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Study of Z Decays to τ Pairs with CMS Detector at √s = 14 TeV

This research focuses on the physics of Z decaying to ττ pairs, studying the Standard Model, Z production at the LHC, τ phenomenology, and identification principles, exploring new physics opportunities with τ leptons. The study includes background processes to Z→ττ, CMS experiment details, generator studies, and analysis of Z decays with relevance to new physics searches. Additionally, it covers τ decay modes, the importance of τ leptons in Higgs searches, and the performance of the CMS detector and trigger system. Detailed information on the CMS experiment's sub-detectors and the trigger performance is provided, with emphasis on the study of Z decays. Also included are the layouts of the Calorimeters, Muon System, and the CMS Trigger Overview. The description ends with insights into the design and specifications of the CMS experiment and its current operational status.

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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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