Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar Karen Bland for the CDF collaboration May 20, 2011
Download ReportTranscript Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar Karen Bland for the CDF collaboration May 20, 2011
Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar Karen Bland for the CDF collaboration May 20, 2011 1 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 7.0 fb-1 • Using 7.0fb-1 for results shown here 2 Outline • • • • Introduction Photon ID and Efficiency SM Hγγ Search Fermiophobic hγγ Search • Summary and Conclusions 3 Outline • Introduction – Theoretical Overview – Motivation • Photon ID and Efficiency • SM Hγγ Search • Fermiophobic hγγ Search • Summary and Conclusions 4 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 Hγγ • Hγγ plays a role in the low mass region… Tevatron exclusion also reaching LEP limit at low mass Hγγ contributes sensitivity here • Which is the favored Higgs mass region from electroweak constraints 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 also reaching LEP limit at low mass Hγγ contributes sensitivity here • 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 6 SM Higgs Production at the Tevatron Gluon Fusion • ggH is largest cross section • Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb) Associated Production Vector Boson Fusion Introduction: Theoretical Overview 7 SM Higgs Production at the Tevatron Gluon Fusion ~ 1000 fb @ 120 GeV is largest cross section •• ggH Produced only rarely: Excluded from channels where Higgs ◦ One out ofdue every 10 collisions decays to quarks to multijet backgrounds Hbb) ◦ That’s (like about 2 Higgs bosons eachallweek • Hγγproduced gains by using three production methods (~1300 fb) 12 Associated Production ~ 225 fb @ 120 GeV Vector Boson Fusion ~ 70 fb @ 120 GeV Introduction: Theoretical Overview 8 SM Hγγ Decay • Dominant low mass decay mode is Hbb • Hγγ Br < 0.25% • Signal expectation @ 120 GeV: N = σ × L × Br = 1300fb × 7.0fb-1 × 0.002 ~ 18 Hγγ events produced (~ 6 reconstructed) Introduction: Theoretical Overview 9 Is a Hγγ search interesting at the Tevatron? • No SM Hγγ observation expected at the Tevatron! • However, contributes sensitivity to Tevatron search in difficult region ~125 GeV • Even has sensitivity comparable to ZHllbb ~140 – 150 GeV: – Br(Hbb) falls steeply for mH > 140 GeV – Br(Hγγ) relatively flat – Translates to relatively flat sensitivity compared to other channels • Many beyond SM scenarios include a larger Br(Hγγ) • New results for one such scenario shown later in the talk Introduction: Motivation 10 Is a Hγγ search interesting at the Tevatron? • Clean signature compared to Hbb – Photons (or electrons from photon conversions) easier to identify/reconstruct than b-jets – Larger fraction of Hγγ events accepted in comparison – Total acceptance: • ~35% accepted for ggH • ~30% accepted for VH and VBF – Largest efficiency losses from fiducial requirements and ID efficiency – Also improves reconstructed mass resolution… Introduction: Motivation 11 Is a Hγγ 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 better than best jet algorithms for Hbb – Great background 2.5% M discrimination using Mγγ alone – Search for narrow resonance – Sideband fits can be used to estimate background Introduction: Motivation 12 Outline • Introduction • Photon ID and Efficiency – – – – Introduction Central Photons Forward Photons Conversion Photons • SM Hγγ Search • Fermiophobic hγγ Search • Summary and Conclusions 13 CDF Detector Electromagnetic Calorimeter Hadronic Calorimeter Central Tracker p p Muon Chambers Silicon Vertex Detector Solenoid Photon ID and Efficiency - Introduction Photon Identification • “Central” – |η|<1.1 • “Plug” Central 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 Cross sectional view of one detector quadrant Photon ID and Efficiency - Introduction 15 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 Signal Inside jets • Unlike that of π0/η γγ decays (the largest background for prompt photons) • Hard to do this with calorimeters alone Background Photon ID and Efficiency - Introduction 16 Photon Identification • ΕΜ calorimeter segmentation: – Δη×Δϕ ~ 0.1×15° (|η|<1) – Not fine enough to fully reject π0/η jets • Shower max detector – ~6 radiation lengths into EM calorimeter – Finely segmented – Gives resolution to better reject π0/ηγγ – Αlso refines EM cluster position measurement to better match associated tracks Hadronic Calorimeter Electromagnetic Calorimeter Shower maximum detector Signal Photon ID and Efficiency - Introduction Background 17 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 • (2) Track veto – Number tracks ≤ 1 – If 1, then pTtrk1 < 1 GeV • (3) Cut on NN Output – More details on next slides • I. ETTot (R 0.4) ETEM • Cut slides with ETEM – Track isolation pTtrk < 5 GeV 0.4 trk|zR 0 ztrk | 5cm Photon ID and Efficiency – Central Photons 18 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 • I. ETTot (R 0.4) ETEM • Cut slides with ETEM – Track isolation pTtrk – pTtrk1 < 5 GeV 0.4 trk|zR 0 ztrk | 5cm • (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 Ze+e– decays (more detail later) Photon ID and Efficiency – Central Photons 19 Central Photon Identification 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 Hγγ vs NN cut gives optimum cut of 0.74 • 98% signal efficiency (8% better than standard ID cuts) • 87% background rejection (23% better than standard ID cuts) 20 Central Photon ID Efficiency • ID efficiency checked in data and MC from Ze+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 21 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 22 Photon Conversions • γe+e– • Colinear tracks moving in approximately same direction • Occurs in presence of detector material • More material, higher the probability of converting port cards, cables L00, L0-L4 L6 ISL outer screen COT inner cylinder L7 23 Photon Conversions • Use central only • Then for two photons, % of events lost from a single central photon converting is: p ≈ 15% for central γ – 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) • Hγγ is the first Run II CDF photon analysis using conversions • Conversion probability at CMS substantially higher*… ~70% of Hγγ 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 Photon ID and Efficiency – Conversion Photons 24 Conversion ID cut ~94% efficient • Base selection: – – – – |η|<1.1 Oppositely signed high quality tracks Proximity: r-fsep and Δcotθ e + (γ e+e–) “trident” veto photon 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 Ze+trident studies r-ϕ separation (cm) cut ~95% efficient cotθ = pz/pT Example trident Photon ID and Efficiency – Conversion Photons Δcotθ 25 Outline • Introduction • Photon ID and Efficiency • SM Hγγ Search – – – – Event Selection Background Modeling Results Tevatron Combination • Fermiophobic hγγ Search • Summary and Conclusions 26 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 Hγγ Search 27 • Widths ~3 GeV (or less) for each channel Signal Shapes • Use 2σ width to determine signal window 12 GeV • Shapes used to fit for signal in the data when setting limits 28 Systematic Uncertainties on Hγγ Signal 29 Primary Background Composition • Regular Photon Backgrounds – Real SM photons via QCD interactions – Jets faking a photon (mostly from π0/ηγγ) – Misidentified electrons such as 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 π0e+e–γ at small radius 30 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 31 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 32 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(Hγγ) CC Channel CC Conversion 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(Hγγ) CP Channel CP Conversion Channel 34 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 Approximate Systematic Errors on Background (%) CC 4 CP 1 CC Conv 8 CP Conv 4 • Model dependence checked by testing alternate fit functions • Variation in normalization as compared to standard found to be within uncertainties already obtained 35 Mass Distributions 36 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 New Limits on Hγγ at CDF using 7.0/fb Will be added to SM Higgs Tevatron combination this summer 37 DØ’s SM H→γγ Search • Uses a boosted decision tree as final Hγγ 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 38 Tevatron SM Hγγ Combination • 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 Hbb • Reaching within one order of magnitude of SM prediction… for an analysis that wasn’t expected to happen at the Tevatron! 39 Tevatron vs LHC • Due to higher jet backgrounds, the LHC is betting on the Hγγ channel rather than Hbb 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 40 ATLAS SM Hγγ • First preliminary result uses 38pb-1 • 95% upper C.L. limits @ ~25xSM 41 ATLAS SM Hγγ • 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 42 Outline • • • • Introduction Photon ID and Efficiency SM Hγγ Search Fermiophobic hγγ Search – Theory Motivation – Differences in search from SM – Results • Summary and Conclusions 43 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 44 Fermiophobic Higgs (hf) Production Gluon Fusion ~ 1000 fb @ 120 GeV No gghf σ ~ 300 fb @ 120 GeV Associated Production ~ 225 fb @ 120 GeV Vector Boson Fusion ~ 70 fb @ 120 GeV Introduction 45 Fermiophobic Higgs (hf) Decay • hbb no longer dominant Suppressed by m2b/m2W • Dominant low mass decay mode is now hγγ Introduction Br ~ 13x (120x) higher than SM @ 120 GeV (100 GeV) • Signal expectation @ 120 GeV: N = σ × L × Br = 300fb × 7.0fb-1 × 0.03 ~63 (22) hfγγ events produced (reconstructed) ~4x higher than SM expectation 46 Fermiophobic Higgs (hf) Decay • hbb no longer dominant Suppressed by m2b/m2W • Dominant low mass decay mode is hγγ • Introduction Br ~ 13x (120x) higher than SM @ 120 GeV (100 GeV) Signal expectation @ 100 GeV: N = σ × L × Br = 560fb × 7.0fb-1 × 0.18 ~700 (245) hfγγ events produced (reconstructed) ~30x higher than SM expectation 47 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 Same as SM Search Greatest sensitivity! • gghf 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 Different for hf search 48 Background Model Example 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 49 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(hfγγ) 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 50 Summary and Conclusions • Improvements for current results: – Inclusion of forward and conversion photons – Better central ID from a NN • SM Hγγ search: – Observed (expected) 95% C.L. upper limits on σ×Br are 28.3xSM (13.0xSM) @ a mass of 120 GeV – Approximate 33% improvement from the 5.4fb-1 result • Tevatron SM combination: – Shows significant gains when combined with DØ – Observed (expected) limits of 13.3 (8.5) at a mass of 120 GeV • Fermiophobic hfγγ: – Excludes hf mass below 114 GeV (111 GeV) from observed (expected) limits – A limit of 114 GeV is currently the best limits on hf mass 51 Backup 52 Used two central photons from cut-based ID 12 GeV/c2 signal region for each test mass used to set upper limits set on σ Br relative to SM prediction Expected and observed limits in good agreement Expected limits of 19.4xSM @ 120 GeV Most sensitive for range 110 – 130 GeV/c2 Previous Limits on Hγγ at CDF using 5.4/fb Added to SM Higgs Tevatron combination this past summer 53 Fermiophobic Higgs (hf) • It’s likely nature doesn’t follow • 5 Physical Higgs Particles: h0, H0, A0, H+, and H– the SM Higgs mechanism… • We also consider a “benchmark” • 2HDM type-I – Scalar field mixing angle α fermiophobic model can lead to different couplings • A Two Higgs Doublet Model to fermions for h0 and H0 (2HDM) extension to SM: – sin(α) for H0 and cos(α) for h0 f1 f1 0 f1 – Limit of απ/2 yields a Higgs with enhanced coupling to bosons: h0hf f2 f2 0 f2 • With vacuum expectation values: • Standard model cross sections assumed 0 0 1 • Not present in MSSM f f 1 1 v1 2 Akeroyd, hep-ph/9511347, 1995 v 2 Introduction 54 Tight Conversion ID • Primary electron: – Fiducial – Had/EM < ~5.5% • Secondary electron: – Fiducial – pT > 1.0 GeV ~24% of events ~72% of events • Conversion photon – pT > 15 GeV – E/P ratio (optimized for Ηγγ) – Calorimeter isolation (optimized for Hγγ) – Rconv > 2.0 cm Photon ID and Efficiency – Conversion Photons 55 Conversion ID Efficiency • Search for “tridents” where one electron leg brems a photon which then converts • Probed conversions of lower momentum range than those from Hγγ • Obtain an uncertainty on conversion ID rather than MC scale factor • In data and MC calculate ratio: N(Z e trid) R N(Z ee) • Rdata/RMC 7% uncertainty Photon ID and Efficiency – Conversion Photons 56