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Study of ZZ→4 Leptons with CMS at the LHC

Explore the analysis and simulation of ZZ→4ℓ decays at the Large Hadron Collider using the CMS detector, with a focus on Higgs boson discovery and beyond the Standard Model implications. Learn about detector components, trigger systems, and LHC operation.

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Study of ZZ→4 Leptons with CMS at the LHC

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  1. Study of ZZ→4 Leptons with CMS at the LHC • Ian Ross • Preliminary Exam

  2. Outline • ZZ→4ℓ • The LHC and CMS • Analysis • Simulation and reconstruction • Comparison to Tevatron data • Future • Next steps of analysis • Conclusions

  3. Standard Model • Describes fundamental particles and interactions • Fermions compose matter • Bosons mediate forces • Higgs boson • Infuses other particles with mass • H→ZZ→4ℓ: “Golden Channel” for discovery at the LHC

  4. Importance of ZZ→4ℓ signals Higgs Decay Modes • Higgs discovery • H→ZZ*→4ℓ • e/μ modes give clean signature across range of mH • BR becomes prominent for MH > 2 MZ • WW branch has higher BR, but more difficult reconstruction • Beyond the Standard Model • ZZ decays of new particles • Anomalous triple gauge couplings

  5. ZZ Production at the LHC • LHC collisions have √s=7 TeV • MC analysis done at √s=10 TeV • ZZ decays • ZZ→4ℓ decays rare, but provide cleanest signal

  6. Large Hadron Collider (LHC) • pp collider • 27 km circumference • Four primary detectors: • ATLAS, CMS – General purpose detectors • ALICE – Heavy ion experiments • LHCb – Forward detector for b-physics LHC Dipole

  7. LHC Proton-Proton Collisions • Designed for: • 14 TeV center of mass energy • 2808 bunches per beam • 1011 protons per bunch • Luminosity up to 1034 cm-2 s-1 • Collect 100 fb-1 per year • 2010-2011 plan: • 7 TeV center of mass energy • 720 bunches per beam • 7∙1010 protons per bunch • Luminosity up to ~1032 cm-2 s-1 • Collect 1 fb-1 of data

  8. Compact Muon Solenoid (CMS) Solenoid Muon System Tracker Electromagnetic Calorimeter (ECAL) Hadronic Calorimeter (HCAL)

  9. Detecting Particles in CMS

  10. CMS: Silicon Tracker • Identifies tracks, measures charge and transverse momentum (pT) • Pixel detectors nearest interaction point, then layers of strip detectors • 66M pixel readout channels • 9.6M strip readout channels • 1.2 m in radius, 5.6 m in length • 205 m2 of Si coverage • Extends to |η| ≤ 2.5 • Resolution:

  11. Tracker Performance – LHC Collisions @ 7 TeV D+→K-π+π+ and D-→K+π-π- MD=1.869GeV • Momentum requirements: • p > 1.5 GeV • pT > 0.1 GeV • Require secondary vertex with total charge ±1 • Shows excellent tracker performance • Momentum resolution • Secondary vertex finding

  12. CMS: ECAL • Measures energy of e/γ • 76,000 lead tungstate crystals • 61,000 in barrel • 15,000 in endcaps • Density ρ=8.28 g cm-3 • Radiation length χ0 = 0.89cm • 2.2 x2.2 x 23 cm • Coverage to |η| ≤ 3 • Resolution: Stochastic Intrinsic Noise

  13. ECAL Performance -- LHC Collisions @ 7 TeV • π0 and η observations indicate excellent early ECAL performance

  14. CMS: HCAL • Measures energy of hadronic showers and missing ET • 9500 readout channels • Central and barrel -- brass and scintillator calorimeter coverage: |η| ≤ 3 • Resolution: • Forward -- steel and quartz calorimeter coverage: 3 ≤ |η| ≤ 5 • Resolution: Stochastic Intrinsic

  15. HCAL Performance -- LHC Collisions @ 7 TeV • Reasonable agreement between MC and data

  16. Dijet event @ 7 TeV • Nice dijet event showing ECAL and HCAL deposits.

  17. CMS: Muon System • Provides muon detection and momentum measurements for high-pT muons • Drift Tube Chambers (DTs) and Cathode Strip Chambers (CSCs) give precise position measurements • DT coverage -- 0 < |η| < 1.2 • CSC coverage -- 0.9 < |η| < 2.4 • Resistive Plate Chambers (RPCs) provide precise timing • 0 < |η| < 1.6 • Muon chamber hits matched to tracker hits for low-pT momentum resolution

  18. Muon System Performance – LHC Collisions @ 7 TeV Muons built in muon system matched to tracker hits • Good muon system performance so far MJ/Ψ=3.096 GeV

  19. CMS Trigger • 25 ns bunch crossings yield up to 1 GHz event rate at 1034cm-2sec-1 (40 MHz ∙ 25 interactions per crossing ) • Level 1 Trigger • Hardware selection reduces rate to 100 kHz • High Level Trigger (HLT) • Software selection utilizing computing farm • Reconstructs events, reduces rate to 300Hz

  20. Calorimeter Trigger Muon Trigger Muon Trigger RPC CSC DT HF HCAL ECAL Local CSC Trigger Local DT Trigger RegionalCalorimeterTrigger PatternComparator Trigger CSC TrackFinder DT TrackFinder GlobalCalorimeterTrigger 40 MHz pipeline, latency < 3.2 μs Global Muon Trigger e, J, ET, HT, ETmiss 4 μ Global Trigger max. 100 kHz L1 Accept CMS: Trigger (Level 1) • Calorimeter Trigger • Regional Calorimeter Trigger (RCT) -- Find e/γ, hadronic deposits • Global Calorimeter Trigger (GCT) • Finds jets, calculates missing transverse energy • Sorts objects from RCT • Muon Trigger • Finds segments, tracks regionally • Global Muon Trigger • Sorts muons, checks isolation • Global trigger makes acceptance decision, passes to HLT

  21. L1 ECAL Trigger Performance @ 7 TeV Barrel Endcap • For 2 GeV trigger threshold, clusters over 4 GeV trigger 100% of the time in both barrel (left) and endcap (right)

  22. CMS: High Level Trigger • e/γ HLT • Start with ECAL objects, match to tracks and pixel hits from tracker • Reconstruction includes bremsstrahlung losses • Apply ET threshold, isolation, other criteria • μ HLT • Find tracks in muon systems • Find consistent tracks in tracker system • Match muon tracks to tracker candidates

  23. HLT Performance (e/γ) @ 7 TeV • HLT efficiency with respect to L1 candidates • Match HLT objects to L1 objects within: • < 0.5 • 10 GeV threshold on this trigger • Reasonable agreement between data and MC

  24. Z candidate @ 7 TeV

  25. Event Simulation Generation Detector Simulation PYTHIA, MADGRAPH GEANT4 • Simulate physical processes • Simulate interaction with matter • Creates detectors hits like real data Reconstruction CMSSW • Rebuild physics objects • Same software is used on real data

  26. Muon Reconstruction • Combined reconstruction • Muon tracks matched to tracker hits • Provides vastly improved low pt resolution • Require < 0.15 to match with generated muons Muon Reconstruction Efficiency (MC) Efficiency Reconstruction Efficiency Muon pt (GeV) Efficiency = (Generated muons matched to reconstructed muons) (All generated muons)

  27. Electron Reconstruction e Reconstruction Efficiency (MC) • Calorimeter Reconstruction • ECAL superclusters generated to include bremmstrahlung photons • Find tracker hits consistent with ECAL deposits • Require < 0.15 to match with generated electrons Efficiency Reconstruction Efficiency Electron pt (GeV) Efficiency = (Generated electrons matched to reconstructed electrons) (All generated electrons)

  28. Important Backgrounds • ZZ→4ℓ (Signal) • σ = 7.3 pb @ 10 TeV • qq/gg→ttbar (Background) • σ = 242.8 pb @ 10 TeV • ttbar→WbWb→ℓνℓνbb • b’s hadronize, can produce (or fake) leptons • Z+jets (Background) • σ = 3600 pb @10 TeV • Z decays leptonically • Jets can produce (or fake) leptons

  29. MC Analysis @ 10 TeV All datasets normalized to 1fb-1 • pp→ZZ→4ℓ Monte Carlo Dataset • σ = 0.1033 pb • 100,000 events generated with PYTHIA6 • Event weight = 10-3 • Most Important Backgrounds: • Z+jets • σ = 3600 pb • 1,000,000 events generated with MADGRAPH • Event weight = 3.6 • ttbar • σ = 242.8 pb • 500,000 events generated with PYTHIA6 • Event weight = 0.5

  30. Preselection • Require 4 leptons (e/μ with pT > 5 GeV) combined into two Z candidates with: • | Mℓℓ1 - 91.2|< 15 GeV • Z candidate with higher pt lepton within 15 GeV of MZ • | Mℓℓ2 - 91.2| < 25 GeV • Second Z candidate within 25 GeV of MZ to remove ZZ* events • Require leptons to be within tracker acceptance of |η| < 2.5 Need to apply criteria to distinguish signal from background Signal Background

  31. Starting M4ℓ Spectrum • Only preselection applied (previous slide) • Stacked Plot

  32. Z mass peaks after preselection • Signal is indistinguishable • Applying discrimination cuts to electrons will allow us to distinguish signal from background

  33. Electron Isolation • Reconstruction process may misidentify particles as electrons • True electrons will be well isolated, with little energy nearby • Track Isolation • Outer cone radius = 0.3; veto cone radius = 0.015 • TrackIso=∑pT>0.7 GeV tracks in outer cone with veto cone removed • Veto cone • Outer Cone • ECAL Isolation • Center cone on electron deposit • Outer cone radius = 0.4 • Veto cone radius = 0.045 (barrel) 0.07 (endcap) • Strip half-width Δη= 0.02 • ECALIso=∑energy deposits in outer cone with veto cone, strip contributions removed • Electrons • Brehmsstrahlung photons • Soft underlying event photon

  34. Electron Isolation (cont.) Outer cone • HCAL Isolation • Sum in outer cone, centered on ECAL supercluster • Use all available HCAL depths • Outer cone radius R=0.4 • Veto cone radius R=0.15 • HCAL Iso=∑energy deposits in outer cone with veto cone removed Veto cone HCAL deposit ECAL deposit • Electron isolation expected to provide discrimination power against jets. • Combined Iso = ECAL Iso + HCAL Iso + Tracker Iso • Measured in GeV

  35. Electron Isolation Cut above 8 GeV Cut above 8 GeV ∑e Iso, Leading electron (GeV) ∑e Iso, second electron (GeV) Leptons ordered by pT Cut above 8 GeV Cut above 8 GeV ∑e Iso, third electron (GeV) ∑e Iso, fourth electron (GeV) • Cuts remove 90% of background and keep 90% of signal

  36. M4l Spectrum After Isolation • With electron isolation applied (stacked plot)

  37. Electron ID • Additional electron discrimination applied via ID variables • Low fraction of energy reaches HCAL (H/E) • φ/η separations between ECAL deposit and tracks (Δφin, Δηin) • Cluster shape covariance (σiηiη) • Expect loose electron ID cuts to further eliminate jets ‘faking’ electrons, while keeping most signal electrons e Reconstruction Efficiency (Signal only) • Default e Reconstruction • e Reconstruction with relaxed loose ID applied

  38. Electon ID (cont.) Efficiency for Electron ID Cuts • σiηiη cuts too many signal electrons in lower pt regions • Apply only H/E, |Δηin|, and |Δφin| cuts Efficiency Electron pt (GeV)

  39. M4l Spectrum After Isolation • With electron isolation and ID applied (stacked plot)

  40. Final Z mass peaks • With electron isolation and ID applied (stacked plot)

  41. ZZ→4ℓ Signal • With only a few cuts, the ZZ→4ℓ (e/μ) signal can be distinguished from relevant backgrounds • 78% of the signal is kept, while 0.6% of total background remains • ZZ→4ℓ signal:background is 6 • Backgrounds will be established using data as it becomes available

  42. Tevatron ZZ→4ℓ (e/μ) Observations • SM predicts σppbar→ZZ = 1.60 pb @ 2 TeV • D0 -- 3 signal events in 1.7fb-1 with 5.3σ significance in observation • σppbar→ZZ = 1.75+1.27-0.86(stat)±0.08 (syst.) ± 0.10 (lumi.) pb • CDF -- 5 signal events in 4.8fb-1 with 5.7σ significance in observation • σppbar→ZZ= 1.56+0.80-0.63(stat)±0.25(syst.) pb LHC yield is expected to be competitive with Fermilab results

  43. ZZ→4ℓ (e/μ) Signal • LHC yield is expected to be competitive with Fermilab results • Signal/Background needs improvement

  44. Conclusions and Next Steps • With the first fb-1 of data at CMS, a handful of ZZ→4ℓ (e/μ) events are expected • Comparable to Tevatron results, but measured at 7 TeV • Next steps: • Improve analysis by further optimizing cuts and extend to other decay modes • Full uncertainty studies. • Data will provide further statistics on backgrounds • Develop data-driven reconstruction efficiencies • Prepare for Higgs search

  45. Backup Slides

  46. Higgs Production BR @ the LHC

  47. Higgs Sensitivity @ CMS • No Higgs masses can be excluded with 1 fb-1 @ 7 TeV using the H→ZZ

  48. Higgs Sensitivity @ CMS • Combined Hγγ, HWW, and HZZ channels provide a mH exclusion range of 145-190 GeV

  49. Tracker Performance – LHC Collisions @ 900 GeV • Λ0 kinematic distributions • First data closely matches Monte Carlo

  50. Tracker Performance – LHC Collisions @ 7 TeV

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