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The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final State

The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final State. David Futyan UC Riverside. Overview. Introduction: LHC and CMS Motivation for Higgs boson searches Decay channels observable at the LHC Signal and background processes: Cross-sections and branching ratios

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The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final State

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  1. The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final State David Futyan UC Riverside David Futyan UC Riverside

  2. Overview • Introduction: • LHC and CMS • Motivation for Higgs boson searches • Decay channels observable at the LHC • Signal and background processes: • Cross-sections and branching ratios • Event generation and simulation • Online selection • Offline reconstruction of electrons and muons • Offline event selection • Evaluation of background from data • Significance with background systematics • Potential for measurement of Higgs boson properties: • Mass, width, cross-section • Experimental systematic uncertainties David Futyan UC Riverside

  3. The LHC (Large Hadron Collider) Geneva CERN Switzerland LHCb France CMS ALTAS ALICE • 17 miles in circumference • Due to begin operation summer 2007 • Proton-proton collider: • √s = 14 TeV • Luminosity = 1034 cm-2s-1 David Futyan UC Riverside

  4. The CMS Detector (Compact Muon Solenoid) Crystal ECAL Silicon Tracker Silicon Pixel Detector Muon Chambers 4 Tesla Superconducting Solenoid HCAL Magnet Return Yoke • General purpose detector • Over 2000 people from 160 institutes • 12500 tonnes David Futyan UC Riverside

  5. CMS Detector Slice David Futyan UC Riverside

  6. CMS Under Construction David Futyan UC Riverside

  7. The Higgs Boson • A key objective of the LHC is to elucidate the origin of mass. • Higgs mechanism: • Provides an explanation for electroweak symmetry breaking in the Standard Model: • Gives rise to the massive Z and W vector bosons and the massless photon. =>Lies at the core of the Standard Model - without the Higgs mechanism the SM is neither consistent nor complete. • Provides mechanism through which gauge bosons and fermions acquire mass. • Predicts the existence of one physical scalar, neutral Higgs boson. David Futyan UC Riverside

  8. Higgs Boson Mass Constraints • Higgs boson mass not predicted by the theory - free parameter of the standard Model: • Must be determined experimentally. • Current limits: • Combined lower limit from direct searches at LEP: mH > 114.4 GeV/c2 (95% CL). • Higgs boson contributes to radiative corrections to electroweak observables. • Consistency fits to electroweak precision measurements from LEP, SLC, Tevatron yield an indirect upper limit: mH < 207 GeV/c2 (95% CL). David Futyan UC Riverside

  9. The Higgs Boson Production at the LHC • Dominant production mechanisms: • Gluon-gluon fusion contributes around 80% of the total • Decay channels: David Futyan UC Riverside

  10. LHC Search Channels for the Higgs Boson EXCLUDED • Decay channels of SM Higgs boson which yield highest sensitivity for discovery at the LHC: • H  W+W-  2l2 • H  ZZ(*)  4l • H  γγ l = electron or muon LEP Limit = 114.4 GeV/c2 David Futyan UC Riverside

  11. The HZZ4l Channel • Most sensitive channel for the discovery of the Higgs boson at the LHC for a wide range of masses. • Exceptionally clean signature of 4 isolated high pT leptons, with relatively small backgrounds. • For mH>2mZ: “Golden channel”, with 2 real Z bosons • Mass of Higgs boson can be directly reconstructed from the invariant mass of the 4 leptons • Direct measurement of mass • Direct measurement of width for large mH (>200 GeV) David Futyan UC Riverside

  12. General Strategy • Detailed analyses have been developed largely independently for each of the three final states: • 4e: LLR (France), Split (Croatia), Rome/INFN • 4Florida, FNAL, Cambridge, • 2e2: UC Riverside, Bari/INFN (Italy) • Common to all channels (allows coherent combination of results): • Event generation, detector simulation. • Signal and background production and decay processes considered and their NLO cross-sections and branching ratios. • Straight forward counting experiment approach - Cut based analyses: • Look for local event excess over expected background. • Details of event selections developed differently in each of the 3 channels. • Analyses are designed as if real data were being analyzed: • Full detailed simulation of CMS detector geometry and response. • Simulation of LHC conditions in first years of running at L = 2×1033cm-2s-1. • Full treatment of systematic errors included in significance evaluation. • Techniques developed to measure the size of the residual background from LHC data. David Futyan UC Riverside

  13. Production Cross-Section and Branching Ratio Sum of gg fusion, WW fusion, ZZ fusion BR(HZZ(*)4l), including BR(t e,m) • BR(HZZ(*)4l) is the branching ratio to a final state containing only e and m, including t decay products. David Futyan UC Riverside

  14. Enhancement for 4e and 4 Final States • For 4e and 4 final states, enhancement of signal cross-section due to constructive final state interference between like-sign electrons or muons originating from different Z(*) bosons: Calculated using CompHEP David Futyan UC Riverside

  15. Signal Monte Carlo Event Generation • Signal samples generated with PYTHIA for 18 mass points between 115 and 600 GeV. • Higgs production mechanisms simulated: • gg fusion, WW fusion, ZZ fusion. • Z bosons forced to decay to e,m,t, with t forced to decay to e, • 10000 events generated per mass point, for each final state (4e, 4, 2e2) • Events re-weighted to correspond to: where David Futyan UC Riverside

  16. Background Processes • Reducible backgrounds: • qq/gg tt W+W-bb 4l + X (PYTHIA) • qq/gg (Z(*)/g*)bb 4l + X (CompHEP interfaced with PYTHIA) • Irreducible non-resonant continuum background: • qq Z(*)/*)(Z(*)/g*) 4l (PYTHIA) { (Z(*)/g*) bb David Futyan UC Riverside

  17. Z(*)/*)(Z(*)/g*) Background q l+ Z(*)/* l+ Z(*)/* q l- l- • LO cross-section 18.07pb (from MCFM generator): • t-channel dominates: • 90% m4l<2mZ, ~100% m4l>2mZ • s-channel simulated for 4 final state only • MCFM generator used to calculate an K-factor to account for all NLO processes: • Function of 4 lepton invariant mass: l+ q Z(*)/* l- q = u,d,s,c or b l+ Z(*)/* q l- David Futyan UC Riverside

  18. Z(*)/*)(Z(*)/g*) Background l+ Z(*)/* g l- q l+ g Z(*)/* l- • Significant NNLO box diagram process not included in the simulation: • TOPREX generator used to obtain ratio (ggZZ4l)/(qqZZ4l): ~20% • Total NLO cross-section = sLO (K(m4l) + 0.2) = 29pb (for average K(m4l)=1.35) • All events re-weighted at analysis level using this m4l dependent K-factor. David Futyan UC Riverside

  19. Other Potential Backgrounds • Zcc can also give 4 leptons in the final state: • Investigated with full detector simulation - found to be negligible • Other potential sources of background investigated at generator level: • Wbb • Wcc • Single top • bbbb • bbcc • cccc • All found to be negligible } One or more fake leptons } All leptons non-isolated David Futyan UC Riverside

  20. Detector Simulation • Generated Monte Carlo events for all generated samples are passed through a highly detailed simulation of the CMS detector, including: • Precise simulation of the complete detector geometry: all material in the detector including cables, services etc. • Detailed simulation of the 4T magnetic field. • Full simulation of detector response for all detector components: information used as input to the analysis fully simulates real LHC data. • Generated events are mixed with pile-up events to simulate the LHC conditions at “low luminosity” (2×1033cm-2s-1) • Several inelastic pp collisions per bunch crossing • Corresponds to conditions during the initial phase of data taking. David Futyan UC Riverside

  21. Cross-Section Times Branching Ratio • Generator level kinematic preselection includes the final state lepton flavor requirement (4e, 2 or 2e2), plus generator level cuts: • Electrons: pT>5GeV, |h|<2.5 • Muons: pT>3GeV, |h|<2.4 • For 2e2 case: David Futyan UC Riverside

  22. 4-lepton Invariant Mass After Generator Pre-selection s-channel ZZ production Same on linear scale mH=140 GeV signal David Futyan UC Riverside

  23. Online Selection • LHC bunch crossing rate is 40MHz. Multiple events per bunch crossing • CMS Trigger consists of a Level-1 trigger followed by a High Level Trigger. HLT is a software trigger involving full reconstruction of physics objects. • Triggers chosen for HZZ4l channels: • Single triggers were also considered for the 2e2 channel but were found not to benefit the final significance. David Futyan UC Riverside

  24. HLT Selection Efficiencies 2e2 4e tt: 0.399 ± 0.001 Zbb: 0.661 ± 0.001 ZZ: 0.896 ± 0.004 • For 4 channel, HLT efficiency is close to 100% for all samples David Futyan UC Riverside

  25. Muon Reconstruction and Selection • Muons are reconstructed with high efficiency with CMS: • Require + and - reconstructed with pT>7 in the barrel and pT>13 in the endcaps. • Require M(μ+μ-)>12GeV for all permutations (excludes low mass resonances). • These cuts have little effect on signal efficiency. David Futyan UC Riverside

  26. Electron Reconstruction • Lowest pT electron in HZZ4e events around 10GeV: • Electrons radiate on average half their energy before reaching the ECAL due to: • Strong magnetic field (4 Tesla) • 1 X0 of material in the inner tracker • Energy is radiated as photons which may in turn convert to e+e- before reaching the ECAL - significant spread of energy in . David Futyan UC Riverside

  27. Electron Reconstruction • Sophisticated algorithms developed, motivated by the HZZ4e analysis, in order to achieve good reconstruction efficiency for low pT electrons: • Use of Gaussian Sum Filter tracking - electron track is reconstructed right out to ECAL surface. Measure bremsstrahlung energy loss: • Categorization of electrons according to amount of radiated energy, ECAL cluster shape, cluster-track matching. • Combine ECAL energy and tracker momentum measurements based on measurement uncertainties: David Futyan UC Riverside

  28. Electron Selection • Electron reconstruction has a significant background from fakes (e.g. p+/p0 overlap from underlying event). • Selection important to exclude potential backgrounds which can fake one or more electrons. • 4e analysis: Cut based selection: • Ecalo/pTrack < 3. • Track cluster matching: || <0.02 and ||<0.1 • EHCAL/EECAL <0.2 • pT> 5 GeV • Loose isolation: pT/pT < 0.5 (cone R=0.2) • 2e2 analysis: • Likelihood developed based on similar variables. • Require likelihood>0.2. • Select electron and positron with highest likelihoods. David Futyan UC Riverside

  29. Electron Reconstruction Efficiency (4e channel) David Futyan UC Riverside

  30. Offline Event Selection • For the signal, and for the irreducible ZZ background, all 4 leptons are isolated and originate from the primary vertex. • For the reducible tt and Zbb backgrounds, 2 of the leptons are associated with b-jets → non-isolated and with displaced vertices. • For all three channels, offline selection consists of two set do cuts: • Vertex/Impact parameter and Isolation cuts - reduce Zbb and tt only. • Kinematic cuts :lepton pT and lepton invariant mass cuts - reduce all backgrounds. • The offline selection for the 2e2 channel is described on the following slides. David Futyan UC Riverside

  31. Vertex and Impact Parameter Cuts (2e2) • 3 variables chosen: High background rejection for 95% signal efficiency. Largely uncorrelated: (1) Transverse distance from m+m- vertex to beam line < 0.011 cm (2) 3D Distance between m+m- and e+e- vertices < 0.06 cm (3) Transverse impact parameter significance of lepton with highest IP significance < 7 David Futyan UC Riverside

  32. Tracker Isolation (2e2) Cut • Cut on SpT of all reconstructed tracks in the event which satisfy: • pT>0.9 GeV • At least 5 hits • Within region defined as the sum of cones of size DR<0.25 around each lepton, excluding veto cones of size DR>0.015 around each lepton. • Consistent with originating from the reconstructed primary vertex to within Dz<0.2cm David Futyan UC Riverside

  33. Kinematic Distributions for Reconstructed Leptons • Shown for events passing HLT and with e+e-m+m- reconstructed David Futyan UC Riverside

  34. Kinematic Cuts (2e2) • Lepton pT cuts: • pT1 > thr1 • pT2 > thr2 • pT3 > thr3 • pT4 > thr4 • m+m- and e+e- invariant mass cuts: • mZ1< thr5 • mZ2 > thr6 • Four lepton invariant mass cuts: • thr7 < mH< thr8 leptons sorted in decreasing order of pT mZ1 = max(mm+m-,me+e-), mZ2 = min(mm+m-,me+e-) David Futyan UC Riverside

  35. Optimization of Selection Cuts (2e2) • Kinematic cuts are optimized simultaneously together with the isolation SpT threshold. • Cut optimization performed using MINUIT by maximizing significance, defined by the Log-Likelihood ratio: • Cuts optimized independently for each Higgs mass. • To exclude effects of limited MC statistics: For each cut obtained from the automatic optimization: • Plot ScL vs cut value with all other cuts fixed. • Assign final cut value by inspection, such that ScL is as close as possible to the maximum whilst retaining smooth variation of cut value as a function of mH. where David Futyan UC Riverside

  36. Optimised Kinematic Cuts (2e2) David Futyan UC Riverside

  37. .BR. After Each Cut 2e2 x-axis categories: Preselection, L1, HLT, 4 leptons, Vertex, Isolation, Lepton pT, Z mass, Higgs mass David Futyan UC Riverside

  38. 4 Lepton Invariant Mass Before/After Offline Selection mH=130 GeV mH=200 GeV Before offline selection 2e2 After offline selection David Futyan UC Riverside

  39. Final Selected Events per fb-1 and NS/NB 2e2 David Futyan UC Riverside

  40. Summary of Offline Selection for 4e Channel • Longitudinal impact parameter significance for all electrons < 13 • Transverse impact parameter significance of reconstructed Z(*) bosons: • < 30 for highest me+e- • < 15 for lowest me+e- • Isolation, required separately for each electron, cone size R<0.2: • Tracker isolation: (pTtracks)/pTe < 0.1 • Hadronic isolation: (ETHCAL)/pTe < 0.2 • Electron quality requirements • Further cuts on track-cluster matching, cluster shape, HCAL/ECAL • Kinematic cuts on lepton pT, mZ1, mZ2, m4e David Futyan UC Riverside

  41. Summary of Offline Selection for 4 Channel • Find that only the following cuts are critical: • Isolation: Tracker and calorimeter: threshold applied to the least isolated muon • Single pT threshold for each mass applied to all but the lowest pT muon • Lowest pT muon already required to have pT>7(13) in the barrel (endcaps) • Four lepton invariant mass cuts • Additional cuts (impact parameter, m+m- inv. mass) do not significantly improve results. • Cut optimization procedure similar to 2e2 analysis, but uses a minimization program named GARCON recently developed by the HZZ4 group. Calorimeter Isolation for least isolated muon David Futyan UC Riverside

  42. Evaluation of the Z(*)/*)(Z(*)/g*) Background • Systematic error on the no. on background events in the signal region enters into the significance calculation. • Direct simulation of Z(*)/*)(Z(*)/g*)4l subject to the following uncertainties: • Theoretical uncertainties: • PDFs and QCD scale variations • NLO and NNLO production cross-section uncertainties • Relies entirely on existing SM constraints and theoretical knowledge • Experimental uncertainties: • LHC luminosity • MC modeling of detector response, material budget etc • Energy scales (ECAL calibration) and resolution • electron and muon reconstruction and kinematic selection efficiencies • Electron and muon islolation efficiencies • Such uncertainties are difficult to evaluate from first principles. • More robust approach is to evaluate the size of the background directly using the LHC data. David Futyan UC Riverside

  43. Evaluation of the Z(*)/*)(Z(*)/g*) Background from Data 2 Approaches: 1) Use single Z boson production: • Single Z bosons will be produced with a high rate at the LHC. • Production cross-section will rapidly be measured directly to a high precision • Can use ratio of production cross-sections for Z(*)/*)(Z(*)/g*) and single Z production to evaluate the Z(*)/*)(Z(*)/g*) background. • Cancellation of luminosity uncertainties. • Reduction of PDF and QCD scale uncertainties for low mH. • Partial cancellation of experimental uncertainties. 2) Direct measurement through counting the number of events in the sidebands (i.e. excluding the signal peak) of the 4-lepton invariant mass distribution: • Full cancellation of all uncertainties except PDF and QCD scale uncertainties (not fully cancelled because may affect the shape of the m(4l) distribution). • Disadvantage: Limited by statistics of the background rate in the sidebands. • Approach 2 is used here as the most robust solution. David Futyan UC Riverside

  44. Evaluation of Z(*)/*)(Z(*)/g*) Background from Sidebands ∫L = 9.2 fb-1 ∫L = 5.8 fb-1 2e2 2e2 • Points represent a simulation of LHC data for the relevant integrated luminosities: • Total no. of events generated randomly from a Poisson distribution with mean = total expected events from all processes (signal and background). • For each event, 4 lepton invariant mass generated randomly according to the histogram formed from the sum of the MC distributions for signal and background. David Futyan UC Riverside

  45. Background Systematic Errors Statistical error on background measurement from data: Theoretical uncertainty on the ratio a High statistical error at high mH due to low statistics in sidebands due to hard lepton pT cuts and large signal width. 2e2 David Futyan UC Riverside

  46. Background Systematic Errors: Theory • Systematic uncertainty from PDFs and QCD scale estimated using the MCFM event generator. • 20 eigenvectors of the CTEQ6M PDFs varied by 1. • QCD normalization and factorization scales varied independently up and down by factor 2 from nominal values R = F = 2mZ. David Futyan UC Riverside

  47. Significance Calculation • Counting experiment significance, ScP: • Defined as no. of sigmas of a Gaussian distribution equivalent to Poisson probability of observing equal to or greater than NObs events, given B expected events: • An extended form of the ScP estimator is used which takes into account the systematic uncertainty on B. David Futyan UC Riverside

  48. Significance for 2e2 Channel David Futyan UC Riverside

  49. Combined Significance for 30 fb-1 where Without systematic Uncertainties: David Futyan UC Riverside

  50. Combined Significance for 30 fb-1 Systematic uncertainties included: David Futyan UC Riverside

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