Reconstruction and identification of hadronic τ decays with ATLAS

Reconstruction and Identification of Hadronic τ Decays with ATLAS Wolfgang F. Mader 1 1 Institut für Kern- und Teilchenphysik TU Dresden DPG Frühjahrstagung München, March 2009

Outline 1 Introduction 2 Reconstruction and Identification of τ Leptons 3 Physics w/ 100 pb 1 4 Summary All Results Published: ATLAS Collaboration, Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics, CERN-OPEN-2008-020, Geneva, 2008. (arxiv:0901.0512)

Part I Introduction

Introduction Taus as Tools in many Areas Understanding of the Detector Measurement of τ Energy Scale Measurement of E miss T Scale Standard Model (SM) Measurement of W /Z Production Cross Section Discovery of the Higgs Boson in ττ Final States Minimal Supersymmetric Standard Model (MSSM) h/h/a ττ Excellent Discovery Potential Searches for Charged Higgs Bosons: H ± τν Exotic Scenarios e. g. Search for Heavy Gauge Bosons In some Models, Large Couplings to ττ Final State

Basic Tau Properties Tau Branching Ratios Leptonic Decay Modes e/µν eµν τ 35% 1Prong Hadronic Decay Modes π ν τ 11% π π 0 ν τ 25% π π 0 π 0 ν τ 9% π π 0 π 0 π 0 ν τ 1% K + Neutrals 1.5% 3Prong Hadronic Decay Modes π π + π 9% π π + π π 0 4.5% K π + π 0.4% Other Modes ( 3%) Tau Reconstruction Means Reconstruction of Hadronic τ Decays Difficult because of Huge QCD Background

Basic Tau Properties Tau Characteristics m τ 1.7 GeV cτ = 87 µm Hadronic Decays are Well Collimated Collections of Charged and Neutral Pions/Kaons Most have 1 or 3 Charged Tracks Leading Pion Direction Reproduces τ Direction Well. Tau Lepton Decays very Well Understood Will be Used as Excellent Probe for New Physics

The Large Hadron Collider (LHC) @ CERN

The Large Hadron Collider (LHC) @ CERN Proton-Proton Collider Nominal Center-of-Mass Energy s = 14 TeV In 2009/10: s = 10 TeV Expected Integrated Luminosity in 2009/10: 100 200 pb 1

The ATLAS Detector

Part II Reconstruction and Identification of τ Leptons

Reconstruction and Identification of τ Leptons Tracking Low Track Multiplicity (1 or 3) Collimated Tracks Secondary Vertex Reconstruction (3-prong) Isolation from other Tracks (Cone) Calorimetry Collimated Energy Deposit in Calorimeter Strong EM Component for 1-prong Possibility to Identify π 0 Clusters Use EM and HAD Components Reconstruction and Identification Separate

Reconstruction and Identification of τ Leptons Track-based Algorithm (Tau1P3P) Reconstruction Find Good Quality Track (p T > 9 GeV) Up to Six Additional Tracks (p T > 2 GeV) inside R < 0.2. Energy Defined Using Energy Flow Algorithm Characteristics Excellent Performance in Low p T Region Optimized for Balanced Efficiency for 1Prong and 3Prong Decays Calorimeter-based Algorithm (TauRec) Reconstruction Cluster with E T > 15 GeV OR Isolated Tracks w/ p T > 2 GeV Collect all Tracks w/ p T > 2 GeV in Cone of R < 0.3. Calibration Using H1-style Procedure Characteristics Almost 100% Efficient Reconstruction Optimized for 1Prong Decays Excellent Performance in high-p T Regions

Reconstruction and Identification of τ Leptons Identification of tau Candidates Variety of Identification Algorithms Available Cut-based Selection Projective Likelihood Neural Network BDT, PRD-RS,... Based on Tracking and Calorimetry Variables Examples:

Reconstruction and Identification of τ Leptons Performance of the Algorithms Tau1P3P NN-based ID TauRec Likelihood-based ID Rejection @ 30% Efficiency

Reconstruction and Identification of τ Leptons Charge Purity (Migration between Categories) Migration Dominated by Effects from Combinatorics... NOT by Charge Misidentification of Individual Tracks Transverse Flight Path Significant Improvement in Rejection of Candidates from Light Jets

Reconstruction and Identification of τ Leptons π 0 Subclusters Reconstructed Using Topological Clustering Clusters within R < 0.2 around Leading Track (p T > 9 GeV) considered. Subtraction Procedure for Energy Deposits from Charged Tracks Apply Quality Criteria (Separation from Tracks, E T > 1 GeV,...)

Reconstruction and Identification of τ Leptons Electron Veto Rejection of Isolated Electrons from W eν and Z ee etc. Processes Vetoing Standard e-identification only 85% Efficient Dedicated Electron Veto Energy Deposit in Hadronic Calorimeter Energy not Associated w/ Track in η-strip Layer E T(EM)/p T Ratio of High- to Low-Threshold Hits in TRT Performance Rejection Factor of 60 against W eν 5% Loss in Efficiency for W τν

Part III Physics w/ 100 pb 1

Physics w/ 100 pb 1 W τν Large Cross Section (σnlo ' 20 000 pb) Low pt of W Boson Low Missing Transverse Energy miss (70% Efficiency) Trigger τ + ET

Physics w/ 100 pb 1 Z ττ Cross Section (σnlo ' 2 000 pb) Cross Check with eτhad and µτhad possible Lepton Triggers used (Possibility to check τ Trigger Efficiency) Determine τ Energy Scale from `τhad visible Mass miss Scale from `τ Determine ET had invariant Mass

Physics w/ 100 pb 1 t t(w τν) High t t Cross Section σ(t t) 800 pb 1 Expect 16 500 Events w/ W τν Establish τ Identification at High p T

Summary Events with τ Leptons will be observed with First Data Excellent Possibility to Understand Detector Performance Reconstruction/Identification Algorithms Calorimeter-based (taurec) for High-p T τ s Track-based (Tau1P3P) for Low-p T τ s Variety of Identification Methods Hadronically Decaying τ Leptons Key to New Physics Discoveries SM Higgs Boson in VBF Production H ττ MSSM Higgs Production bbh/h/a ττ Charged Higgs H ± τ ± ν SUSY Signatures with τ Final State Exotic Models...