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Study of Z decays to τ pairs with CMS detector at √ s = 14 TeV

Study of Z decays to τ pairs with CMS detector at √ s = 14 TeV. Michail Bachtis CMS Group University of Wisconsin - Madison. Outline. Physics of Z  ττ The Standard Model Z production at the LHC τ phenomenology and identification principles New Physics with τ

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Study of Z decays to τ pairs with CMS detector at √ s = 14 TeV

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  1. Study of Z decays to τ pairs with CMS detector at √s = 14 TeV Michail Bachtis CMS Group University of Wisconsin - Madison

  2. Outline • Physics of Z ττ • The Standard Model • Z production at the LHC • τ phenomenology and identification principles • New Physics with τ • Background processes to Z ττ • The CMS experiment • Design and sub-detectors • The CMS Trigger system • Z ττ study in CMS • Generator studies • Detector and Trigger Performance studies • Zττ Analysis • Summary/Next plans

  3. 12 Elementary Particles (fermions) Three generations of quarks. Three charged leptons and corresponding neutrinos 4 Force Carriers (bosons) Gluon (Strong) Photon (EM) W,Z (Weak) Not a complete theory Higgs boson to be discovered The Standard Model

  4. Importance of Z boson studies • Test of the Standard model in the new energy domain. • Detector performance studies • Optimization of τ trigger and offline reconstruction • Background for new physics • Higgs, Z’ Z production via Drell-Yan in proton collisions

  5. τ Decays Leptonic Decay Hadronic Decay • τlepton • mass = 1.8 GeV • meanlifetime ~10-13s • Decays to lighter particles • Leptonic decays (~35%) • Electron/Muon + 2 Neutrinos • Lepton+ Missing Et signature • Hadronic decays (~65%) • Mostly one or three charged particles (prongs) +neutrals+ neutrino • “Narrow” jet signature in the detector Most Relevant τ decays and BRs

  6. Z  ττ τe+τμ BR = 6.2% • Three decay modes • Both τ decay leptonically • electron + muon • electron +electron • muon+muon • One τ decays leptonically and one hadronically • One τ can give one/three prongs • Most favored mode • Both τ leptons decay hadronically • Large Jet Background τh+τl BR = 45.3% τh+τh BR = 41.4%

  7. Background processes • QCD Jets • Extremely large cross-section (order of mb) • Narrow jets fake hadronic τ • Drell-Yan • Background for e,μ from τ • Leptons fake one prong τ too! • W+Jets • Wlν • Jets fake hadronic τ • Top quark pairs • Contain τ,W,Jets

  8. New Physics with τ • Standard Model Higgs searches (Hττ) • Higgs couples to mass • ττBR=~10% for low Higgs mass • Example:Vector boson fusion • Two τ + forward Jet Signature • Higgs mass limits • >114 GeV (LEP) • <160 GeV from indirect searches • MSSM Higgs • Both charged and neutral Higgs possible • Large Branching ratio to τ • Heavy neutral Higgs to τ pair • Charged Hτν

  9. The CMS Experiment Calorimeters HCAL ECAL Plastic scintillator/brass sandwich 76k scintillating PbWO4 crystals Iron Yoke Muon Endcap Cathode StripChambers (CSC) Resistive PlateChambers (RPC) Inner Detector PixelSilicon Tracker 210 m2 of silicon sensors 9.6M channels Weight: 12,500 T Diameter: 15.0 m Length: 21.5 m Solenoid Magnet 4 T Magnetic Field Muon Barrel Resistive Plate Drift Tube Chambers Chambers

  10. The CMS Experiment Today Lowering the last heavy element Tracker in position Solenoid

  11. Silicon Tracker • Silicon Technology • Pixel Detector near the interaction point • Strips in surrounding area (Barrel, Endcap) • Performance • High tracker granularity, large size + strong B-field make the tracker efficient for a broad Pt spectrum. • Resolution : Tracker installation

  12. Electromagnetic Calorimeter • Crystal Technology • Lead Tungstate Crystals (~76000) • High density (8.2 g/cm3) • Short radiation length (8.9 mm) • Small Moliere radius (22 mm) • High segmentation for precise position measurement • Acceptance to |η|<3.0 • Resolution:

  13. Hadronic Calorimeter • Barrel and Endcap part (|η|<3) • Brass / Scintillation layers Resolution: • Forward Region (3<|η|<5) • Steel plates / Quartz fibers Resolution: • Absorber geometry • 7 Interaction lengths at η = 0 • 11 Interaction lengths at η = 1.3

  14. Muon System • Operation Principles • Muons are identified in Muon System • For low Pt muons, Pt is assigned by the tracker • For high Pt muons, Muon system contributes to the measurement • All muon sub-detectors contribute to the trigger • Layout • Barrel • Drift Tube chambers (DT) |η|<1.3 • Resistive Plates (RPC) |η|<1.3 • Endcap • Cathode Strip Chambers (CSC) 0.9<|η|<2.4 • Resistive Plate Chambers (RPC) |η|<2.1 Endcap Disc made in UW

  15. CMS Trigger Overview • 2-Level Trigger Design • Level 1 Trigger • Hardware • 100 kHz output (50kHz at first runs) • Latency = 3 μs • High Level Trigger • Software running on Processor Farm • Algorithms similar to offline reconstruction • ~100Hz output Crossing rate =40 MHz Trigger Rejection ~ 4x105

  16. L1 Trigger Design • Calorimeter Trigger • Regional Calorimeter Trigger (RCT) • Finds e/γ,regional energy deposits • Forwards RCT objects to GCT • Global Calorimeter Trigger (GCT) • Finds jets,τ • Sorts RCT Objects, • Calculates Missing Et • Forwards Calorimeter quiet regions to Muon Trigger • Muon Trigger • Regional Triggers • Find Segments on chambers • Tracks are created in DT,CSC • Global Muon trigger • Sorts muons • Checks Muon Isolation • Global Trigger • Applies selection criteria • Communicates L1 decision

  17. Analysis Outline • Zττ predicted cross section is 530 pb • Main background for τ hadronic decays: QCD Jets • QCD cross section = ~108 pb!! • Leptonic τ decays faked by Electroweak Processes • Z,W, Drell-Yan • Optimization of Trigger and τ-ID • Important for suppressing backgrounds • Zττanalysis procedure • Trigger and detector performance studies • Zττ analysis • Monte Carlo Samples (Pythia) • Zττ(500K events) • QCD • 1B events • σ=1.8x108 pb • Electroweak (EWK) 50M events • W+Jets, Z (excluding τ)+Jets, Drell-Yan • σ=2.1x105 pb Expected Events @ ∫ L=100pb-1 Ratio of Produced Signal to Background Events (Before Trigger) 1:350000 Events! Zττ : 53000 QCD : 18B EWK : 21M

  18. Generator Level Cuts • Require visible τPt>10 GeV (73% accepted) • Visible Pt much smaller • 73% of the generated τaccepted • Require visible τ |η|<2.5 • τmust be in tracker acceptance • 65% of the generated τaccepted • 48% of generated events accepted MC Fiducial Cut (Accept) Tracker Acceptance

  19. Generated Z invariant Mass • Broad mass distribution • Mass peak shifted 15-25 GeV in leptonic τ decays • Neutrinos from e,μ • Mass window expected in 20-100 GeV • Zττdecays to μμ,ee • Drell Yan μμ,ee is irreducible background • S:B ~ 50:3000 events! Pythia, Zττ hh hμ/he μe μμ/ee Pythia, Drell-Yan

  20. Calorimeter Geometry Crack [Tracker Cabling/ Services] η η

  21. L1 e/γ Trigger Algorithm • e/γ Triggers • Large energy deposit in 2 adjacent towers. • Shower profile • Fine Grain spread in central cell of 3x3 • Longitudinal Profile • Ratio of HCAL-ECAL energies • Isolation on nearest neighbors for isolated object triggers • Efficiency per electron candidate = 98% for Pt>10 GeV Pythia, Zττ

  22. L1 τ Trigger Algorithm • L1 τ algorithm • Uses towers in 12x12 region • Specific isolated energy patterns allowed in 4x4 region • Non isolated patterns set a veto • τ accepted if all vetos are off. • Additional Isolation • Requires Et<2 GeV in 7 of 8 neighboring 4x4 regions • Efficiency per τ candidate = 78% for Et>10 GeV Pythia, Zττ τ Candidate

  23. Muon Geometry • Full coverage to |η|<2.4 • Overlaps with Tracker Coverage. • Three main coverage regions • |η|<0.8: Barrel only • 0.8<|η|<1.3: Barrel and endcap • 1.3<|η|<2.4: Endcap only.

  24. L1 μ Trigger Algorithms • Local Tracking on Chambers • Segment Reconstruction • Track Finders • Cathode Strip Chamber and Drift Tubes • Segments combined to global tracks • Momentum assigned to the tracks • Efficiency per Muon Candidate = 99% for Pt>10 GeV CSCs , 0.9<|η|<2.4 Pythia, Zττ DTs , |η|<1.3

  25. L1 Global Trigger Paths • Zτh+τh • Double τ Trigger • Requires 2 τ with Et>20 GeV • Zτl+τh • muon + hadronic τ • Requires an isolated muon with Pt>5 GeV and a hadronic τ with Et>10GeV • electron + hadronic τ • Requires an isolated e/γ object with Et>10GeV and a hadronic τ with Et>10GeV • Zτμ+τe • An “electron + muon” L1 trigger is required • Requires an isolated e/γ object with Et>10GeV and a muon with Pt> 5GeV

  26. τ High Level Trigger L1 Seeding L2 Calorimeter Isolation L2.5 Pixel Isolation L3 Tracker Isolation Regional Jet Reconstruction around L1 tagged τ Isolation using Calorimeter only Isolation using Pixel Tracks Isolation using Silicon Tracks Those algorithms won’t be used with first data

  27. Isolation annulus This is a τ This is a Jet φ φ φ η η η ECAL Clusters Signal Cone Isolation Cone L2τ High Level Trigger My work.. • Three Trigger Algorithms • Start with L1 seeded Jet and a cone around jet axis • ECAL Isolation • Sum of Crystal Et in isolation annulus • Tower Isolation using CaloTowers • Sum of Tower Et of isolation annulus • “Fast” ECAL Clustering • Clustering using ECAL crystals • Number of Clusters • Cluster spreading around jet center

  28. Cone Isolation • Hadronic τ : narrower than QCD jets • ECAL Isolation Algorithm • Measures total ECAL Crystal Et in isolation annulus • Require ECAL Et<3 GeV • QCD Jets Removal of 40% • τEfficiency = 98% • Tower Isolation Algorithm • Measures tower Et (ECAL+HCAL) in the isolation annulus • Require Tower Et<5 GeV • QCD Jets Removal of 50% • τEfficiency = 97% • Important cut for candidates without ECAL contribution Zтт QCD Reject Zтт QCD Reject

  29. ECAL Clustering • For further background removal, a Clustering algorithm on ECAL Crystals is applied • Clusters are created by ECAL crystals • Cuts • Number of clusters<7 • QCD Rejection =55% • τEfficiency =96% • η RMS<0.04 • QCD Rejection 60% • τEfficiency =93% Zττ QCD Reject Zττ QCD Reject

  30. HLT Performance • Results after applying all the previous algorithms • τ Efficiency = 90% • QCD Rejection = 75% • Cuts can be tuned to provide tighter (looser) configurations • Maximum QCD Rejection by a factor of 10 with 83% of τpreserved • Maximum τ Efficiency of 99% with 40% of QCD rejection Pythia, Zττ

  31. Inclusive Trigger Performance Trigger Acceptance (Events at ∫ L=100pb-1) Triggers reduce background rates but we can do even better Recall: We started with 1:350000 events

  32. • e/μ Offline Reconstruction • Electrons • Calorimeter Reconstruction • Create “super-clusters” of clusters to include radiated photons • Apply Et thresholds • Tracker Reconstruction • Electron is matched to a track. • Cuts are applied on e/p and HCAL energy deposits • Muons • Standalone Reconstruction • Muon tracks reconstructed from the muon system • Combined Reconstruction • Muon Tracks are matched to tracker tracks and combined muons are created • Isolation can be applied in both cases • High Level trigger algorithms are similar. ET/pT cut ET γ e- Tracker Strips pT Pixels Inner Detector Track Standalone Muon Track

  33. Lepton Offline Reconstruction Efficiency • Muons (from τ decays) • Muon efficiency = ~95% for Pt > 5 GeV • Coverage of the Muon Detector up to η = 2.4 for Pt > 5 GeV • Electrons (from τ decays) • Electron efficiency = ~88% for Pt>15 GeV • Future improvement: Optimizing Electron Offline Performance • Geometrical acceptance • “Crack” reduces efficiency in the transition region (Barrel – Endcap) • Reconstruction harder near the tracker boundary (η=2.5) Muons Electrons Pythia, Zττ Muons Electrons Pythia, Zττ

  34. e,μ Resolution • Muons • Curvature resolution provided by Tracker • Curvature Resolution = 1.9% • Electrons • Bremsstrahlung blurs Resolution for electrons • Peak shifted by ~0.04 • Curvature Resolution = 4.9% Pythia,Zττ Pythia,Zττ

  35. τ Identification with Cone Isolation Leading Track axis • Two algorithms • CaloTau Algorithm • Associates tracks to jets • Identifies τ by track isolation • Particle Flow • The algorithm • Reconstructs particles • Applies Pt corrections in particle level • Forms jets from particles γ Jet Axis Signal Cone ΔR=0.15 Isolation Cone ΔR=0.5 Jet cone Require no charged,γ candidates in isolation annulus

  36. Hadronic τ Performance • Particle Flow improves Resolution • Distribution is better centered. • Peak is sharper • CaloTau Efficiency is Higher • Particle flow can miss a High Pt candidate • Particle Flow τ-ID still under basic development • Tail under investigation to raise efficiency Pythia, Zττ PFTau CaloTau Pythia, Zττ PFTau CaloTau

  37. Calorimeter Missing Et • Estimates Et of particles undetected in Calorimeter • Muons • Neutrinos • Particles outside geometrical acceptance • It is defined as: • where the sum is evaluated over • all the Calorimeter towers • Critical for many physics studies • Top Studies • SUSY Studies • τStudies Pythia, Zττ • Poor Resolution • MET Higher • Underestimation of jet energies • Need to be adjusted by calibration

  38. Zτμ+τh studies • MC Samples (PYTHIA) • Zττ • 500K events (Full Simulation) • QCD Jets (Pt >30 GeV) • Muon preselection in MC Level • Require 1 final state muon • Generated: 1 Billion events (Fast Simulation) • Electroweak • W+Jets • Z+Jets (excluding ττ) • Drell-Yan • 50 Million Events (Fast Simulation)

  39. μ,τ and MET spectra • EWK & QCD :dominant backgrounds • Muon Pt and MET larger for EWK sample • W decays • Requirements • Event has passed μ+τHLT Path • τ tagged by PF-TauID • Only one isolated μ, Pt > 10 GeV • Only one τ, Et > 20 GeV Zττ Accept EWK QCD EWK X 10 Signal X 20 Zττ EWK Accept QCD EWK X 10 Signal X 20 Zττ Process Events(100pb-1)S:B EWK Zττ 827 QCD 1:11 EWK X 10 EWK 9046 Signal X 20 QCD 20 Not enough QCD statistics to populate spectrum!

  40. Opposite direction and sign • Opposite direction • τ expected back to back in r,φ • Neutrinos blur τalignment • Require |Δφ|>2.5 • Require opposite sign between τ,μ • 83% of Electroweak Background rejected Zττ EWK Reject QCD Process Events(100pb-1)S:B Zττ 637 1:2.8 EWK 1809 QCD <20

  41. Rejection of W decays • Apply (μ,MET) transverse mass cut • Expected to be larger for W • MET larger in W decays • W mass larger • Require • Mt < 30 GeV Zтт EWK Reject Process Events (100pb-1)S:B Zττ 495 2:1 EWK 251 QCD <20

  42. μ+τ Invariant Mass • Z peak visible in mass spectrum • 495 events at ∫L=100pb-1 • Mass Window • 32-104 GeV • For M(μ,τ) >110GeV • Background < 2 events @100pb-1 • Expect good results for Hττ with similar analysis Zττ EWK Signal Efficiency = 53% EWK Rejection = 97.7%

  43. Conclusion • Summary • Improved τTrigger • Achieved maximum QCD Suppression by a factor of 10 using Calorimeter cuts • Implemented ZττAnalysis • Achieved a S/B ratio of 2 • Conclusions • Zττ is detectable at ∫L=100pb-1 • If a SUSY Higgs appears in low luminosity (large (tanβ)2 ), it possibly can be observed • This is the first step for a SM Hττstudy

  44. Next plans • Next plans • Work on L1 and High Level trigger • Improve Trigger and Reconstruction performance for leptons and hadronic τ • Optimize Zττ analysis and measure σ(Zττ) • Optimization with Linear Fisher Discriminant slightly improves performance (S/B = 2.8) • Search for the Higgs

  45. Backup Slides

  46. Linear Fisher Discriminant • Take two sets of points x in a N dimensional space • (one for signal, one for background). • Define a linear transformation y=wt·x : RNR • We need the transformation w such that the clusters will be best separated in the 1D space. Best separation |μ1-μ2|2 One idea is to maximize J= where μ,σ σ12+σ22 is the mean and variance in 1D space. (maximum distance and minimum spreading in the final space) wt M w with M=(m1-m2)(m1-m2)t S=Σ(x-m1)(x-m2)t J can be written as : J= in ND space wt S w Setting J = λgives: wtMw = λ wtSw  Mw = λSw S-1Mw = λw So we have an eigenvalue equation for S-1M. The maximum eigenvalue gives maximum separation and the corresponding eigenvector gives the linear transformation.

  47. Further Optimization • Using Linear Fisher Disciminant • Fisher Discriminant projects the variable space in one dimension • Projection with maximum separation • Input Variables [4 Dimensions] • Δφ (μ,τ) • Mt(μ,MET) • Δφ (μ,METu) • MET • Cut value computed by maximizing: • Sig = S / (S+(Ls/Lb)B)1/2 where: • S: # of signal events that satisfy cut • B: # of background events that satisfy cut • Li:integrated Luminosity of sample • Optimized value: 4.1 Zтт EWK Reject

  48. Cut on Fisher Discriminant • Optimization easier in one dimension • Discriminant provides one dimensional variable • Similar results with cut based analysis • Signal efficiency increased to 61% (+10%) • Background Acceptance decreased to 1.9% (-1.3%) Process Events (100pb-1)S:B Zττ 574 2.8:1 EWK 207 QCD <20

  49. Muon Isolation • QCD Jets often contain leptonic quark decays • μ+narrow Jet can fake Zτμτh • Apply Muon Isolation • Sum of the ECAL Et< 3 GeV in Cone of ΔR = 0.3 • Sum of Track Pt< 3 GeV in cone of ΔR = 0.3 Zττ Reject EWK QCD Zττ EWK Reject QCD

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