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Lighting up the Higgs sector with photons at CDF

This seminar explores the use of photons for studying the Higgs sector at the CDF Fermilab experiment, including photon identification and efficiency, as well as the search for the Standard Model Higgs decaying to γγ and fermiophobic Higgs decaying to γγ.

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Lighting up the Higgs sector with photons at CDF

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  1. Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar Karen Bland for the CDF collaboration May 20, 2011

  2. Tevatron and CDF Performance • Thanks to accelerator division for delivering luminosity! • And CDF for keeping the detector running well! • Delivered luminosity ~11.0 fb-1 !! • CDF acquired luminosity: ~9.2 fb-1 • Using 7.0fb-1 for results shown here 7.0 fb-1

  3. Outline • Introduction • Photon ID and Efficiency • SM Hγγ Search • Fermiophobic hγγ Search • Summary and Conclusions

  4. Outline • Introduction • Theoretical Overview • Motivation • Photon ID and Efficiency • SM Hγγ Search • Fermiophobic hγγ Search • Summary and Conclusions

  5. The Standard Model • Higgs boson is only SM particle that hasn’t been observed! • Through the Higgs mechanism: (1) Electroweak symmetry is broken (2) Other SM particles acquire mass • But mass of Higgs boson a free parameter… • Has to be determined experimentally if exists • What we know so far: • Tevatron exclusion regions mostly based on search channels other than Hγγ • Hγγ plays a role in the low mass region… • Which is the favored Higgs mass region from electroweak constraints Tevatron exclusionalso reaching LEP limit at low mass  Hγγ contributes sensitivity here

  6. The Standard Model • Higgs boson is only SM particle that hasn’t been observed! • Through the Higgs mechanism: (1) Electroweak symmetry is broken (2) Other SM particles acquire mass • But mass of Higgs boson a free parameter… • Has to be determined experimentally if exists • What we know so far: • There is a lot of work being done still at the Tevatron experiments to extend this exclusion region, so stay tuned! • A new combination will come out this summer Tevatron exclusionalso reaching LEP limit at low mass  Hγγ contributes sensitivity here

  7. SM Higgs Production at the Tevatron Gluon Fusion • ggH is largest cross section • Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb) Associated Production Vector Boson Fusion Introduction: Theoretical Overview

  8. SM Higgs Production at the Tevatron Gluon Fusion~ 1000 fb @ 120 GeV • ggH is largest cross section • Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb) • Hγγ gains by using all three production methods (~1300 fb) • Produced only rarely: • One out of every 1012 collisions • That’s about 2 Higgs bosons produced each week Associated Production~ 225 fb @ 120 GeV Vector Boson Fusion ~ 70 fb @ 120 GeV Introduction: Theoretical Overview

  9. SM Hγγ Decay • Dominant low mass decay mode is Hbb • Hγγ Br < 0.25% • Signal expectation @ 120 GeV:N = σ×L × Br = 1300fb × 7.0fb-1 × 0.002 ~ 18 Hγγ events produced (~ 6 reconstructed) Introduction: Theoretical Overview

  10. Is a Hγγ search interesting at the Tevatron? • No SM Hγγ observation expected at the Tevatron! • However, contributes sensitivity to Tevatron search in difficult region ~125 GeV • Even has sensitivity comparable to ZHllbb~140 – 150 GeV: • Br(Hbb) falls steeply for mH > 140 GeV • Br(Hγγ) relatively flat • Translates to relatively flat sensitivity compared to other channels Many beyond SM scenarios include a larger Br(Hγγ) New results for one such scenario shown later in the talk Introduction: Motivation

  11. Is a Hγγ search interesting at the Tevatron? • Clean signature compared to Hbb • Photons (or electrons from photon conversions) easier to identify/reconstruct than b-jets • Larger fraction of Hγγ events accepted in comparison • Total acceptance: • ~35% accepted for ggH • ~30% accepted for VH and VBF • Largest efficiency losses from fiducial requirements and ID efficiency • Also improves reconstructed mass resolution… Introduction: Motivation

  12. Is a Hγγ search interesting at the Tevatron? • Great mass resolution: • Mass resolution limited only by electromagnetic (EM) calorimeter • 1σ width ~3 GeV or less(Mjj width is ~16 GeV) • Resolution ~5x betterthan best jet algorithms for Hbb • Great background discrimination using Mγγ alone • Search for narrow resonance • Sideband fits can be used to estimate background Introduction: Motivation

  13. Outline • Introduction • Photon ID and Efficiency • Introduction • Central Photons • Forward Photons • Conversion Photons • SM Hγγ Search • Fermiophobic hγγ Search • Summary and Conclusions

  14. Electromagnetic Calorimeter p Muon Chambers Central Tracker Silicon Vertex Detector CDF Detector Hadronic Calorimeter p Solenoid Photon ID and Efficiency - Introduction

  15. Photon Identification • “Central” • |η|<1.1 • “Plug” • 1.2<|η|<2.8 • Tracking efficiency lower than in central region • Easier to miss a track and reconstruct fake object as a photon • Higher backgrounds then for plug photons Central Plug Cross sectional view of one detector quadrant Photon ID and Efficiency - Introduction

  16. Photon Identification • Basic Photon Signature: • Compact EM cluster • Isolated • No high momentum track associated with cluster • Profile (lateral shower shape) consistent with that of a prompt photon • Unlike that of π0/ηγγ decays (the largest background for prompt photons) • Hard to do this with calorimeters alone Signal Inside jets Background Photon ID and Efficiency - Introduction

  17. Photon Identification • ΕΜ calorimeter segmentation: • Δη×Δϕ ~ 0.1×15° (|η|<1) • Not fine enough to fully reject π0/η jets Hadronic Calorimeter • Shower max detector • ~6 radiation lengths into EM calorimeter • Finely segmented • Gives resolution to better reject π0/ηγγ • Αlso refines EM cluster position measurement to better match associated tracks Electromagnetic Calorimeter Shower maximum detector Photon ID and Efficiency - Introduction Signal Background

  18. Central Photon Identification • Three level selection • (1) Loose requirements • Fiducial in shower max detector • Ratio of hadronic to electromagnetic transverse energy (Had/EM) < 12.5% • Calorimeter isolation • . • Cut slides with • Track isolation < 5 GeV • (2) Track veto • Number tracks ≤ 1 • If 1, then pTtrk1 < 1 GeV • (3) Cut on NN Output • More details on next slides Photon ID and Efficiency – Central Photons

  19. Central Electron Identification • Three level selection • (1) Loose requirements • Fiducial in shower max detector • Ratio of hadronic to electromagnetic transverse energy (Had/EM) < 12.5% • Calorimeter isolation • . • Cut slides with • Track isolation – pTtrk1 < 5 GeV • (2) Track veto • Number tracks ≤ 2 • If 2, then pTtrk2 < 1 GeV • (3) Cut on NN Output • More details on next slides • No pure high statistics data sample of photons to validate ID efficiency • Selection chosen so can be modified for electrons • Then use Ze+e– decays (more detail later) Photon ID and Efficiency – Central Photons

  20. Central Photon Identification • 98% signal efficiency (8% better than standard ID cuts) • 87% background rejection (23% better than standard ID cuts) NN discriminant constructed from seven well understood variables: • Ratio of hadronic to EM transverse energy • Shape in shower max compared to expectation • Calorimeter Isolation • Track isolation • Ratio of energy at shower max to total EM energy • Lateral sharing of energy between towers compared to expectation Trained using inclusive photon MC and jet MC (with ISR photons removed and energy reweighting) s/sqrt(b) for Hγγ vs NN cut gives optimum cut of 0.74

  21. Central Photon ID Efficiency • ID efficiency checked in data and MC from Ze+e– decays • Z mass constraint applied to get a pure sample of electrons to probe • Effect of pile-up seen through Nvtx dependence • Net efficiencies obtained by folding εvtx into Nvtx distribution of diphoton data and signal MC (a weighted average) • Net photon ID efficiency: Data: 83.2% MC: 87.8% • MC scale factor of 94.8% applied • Total systematic uncertainty of 2% applied from: • Differences between electron vs photon response (checked in MC) • Data taking period dependence • Fits made to Z mass distribution • Small uncertainties using this method! Photon ID and Efficiency – Central Photons

  22. Plug Photon ID and Efficiency Standard CDF Cut-Based ID Same Efficiency Technique as for Central Photons • Fiducial in shower max detector • Ratio of hadronic to EM transverse energy* < 5% • Calorimeter isolation* < 2 GeV • Track isolation* < 2 GeV • Shape in shower max compared to expectation • Net photon ID efficiency: • Data: 73.2% • MC: 80.6% • MC scale factor of 90.7% applied • Total systematic uncertainty of 4.5% * Slides with EM energy or ET Photon ID and Efficiency – Foward Photons

  23. Photon Conversions • γe+e– • Colinear tracks moving in approximately same direction • Occurs in presence of detector material • More material, higher the probability of converting port cards, cables COT inner cylinder ISL outer screen L00, L0-L4 L6 L7

  24. PhotonConversions • Conversion probability at CMS substantially higher*… • ~70% of Hγγ events have at least one photon that converts!! • Similarly for ATLAS • Much more important at LHC experiments! * J. Nysten, Nuclear Instruments and Methods in Physics Research A 534 (2004) 194-198 p ≈ 15% for central γ • Use central only • Then for two photons, % of events lost from a single central photon converting is: • 26% for CC channel • 15% for CP channel • CDF had only one Run I measurement using converted photons: γ cross section  PRD,70, 074008 (2004) • Hγγ is the first Run II CDF photon analysis using conversions Photon ID and Efficiency – Conversion Photons

  25. Conversion ID • Base selection: • |η|<1.1 • Oppositely signed high quality tracks • Proximity: r-f sep and Δcotθ • e + (γe+e–) “trident” vetophoton radiated via bremmstrahlung • Other tighter selection on calorimeter and tracking variables applied to further reduce backgrounds • 7% uncertainty in conversion ID taken as systematic from Ze+trident studies cut ~94% efficient r-ϕ separation (cm) cut ~95% efficient cotθ= pz/pT Example trident Photon ID and Efficiency – Conversion Photons Δcotθ

  26. Outline • Introduction • Photon ID and Efficiency • SM Hγγ Search • Event Selection • Background Modeling • Results • Tevatron Combination • Fermiophobic hγγ Search • Summary and Conclusions

  27. Event Selection • Inclusive photon trigger • Single photon ET > 25 GeV • Trigger efficiency after offline selection obtained from trigger simulation assuming zvtx = 0 and trigger tower clustering • Use photon ID as previously described • Photon pT > 15 GeV • Four orthogonal diphoton categories: • Central-central photons (CC) • Central-plug photons (CP) • Central-central conversion photons (CC conv) where one converts • Central-plug conversion photons (CP conv) where central converts SM Hγγ Search

  28. Signal Shapes • Widths ~3 GeV (or less) for each channel • Use 2σ width to determine signal window  12 GeV • Shapes used to fit for signal in the data when setting limits

  29. Systematic Uncertaintieson Hγγ Signal

  30. Primary Background Composition • Regular Photon Backgrounds • Real SM photons via QCDinteractions • Jets faking a photon (mostly from π0/ηγγ) • Misidentified electrons suchas in Drell-Yan Z/γ*e+e– • Conversion backgrounds • Real SM photons converting • Photons from π0/η jets converting • Combinatorics • Prompt conversions from Dalitz decays π0e+e–γ at small radius

  31. Data-Driven Background Model • Assume a null hypothesis • Fit made to sideband regions of Mγγ distribution • We use a 6 parameter polynomial times exponential to model smooth portion of the data • Fit is then interpolated into the 12 GeV signal region • Example shown here for a test mass at 115 GeV for CC channel

  32. Data-Driven Background Model • CP and CP conversion channels also contaminated by Z background • Breit-Wigner function added to smooth distribution to model this, where mean and width are bounded in fit • Example shown here for a test mass at 115 GeV for CP channel

  33. Background Model • Windowed fit shown to indicate Higgs mass region being tested • Interpolated fit used to obtain data-fit residuals • Used to inspect for signs of a resonance for each mass and channel • No significant resonance observed so will set limits on σ×Br(Hγγ) CC Channel CC Conversion Channel

  34. Background Model • Windowed fit shown to indicate Higgs mass region being tested • Interpolated fit used to obtain data-fit residuals • Used to inspect for signs of a resonance for each mass and channel • No significant resonance observed so will set limits on σ×Br(Hγγ) CP Channel CP Conversion Channel

  35. Background Rate Uncertainty • Parameters of fit function varied within uncertainties to obtain a new test fit • Integral in 12 GeV signal region calculated for test fit • Repeated millions of times • Largest upper and lower differences from standard fit stored • Then symmetrized to obtain rate uncertainty for each test mass and channel • Model dependence checked by testing alternate fit functions • Variation in normalization as compared to standard found to be within uncertainties already obtained

  36. Mass Distributions

  37. 12 GeV/c2 signal region for each test mass used to set upper limits set on σ×Br relative to SM prediction • Expected limit of 13.0xSM @ 120 GeV • An improvement of ~33% on last result! • Observed limit outside 2σ band @ 120 GeV,but reduced to < 2σ after trial factor taken into account Will be added to SM Higgs Tevatron combination this summer New Limits on Hγγ at CDF using 7.0/fb

  38. DØ’s SM H→γγ Search • Uses a boosted decision tree as final Hγγ discriminant • Βased on five kinematic inputs: Mγγ, pTγγ, ET1, ET2, Δφγγ • Example output shown for mass of 120 GeV • From March 2011 • Using 8.2fb-1 • Observed @ 120: 12.4xSM • Expected @ 120: 11.3xSM

  39. Tevatron SM Hγγ Combination • Reaching within one order of magnitude of SM prediction… for an analysis that wasn’t expected to happen at the Tevatron! • Observed @ 120: 16.9xSM • Expected @ 120: 9.1xSM • Combination significantly extends the sensitivity of the separate CDF/DØ results • This is deepest existing investigation into this channel – a channel that’s very different from Hbb

  40. Tevatron vs LHC • Due to higher jet backgrounds, the LHC is betting on the Hγγ channel rather thanHbb for a low mass Higgs discovery… • But how are LHC experiments currently doing compared to the Tevatron? • CMS has no public results yet, so we can look at ATLAS

  41. ATLAS SM Hγγ • First preliminary result uses 38pb-1 • 95% upper C.L. limits @ ~25xSM

  42. ATLAS SM Hγγ • Here with 94pb-1 from 2011 only • Here with 131pb-1 from both 2010 and 2011 Atlas expecting to be near ~4 times SM prediction with 1 fb-1

  43. Outline • Introduction • Photon ID and Efficiency • SM Hγγ Search • Fermiophobic hγγ Search • Theory Motivation • Differences in search from SM • Results • Summary and Conclusions

  44. Fermiophobic Higgs (hf) • It’s likely nature doesn’t follow the SM Higgs mechanism… • We also consider a “benchmark” fermiophobic model • A two-Higgs doublet model extension to the SM • Spontaneous symmetry breaking mechanism different for fermions and bosons  5 Higges • We search for one in which: • No Higgs coupling to fermions • SM Higgs coupling to bosons • SM production cross sections assumed Introduction

  45. Fermiophobic Higgs (hf) Production Gluon Fusion~ 1000 fb @ 120 GeV • No gghf • σ ~ 300 fb @ 120 GeV Associated Production~ 225 fb @ 120 GeV Vector Boson Fusion ~ 70 fb @ 120 GeV Introduction

  46. Fermiophobic Higgs (hf) Decay Suppressed by m2b/m2W • Dominant low mass decay mode is now hγγ • Signal expectation @ 120 GeV:N = σ×L × Br • = 300fb × 7.0fb-1× 0.03 ~63 (22) hfγγ events produced (reconstructed) ~4x higher than SM expectation • hbb no longer dominant Introduction Br ~ 13x (120x) higher than SM @ 120 GeV (100 GeV)

  47. Fermiophobic Higgs (hf) Decay Suppressed by m2b/m2W • Dominant low mass decay mode is hγγ • Signal expectation @ 100 GeV:N = σ×L × Br = 560fb ×7.0fb-1× 0.18 ~700 (245) hfγγ events produced (reconstructed) ~30x higher than SM expectation • hbb no longer dominant Introduction Br ~ 13x (120x) higher than SM @ 120 GeV (100 GeV)

  48. Event Selection • Inclusive photon trigger • Single photon ET > 25 GeV • Trigger efficiency after offline selection obtained from trigger simulation assuming zvtx = 0 and trigger tower clustering • Use photon ID as previously described • Photon pT > 15 GeV • Four orthogonal diphoton categories: • Central-central photons (CC) • Central-plug photons (CP) • Central-central conversion photons (CC conv) where one converts • Central-plug conversion photons (CP conv) where central converts • gghf suppressed • Optimize for VH/VBF • Split into three diphoton pt bins: • High: pT> 75 GeV • Medium: 35 < pT < 75 GeV • Low: pT < 35 GeV • 4 diphoton categories x 3 Pt bins = 12 total channels Greatest sensitivity! Same as SM Search Different for hf search

  49. Background ModelExample fits for CC for each pTγγ bin Same approach for background model as done for SM High pTγγ Bin N signal = 2.9 s/sqrt(b) = 0.66 Medium pTγγ Bin N signal = 2.5 s/sqrt(b) = 0.37 Low pTγγ Bin N signal = 1.3 s/sqrt(b) = 0.09

  50. Results • For comparison: • LEP Limit: 109.7 GeV • Previous CDF PRL result (3.0/fb): 106 GeV • DØ’s recent (8.2/fb): 112 GeV • Observed (expected) 95% C.L. limits on σ×B(hfγγ) exclude a Fermiophobic Higgs boson with a mass < 114 GeV (111 GeV) • A limit of 114 GeV is currently the world’s best limit on a hf Higgs

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