 GeVT3-prong, EATLASFIGURE 1. Left: The distribution of the reconstructed radius in the EM calorimeter (REM) for τhadleptons and hadronic jets. Right: Rejection against jets as a function of τ lepton efficiency for the projectivelikelihood identification method for 1-prong and 3-prong candidates with pτT = 60−100 and > 100 GeV.in Fig. 1 (right) for the projective likelihood for 1-prong and 3-prong candidates.Standard Model processes with τ final statesThe process W → τν has a cross-section of 1.7×104 pb at ECM = 14 TeV and it willbe the most abundant source of τ leptons at ATLAS. Events from this process will beselected by a τhad lepton + EmissT trigger, which has an efficiency of 70% with respect tooffline selection, and can only be run during luminosities less than 1032cm−2s−1. Eventsare selected by requiring a τhad lepton with 20 < pT < 60 GeV, an additional jet withpT > 15 GeV, and EmissT > 60 GeV not pointing in the direction of the jet or τhad leptoncandidate. Events that have an isolated electron or muon are also vetoed.The dominant background to this process is QCD dijet production, which has aproduction cross-section that is approximately six orders of magnitude greater than thatfor W → τν . Other backgrounds that contribute include W → eν , W → µν , t ¯t, Z → ττ ,and Z → ee. The signal to background ratio is very sensitive to the cut on EmissT . Acut of EmissT > 50 GeV leads to a signal to background ratio of 1:1, while increasingthis threshold to 60 GeV yields 3:1 for this ratio with 1550 signal events expected in100 pb−1 of integrated luminosity. Therefore the scale and systematic effects of EmissTwill have to be well under control. Fig. 2 shows the track multiplicity spectrum for τcandidates in signal and background processes for 100 pb−1 of integrated luminosity.The Z → ττ process will be the main channel of interest in early data, despite havinga cross-section approximately an order of magnitude less than W → τν . The presenceof an additional τ lepton can be used to further suppress backgrounds. Studies involvingthis process usually consider the case when one τ lepton decays hadronically and theother leptonically (τlep). This also allows the events to be selected using an electron ormuon trigger, leading to an unbiased sample of τhad candidates.Backgrounds to Z → ττ include dijet production, W → lν (l = e,µ,τ), Z → ee, andt ¯t events. Events are selected by requiring an isolated electron or muon with pT > 15 Track multiplicity0 1 2 3 4 5 6 7 8 9−1 Events 100 pb020040060080010001200ντ→W QCD (J0+J1+J2+J3)ν e →W νµ→W ττ→Z  e e→Z ttbarATLASFIGURE 2. The track multiplicity spectrum for τhad candidates in 100 pb−1 for W → τν and back-ground processes.GeV, an identified τhad lepton with pT > 15 GeV, EmissT > 20 GeV, transverse mass ofthe lepton and EmissT satisfying MT(lep,EmissT ) < 30 GeV, sum of calorimeter energydeposits ΣEcaloT < 400 GeV, and a veto on b-jets. The lepton and τhad candidate mustalso be of opposite sign, with requirements placed on ∆φ(lep,τhad) to ensure that theyare not back-to-back.With 100 pb−1, 520 events are expected with a visible mass 37 < Mvis(lep,τhad) <75 GeV with a signal to background ratio of 5:1. The distribution of Mvis(lep,τhad) isshown in Fig. 3 (left). Backgrounds from dijet and W production for this channel can beevaluated using data driven techniques, using control samples of same sign events, andthe high MT(lep,EmissT ) region. (GeV)ττVisible m0 20 40 60 80 100 120 140 160 180Expected events in 100pb-1050100150200250ATLASMean= 53.8 GeV= 10.6 GeVσSignalBackgroundTau Energy Scale0.9 0.95 1 1.05 1.1 (GeV) ττVisible m5051525354555657ATLASFIGURE 3. Left: The visible mass Mvis(lep,τhad) distribution for Z → ττ events and backgrounds in100 pb−1 of data. Right: The reconstructed visible mass Mvis(lep,τhad) as a function of the τhad energyscale (uncertainties shown are statistical only).Since this channel provides an unbiased sample of τhad candidates, tag and probemethods can be used to measure the identification efficiency for both the offline recon-struction and trigger. In addition, the visible mass distribution can be used to evaluatethe τhad lepton energy scale, shown in Fig. 3 (right).Searches for new phenomena with τ final statesMany models that describe phenomena beyond the Standard Model predict signaturesthat are observable at the LHC with τ leptons in the final state. For instance, some SUSYmodels predict an excess of events with τ leptons, particularly if the parameter tanβ islarge [6], when the τ˜ is predicted to be the next-to-lightest SUSY particle.Strategies for SUSY searches with τ lepton final states have been evaluated assuming1 fb−1 of data, requiring a jet with pT > 100 GeV, three other jets with pT > 50 GeV, aτhad candidate with pT > 40 GeV, EmissT > 100 GeV, and a veto on isolated electrons ormuons. The EmissT is separated from the jets by requiring ∆R(EmissT , jet) > 0.2 and alsomust satisfy EmissT > 0.2×Meff where the effective mass Meff is the scalar sum of the pTof the jets, the EmissT and pτhadT . Additionally, the transverse sphericity greater than 0.2 isrequired, and a requirement of MT(EmissT ,τhad) > 100 GeV is also imposed.The main backgrounds for this channel include t ¯t, W + jets, and Z + jets processes.The discovery reach for this event selection in the mSUGRA m0 −m1/2 plane withtanβ = 50 is shown in Fig. 4 for 1 fb−1 of integrated luminosity. The sensitivity ofthis search is comparable to similar search strategies without τhad lepton selection. [GeV]0m500 1000 1500 2000 2500 3000 [GeV]1/2m20040060080010004 jets 0 lepton4 jets 1 leptonτ4 jets 1 ATLAS = 50 βMSUGRA tan  discoveryσ5 NO EWSB LSP1τ∼ (0.5 TeV)q~ (1.0 TeV)q~ (1.5 TeV)q~ (2.0 TeV)q~ (0.5 TeV)g~ (1.0 TeV)g~ (1.5 TeV)g~ (2.0 TeV)g~FIGURE 4. The 5σ discovery contours in the mSUGRA m0−m1/2 plane with tanβ = 50 for 1 fb−1.The 4 jets + 1 τhad has similar sensitivity to the 4 jets search and the 4 jet + 1 lepton search.Other models posit the existence of heavier gauge bosons [7] such as the Z′ boson,with some predicting enhanced couplings to τ leptons. The most promising prospectsoccur in the Z′ → τhadτlep channel. In this case, an isolated electron (muon) with pT >27(22) GeV, an identified τhad lepton with pT > 60 GeV and opposite charge, EmissT >30 GeV and MT(lep,EmissT ) < 35 GeV are required. The vector sum of EmissT , pτhadT andplepT must be less than 70 GeV, and a visible mass cut of Mvis(lep,τhad) > 300 GeV isapplied. To extract the invariant mass of the ττ system, the collinear approximation isused, which infers the direction of neutrinos from τ decay by projecting the componentsof the EmissT on the axes defined by the visible τ lepton decay products. A cut oncos∆φ(lep,τhad) > −0.99 ensures that the lepton and τhad candidates are not back-to-back so that the collinear approximation can be applied.The distributions for the visible mass Mvis(lep,τhad) and invariant mass Minv(lep,τhad)attained through the collinear approximation are shown in Fig. 5, where Standard Modelcouplings for the Z boson are extended to the Z′ boson, and the mass mZ′ = 600 GeV isassumed. A clear excess of Z′ events can be seen over Standard Model backgrounds.Systematic uncertainties are dominated by uncertainties in the integrated luminosityacquired and the τhad lepton energy scale.Visible Mass GeV0 200 400 600 800 1000 1200 1400-1Events/fb020406080100120-1Events/fbττ→Z’ttbarQCDW+jetsDrell-YanATLASCollinear Mass GeV0 200 400 600 800 1000 1200 1400-1Events/fb012345678-1Events/fbττ→Z’ttbarQCDW+jetsDrell-YanATLASFIGURE 5. Left: The visible mass Mvis(lep,τhad) distribution for Z′ events and associated backgrounds(mZ′ = 600 GeV). Right: The invariant mass Minv(lep,τhad) distribution attained from the collinearapproximation for Z′ events and associated backgrounds (mZ′ = 600 GeV).ConclusionsThe ATLAS experiment has developed a reconstruction algorithm for hadronicallydecaying τ leptons that exploits their well known properties to suppress fake τhadcandidates coming from hadronic jets, electrons or muons. Standard Model processeslike W and Z boson production will be an abundant source of τ leptons, enablingearly data measurements of τhad lepton identification, τhad energy scale, and cross-section measurements of W and Z boson production in τ lepton channels. Once τhadlepton reconstruction is well understood in data, promising searches for new phenomenabeyond the Standard Model with τ lepton final states can be performed.REFERENCES1. C. Amsler et al. (Particle Data Group), Physics Letters B667, 1 (2008).2. The ATLAS Collaboration, G. Aad et al., “Expected Performance of the ATLAS Experiment, Detector,Trigger, and Physics”, CERN-OPEN-2008-020, Geneva, (2008).3. The ATLAS Collaboration, G. Aad et al., “The ATLAS Experiment at the CERN Large HadronCollider”, JINST 3 (2008) S08003.4. S.D. Ellis et al., “Jets in hadron-hadron collisions”, Prog.Part.Nucl.Phys. 60:484-551, (2008).5. C. Schwanenberger (for the H1 Collaboration), “The Jet Calibration in the H1 Liquid Argon Calorime-ter”, arXiv:physics/0209026v1 [physics.ins-det] (2002).6. S.P. Martin, “A Supersymmetry Primer”, arXiv:hep-ph/9709356v5, (1997).7. H. Georgi, S. Weinberg, “Neutral currents in expanded gauge theories”, Phys. Rev. D, 17, 275, (1978).

Tau physics at the LHC with ATLAS

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