Measurement of the Z ττ cross-section in the semileptonic channel in pp collisions at s = 7 TeV with the ATLAS detector

Transcription

1 Measurement of the Z ττ cross-section in the semileptonic channel in pp collisions at s = 7 TeV with the ATLAS detector Sofia Consonni Università di Milano & INFN XCVII Congresso Nazionale della Società Italiana di Fisica, L Aquila, Settembre 2011

2 The Z ττ process at hadron colliders The Z ττ process Drell-Yan mechanism NNLO cross section pp Z x BR(Z ττ) = 990 pb at s = 7 TeV Benchmark process for physics with taus Z τ + τ - The τ lepton Third generation lepton m = ± 0.16 MeV τ = ± 1.0 fs Main decay modes: τ + e + ν e ν τ 17.8% τ + μ + ν μ ν τ 17.4% τ + h + ν τ + neutral hadrons 49.5% τ + h + h - h + ν e ν τ + neutrals 14.6% q q Why taus? In MSSM A/H Higgs states decay preferentially to third generation fermions in large regions of the parameter space Charged H ± τν may provide a unique evidence of an extended Higgs sector Standard Model H ττ decay is an interesting channel in association with VBF production SUSY decay cascades may contain taus 2

3 The ATLAS experiment 20 data: recorded luminosity 45 pb -1 Tracking Physics objects in the detector Muon system Electromagnetic and hadronic calorimeters 2011 data: 2.97 fb -1 Hadronically decaying tau: collimated calorimeter energy deposition, 1 or 3 associated tracks. Identification by shower width and composition variables. 3

4 Z ττ lτ jet at ATLAS Lepton-hadron channel: clean, large BR (45%), possibility to use single lepton triggers Cross section measurement: - global check of the comprehension of analyses involving tau decay channels - complete electron and muon channels - possibly tests of universality Trigger unbiased control sample for tau performance studies Important control sample for many Higgs (A/H ττ), SUSY, exotic physics searches needs to be well measured and understood Electron-muon and muon-muon channels were studied as well and the cross section measurements were combined Events / 5 GeV Transverse missing energy Visible mass m vis = ( p l + p ) 2 τ jet 60 ATLAS s = 7 TeV Data */Z -1 Multijet 50 Ldt = 36 pb W l W ll 40 tt*/z m vis (µ, ) [GeV] h 4

5 Cross section measurement Kinematic acceptance Fiducial region defined by: Full lepton and τ-jet kinematics Angular cuts, transverse mass and visible mass cuts Total cross section σ tot = N obs N bkg A Z C Z L Efficiency Observed signal events Luminosity: from 20 data 36 pb-1 after DQ requirements Basic selection criteria Data quality Single lepton triggers (pt threshold 15 GeV for electrons -13 GeV for muons) Criteria against cosmics and noise bursts in the calorimeters Objects selection: Electron channel: electron satisfying tight identification criteria pt > 16 GeV, η < 2.5, isolated Muon channel: muon reconstructed both in the inner dector and muon system pt > 15 GeV, η < 2.4, isolated Hadronically decaying tau, that satisfies tight identification criteria, pt > 20 GeV, η < 1.5 5

6 Background suppression Further requirements to suppress the background Multijet suppression: Tight lepton isolation Sum of transverse momentum of tracks in ΔR = 0.4 cone divided by lepton pt Sum of energy in calorimeter in ΔR = 0.4/0.3 cone divided by lepton pt hadronically decaying tau required to have unitary charge and 1 or 3 associated tracks W lν suppression: angular correlations cos(δϕ(lep, Etmiss)) + cos(δϕ(τ-jet, Etmiss)) > , Transverse mass (lep, Etmiss) < 50 GeV Z ll suppression: lepton vetoes Opposite charge (OS) of the lepton and τ-jet and invariant mass of the visible decay products in [35, 75] GeV Electrons / 0.01 Events / ATLAS s = 7 TeV -1 Ldt = 36 pb Data */Z Multijet W l W ll tt*/z I ET / E T (e) 140 ATLAS s = 7 TeV Data */Z Multijet Ldt = 36 pb W l 0 W ll tt*/z requirements are tight! AzCz 2.3% (muon channel), 1.2% (electron channel) cos 6

7 Background evaluation Z ll and ttbar from Monte Carlo normalized to NNLO W lν and W τν from Monte Carlo, with scale factors from data Multijet from data driven estimate: 2D sidebands method OS-SS method: control regions are defined reverting the requirement of opposite charge of the lepton and hadronically decaying tau and lepton isolation Muons / 5 GeV Muons / 5 GeV W control region ATLAS s = 7 TeV -1 Ldt = 36 pb Data */Z W l W */Z ll tt p (µ) [GeV] T 120 ATLAS s = 7 TeV Data Ldt = 36 pb */Z W l W */Z ll tt p (µ) [GeV] T Considering all backgrounds S/B = 3.8 muon channel S/B = 2.6 electron channel Before TauID After TauID 7

8 Systematic uncertainties Signal yield systematics: uncertainties due to background subtraction, Monte Carlo subtraction main systematics: - Tau energy scale - Tau fake rates Data driven background estimate - Correlation between variables in the OS-SS method Signal efficiency main systematics: - Tau energy scale - Tau identification efficiency - Lepton identification and isolation efficiency (especially for electrons) Acceptance main systematic: PDF sets OS/SS Electron isolation efficiency ATLAS -1 Ldt = 36 pb s = 7 TeV Data EW Subtracted Data EW Contamination Nominal Ratio Statistical Error OS-SS vs isolation muon channel µ h final state 0.4 I ET / p (µ) T ATLAS Work in progress MC data SF [GeV] p T 8

9 Cross section measurement Muon Hadron channel N oss, N oss -N bkg ± 16 (stat) ± 4 (syst) σtot [nb] 0.86 ± 0.08 (stat) ± 0.12 (syst) ± 0.03 (lumi) Electron Hadron channel N oss, N oss -N bkg ± 14 (stat) ± 3 (syst) Z combined 36pb -1 Z ee/µµ 33-36pb µ h -1 ATLAS σtot [nb] 1.14 ± 0.14 (stat) ± 0.20 (syst) ± 0.04 (lumi) Electron Muon channel c N oss, N oss -N bkg ± (stat) ± 1 (syst) σtot [nb] 1.06 ± 0.14 (stat) ± 0.08 (syst) ± 0.04 (lumi) Muon Muon channel N oss, N oss -N bkg ± (stat) ± 3 (syst) σtot [nb] σtot [nb] 0.96 ± 0.22 (stat) ± 0.12 (syst) ± 0.03 (lumi) Combination 0.97 ± 0.07 (stat) ± 0.06 (syst) ± 0.03 (lumi) e h e µ µ µ Stat Syst Stat Syst Stat Lumi Theory (NNLO) (Z ll, 66<m <116 GeV) [nb] Total cross section in [66, 116] GeV invariant mass window Good agreement among Z ττ channels Complete ATLAS measurment of all Z ll channels Consistency with NNLO theory Similar result by CMS [60, 120] GeV σ = 1.00 ± 0.05 (stat) ± 0.08 (syst) ± 0.04 (lumi) nb (CERN-PH-EP/ ) inv 9

10 Conclusion The cross section of the process pp Z ττ was measured in the semileptonic channels as 0.86 ± 0.15 nb in the muon channel and 1.14 ± 0.24 in the electron channel, in good agreement with the NNLO prediction The semileptonic channels were combined with the leptonic channels to give a cross section of 0.97 ± 0.07 (stat) ± 0.06 (syst) ± 0.03 (lumi) nb Results are reported in, submitted to Phys. Rev. D This is a pioneering analysis for di-tau studies in ATLAS. - Tau performance was better understood - Methods for data-driven estimates of the background were designed and tested These methods will be fundamental in other physics channels including taus

11 Backup

12 Cross section methodology Total cross section: σ tot = N obs N bkg A Z C Z L C Z = N reco pass N gen kin dressed Efficiency correction factor: the denominator of Cz defines the fiducial phase space as close as possible to what we are looking at experimentally on truth level. The numerator is determined from full simulated MC. This factor encloses all the trigger, reconstruction and identification efficiencies for the various objects. Fiducial cross section: defined only for the fiducial phase space. It does not include tentatively any extrapolation to other regions, and therefore it should be virtually model independent. σ fid = N obs N bkg C Z L A Z = N gen kin dressed N gen m inv born Acceptance factor: it allows to correct the fiducial cross section back to other channels and theory. Correction back to Born (pre FSR) level (as WZ inclusive analyses) 12

13 Details of selection criteria Data quality: GRL WZ + cp_tau Trigger: - Electron channel: EF_e15_medium, - Muon channel (35.5 pb-1): EF_mu_MG (E4-G1), EF_mu13_MG (G2-I1), EF_mu13_MG_tight (I1-I2) Cleaning: medium with tau modification Objects selection: Electron channel: tight electron pt > 16 GeV not in the crack, isolated Muon channel: combined muon pt > 15 GeV, isolated ID τ-jet (medium 1-prong, tight multi-prong), pt 20 > GeV, not in the crack W suppression: angular correlations cos(δϕ(lep, Etmiss)) + cos(δϕ(τ-jet, Etmiss)) > , mt < 50 GeV QCD rejection: τ-jet unitary charge and 1 or 3 associated tracks Z ll suppression: lepton vetoes (τ-jet object lepton vetoes and event level veto dilepton veto with loosely selected objects) Opposite charge of the lepton and τ-jet and invariant mass of the visible decay products in [35, 75] GeV 13

14 Tau identification Hadronically decaying tau candidates seeded anti-kt R = 0.4 calorimeter jets and tracks matched to candidates Cut based and multivariate (BDT, LLH) identification criteria Variables used: - Shower width in the electromagnetic calorimeter - Track width - Leading track momentum - more (for MV discriminants) Rejection of electrons passing tau selection (electron veto) Details in ATLAS CONF Inverse Background Efficiency Inverse Background Efficiency ATLAS Preliminary 1-Prong p >20GeV T Cuts BDT Likelihood ATLAS Preliminary 3-Prong p >20GeV T Cuts BDT 1-prong Likelihood Signal Efficiency multi-prong ATLAS CONF ATLAS CONF Signal Efficiency 14

15 Details of systematics 15

16 Combination (BLUE) 16