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Low Mass Higgs Searches at ATLAS

Low Mass Higgs Searches at ATLAS. B.Mellado University of Wisconsin-Madison Especial Thanks to F.Gianotti for slides "Light Higgs: Implications for the Search for New Physics at the LHC ” January 13 th 2012. Integrated Luminosity. December 13 th Seminar ~ 4.9 fb -1. LP, end August

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Low Mass Higgs Searches at ATLAS

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  1. Low Mass Higgs Searches at ATLAS B.Mellado University of Wisconsin-Madison Especial Thanks to F.Gianotti for slides "Light Higgs: Implications for the Search for New Physics at the LHC” January 13th 2012

  2. Integrated Luminosity December 13th Seminar ~ 4.9 fb-1 LP, end August ~ 2.5 fb-1 EPS, July ~ 1.2 fb-1

  3. Higgs to WW Results with 2011 Data

  4. Higgs decay to W+W- Two leptons + neutrinos No mass peak Event counting experiment W+W- backgrounds + Single top & non-resonant WWbb

  5. The reconstruction of missing energy is real is a crucial element to the search. Shown are the METRel distribution of the neutrino momenta in the presence of two charged leptons Overall, good control of the data

  6. H+0j Dilepton invariant mass spectra in bins of jet multiplicity H+1j

  7. H+0j Dilepton azimuthal angle differnces in bins of jet multiplicity H+1j

  8. Counting experiment with the transverse mass as the final discriminator H+0j H+1j

  9. The expected numbers of signal (mH=150 GeV) and background events after the requirements listed in the first column, as well as the observed numbers of events in data. All numbers are summed over lepton flavor.

  10. After all cuts (selection for mH=130 GeV) 2.1 fb-1 Observed in data 94 events 10 ee, 42 eμ, 42 μμ Expected background 76 (±11) Expected signal mH=130 GeV 19 (±4) ATLAS Consistency of the data with the background-only expectation Expected from SM Higgs at given mH Vertical lines indicate points where selection changes • Excluded (95% CL): 145 < mH < 206 GeV (expected: 134-200 GeV) • Observed limit within 2σ of expected: max deviation 1.9 σ for mH ~ 130 GeV

  11. Status of Low mass SM Higgs with Hγγ

  12. Higgs decay to   Backgrounds Reducible j and jj Backgrounds q0

  13. Diphoton Invariant Mass

  14. Categorization

  15. Number of events in 4.9 fb-1 of data for the nice categories

  16. Background Composition

  17. After all selections: kinematic cuts, γ identification and isolation • 22489 events with 100 < mγγ < 160 GeV observed in the data • expected signal efficiency: ~ 35% for mH=125 GeV • mγγ spectrum fit with exponential function for • background plus Crystal Ball + Gaussian for signal • background determined directly from data Systematic uncertainties on signal expectation Main systematic uncertainties Expected signal yield : ~ 20% H γγ mass resolution : ~ 14% H γγ pT modeling : ~ 8% Background modeling : ±0.1-5.6 events

  18. Excluded (95% CL): 114 ≤ mH ≤ 115 GeV, 135 ≤ mH≤ 136 GeV Consistency of the data with the background-only expectation • Maximum deviation from background-only • expectation observed for mH~126 GeV: • local p0-value: 0.27% or 2.8σ • expected from SM Higgs: ~ 1.4σ local • global p0-value: includes probability for • such an excess to appear anywhere in • the investigated mass range (110-150 GeV) • (“Look-Elsewhere-Effect”): ~7% (1.5σ) Expected from SM Higgs at given mH

  19. Status of Low mass SM Higgs with HZZ

  20. Continuum Irreducible Non-Resonant reducible l l l l l ν l b b b b ν l l l l l l l Resonant reducible tt WbWb Zbb ZZ*/ *4l Side-bands or Z(*) Isolation techniques ν l ν Backgrounds HiggsZZ(*)4l(l=e) τ ZZ*/ *2l 2τ τ l ν ν l

  21. Invariant mass distributions for simulated H → ZZ(∗) → 4μ and H → ZZ(∗) → 4e for mH = 130 GeV. The fraction of events outside the ±2σ region is found to be 15% for 4μ and 18% for 4e for mH =130 GeV.

  22. Invariant mass distribution of the second lepton pair: μμ and ee. The kinematic selections of the analysis have been applied. Isolation requirements have been applied on the first lepton pair. No charge requirements were applied to the second lepton pair.

  23. The expected number of signal and background events, with their systematic uncertainty, sepa- rated into “Low m4l”(m4l < 180 GeV) and “High-m4l” (m4l ≥ 180 GeV) regions. The observed numbers of events are also presented.

  24. After all selections: kinematic cuts, isolation, impact parameter Full mass range Observed: 71 events: 24 4μ + 30 2e2μ + 17 4e Expected from background: 62±9 m(4l) < 180 GeV Observed: 8 events: 3 4μ + 3 2e2μ + 2 4e Expected from background: 9.3±1.5 In the region mH < 141 GeV (not already excluded at 95% C.L.) 3 events are observed: two 2e2μ events (m=123.6 GeV, m=124.3 GeV) and one 4μ event (m=124.6 GeV) • In the region 117< m4l <128 GeV • (containing ~90% of a mH=125 GeV signal): • similar contributions expected from signal and • background: ~ 1.5 events each • S/B ~ 2 (4μ ), ~ 1 (2e2μ), ~ 0.3 (4e) • Background dominated by ZZ* (4μ and 2e2μ), • ZZ* and Z+jets (4e) Main systematic uncertainties Higgs cross-section : ~ 15% Electron efficiency : ~ 2-8% ZZ* background : ~ 15% Zbb, +jets backgrounds : ~ 40%

  25. 4μ candidate with m4μ= 124.6 GeV pT (μ-, μ+, μ+, μ-)= 61.2, 33.1, 17.8, 11.6 GeV m12= 89.7 GeV, m34= 24.6 GeV

  26. 2e2μ candidate with m2e2μ= 124.3 GeV pT (e+, e-, μ-, μ+)= 41.5, 26.5, 24.7, 18.3 GeV m(e+e-)= 76.8 GeV, m(μ+μ-) = 45.7 GeV

  27. From signal and background expectations to 4l mass spectrum Excluded (95% CL): 135 < mH < 156 GeV and 181 < mH < 415 GeV (except 234-255 GeV) Expected (95% CL): 137 < mH < 158 GeV and 185 < mH < 400 GeV

  28. Status of Low mass Higgs Limits with Hττ

  29. These results were used for SM Higgs limit setting

  30. We’ll see a strong improvement in Moriond!

  31. Status of Overall Higgs Boson Combination

  32. Hγγ, H ττ H WW(*) lνlν H ZZ(*)  4l, H ZZ  llνν H ZZ  llqq, H WWlνqq W/ZH lbb+X not included Putting all channels together  combined constraints LEP ATLAS+CMS Combination ATLAS today 112.7 < mH < 115.5 GeV 131 <mH < 453 GeV, except 237-251 GeV Excluded at 95% CL 124.6-520 GeV Expected if no signal Excluded at 99% CL 133 <mH < 230 GeV, 260 < mH < 437 GeV

  33. Consistency of the data with the background-only expectation Maximum deviation from background-only expectation observed for mH~126 GeV Expected from 126 GeV SM Higgs • Local p0-value: 1.9 10-4 • local significance of the excess: 3.6σ • ~ 2.8σ H γγ, 2.1σ H 4l, 1.4σ H lνlν Expected from SM Higgs: ~2.4σ local (~1.4σ per channel) Global p0-value : 0.6% 2.5σ LEE over 110-146 GeV Global p0-value : 1.4% 2.2σ LEE over 110-600 GeV

  34. Compatibility of the observation with the expected strength of a SM Higgs signal Total H  γγ H  lνlν H  4l The observed excess is slightly larger (2±0.8) than expected in the Hγγ channel and compatible within 1σ for the other channels and the combined result

  35. Outlook and Conclusions • CMS and ATLAS have accumulated close to 5 fb-1 of useable data: kudos to the LHC! • This more than doubles de data set of the summer shown in Mumbai • Combined preliminary results shown by the CMS and ATLAS spokespeople on Dec. 13th • Strong enhancement of exclusion in almost entire range of 115 – 600 GeV except for: • ATLAS sees an excess, but need more data 115.5-131 GeV 126 GeV with 3.6σ (local) 2.3σ (global)

  36. Additional Slides

  37. The Standard Model Rediscovered by the LHC: What about the Higgs? ?

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