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Search for SM Higgs via ATLAS detector

Search for SM Higgs via ATLAS detector. 方亚泉 威士康辛大学麦迪逊分校 University of Wisconsin, Madison yaquan.fang@cern.ch 山东大学学术交流. May 29th, 2012. introduction. LHC and ATLAS detector. Standard Model, Higgs Mechanism and its cross-section and branching ratio.

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Search for SM Higgs via ATLAS detector

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  1. Search for SM Higgsvia ATLAS detector 方亚泉 威士康辛大学麦迪逊分校 University of Wisconsin, Madison yaquan.fang@cern.ch 山东大学学术交流 May 29th, 2012

  2. introduction • LHC and ATLAS detector. • Standard Model, Higgs Mechanism and its cross-section and branching ratio. • Show the most updated results channel by channel. • H→γγ • H→ZZ→4l • H→ZZ→llνν,llqq, H→WW, H→bb, H→tt (very brief). • Combined results for full 2011data. • Conclusion and outlook for 2012

  3. LHC (Large Hadron Collider) 4 TeV Super proton synchrotron : 450 GeV • 100 meters underground, ring with radius 4.3 kilometers • CM energy : 2012，8 TeV, 2011, 7 TeV • Four experiment ：ATLAS，CMS, ALICE，LHCb proton synchrotron : 26 GeV 4.3 kilometers Higgs, “God Particle” 4 TeV 4 TeV in 2012

  4. ATLAS detector • Long: 44 meters，25 meters in diameter ~7000 tons. • (One Eiffel tower ,~100 jet 747). • components built within 35 countries : • Muon Spectrometer, Hadronic Calorimeter, • Electromagnetic (EM) Calorimeter ，Inner Detector, • Barrel Toroid, Solenoid.

  5. Standard Model • Standard Model explains what and how • the matter is built at the subatomic level : • Subatomic particle : • 6 quarks : u, d, c, s, t, b • 3 leptons e, μ, t and 3 neutrino • Three fundamental forces to describe the • interactions between particles : • Electromagnetic (EM) force • Weak • Strong • Three sets of mediators to mediate the forces : • g (EM). • W/Z, Higgs (Weak). • gluons (Strong). • NOT covered: • Dark matter (energy). • Neutrino Oscillations and its non-zero mass. • Gravitons not included in the frame (GUT). It cannot tell who I am and where I am going to…..

  6. Motivation for Higgs Mechanism • Gauge Symmetry Lagrangian is invariant under local phase transformation • QED : local gauge invariance → massless photon field Aμ • QCD: local gauge invariance → 8 massless vector gluon fields • Weak interaction: massive W/Z instead of massless (1983). • A solution: spontaneous breaking of a local gauge symmetry (introduce mass without breaking gauge invariance) (1960s). • Or ignore the experiment factor that massive W/Z mediators have been discovered. U(1) SU(3) Recommendation of this book except its price.

  7. V(f) f Higgs Mechanism Peter Higgs 2008 at CERN correct and promising ? (Where μ2<0,λ2>0) We substitute Φ and Aμwith : So a vector gauge boson Am and massive scalar h (higgs particle) are produced Similarly, for SU(2), three massive gauge fields (W± ,Z) and one massive scalar H are produced Higgs particle : The last particle in SM that hasn’t been shown experimentally.

  8. SM Higgs Production and decay for LHC Associated (small cross-sections) Vector Boson Fusion (VBF): Second largest ggFusion : dominant • Most sensitive channels are : <130: γγ; 125-300: ZZ*→4l; 300-600: ZZ→llvv, • 125-180: WW*→lvlv. • H→tt, H→ are significantly affected by QCD backgrounds. Try associated/VBF mode 8

  9. Previous limits from LEP and TEVATRON • Before LHC’s 2011 results, some Higgs mass regions have been excluded by • TEVATRON (July, 2010) and LEP. • LEP : excludes <114.4 GeV. • TEVATRON : excludes 158-175 GeV.

  10. Data taken with ATLAS detector in 2011 high lum. low lum. • In 2011, LHC operated successfully with high luminosity. • Bunch spacing 50 ns, peak lumi : 3.65X1033 /cm2/s • Delivered 5.61 fb-1, Recorded 5.25 fb-1. • Precise understanding pile-up effect is crucial for the analyses. • Especially for analyses related with ETmiss and jets.

  11. H→γγ channel Phys. Rev. Lett., 2012,108,11803 Phys. Lett. B, 2011, 705, 452-470 ATLAS-CONF-2011-161 ATLAS-CONF-2011-085 ATLAS-CONF-2011-071 ATLAS-CONF-2011-025 ATLAS-CONF-2011-004 PRL cover

  12. Signal and backgrounds for H→γγ • Signal : • Higgs decays to diphoton via top/W triangle • Small branching ratio as page 9 shows. • Expect ~200 events (120 GeV) with 5 fb-1 before any selection. • Backgrounds : • Irreducible • Born, Box • Reducible : • Photon-jet/di-jet with one/two jets faking as a photon/photons. • Advantage : side-band to fit the signal (the most important channel) g g q + Born Box + ······· Photon-jets + ······· diphoton Fragmentation + ······· Di-jet

  13. Requirements for H→ channel • Need good energy and angular resolution to achieve ~1-2% resolution in the Higgs mass reconstruction. • σ/mH ~ 1.4% • Need good particle identification : ~85% for real photon and reject the large QCD background (p0 et al.) with rejection above 1000. (9 EM shower shape variables+ isolation are applied to separate reducible backgrounds. (γ-jet,jet-jet). Purity: 70%

  14. Energy Calibration and vertex correction • Energy Calibration: • MC-based calibration (experience from beam-test) • After that, energy scale correction obtained from electrons using Z→ee events from data. • Vertex reconstruction : • Unconverted photon : 1st+2nd layer EM calorimeter • Converted photon : • 1st layer EM calorimeter + track from converted e+/e- • Robust against pileup (not use primary vertex) .

  15. Analysis strategy and selections • Selection : Two photons passing trigger, identification, isolation with pTγ1,γ,2 >40, 25 GeV. • Strategy : • Based on different ratio of S/B and resolution, divide events into 9 categories: unconverted – converted pseudorapidity (central, transition, rest) and pTt lower/higher than 40 GeV. where pTt is nothing but the transverse component of pTgg w.r.t. thrust axis : which provides a better resolution than pTgg .

  16. Signal modeling • Signal MC are available at 11 mass points : • 100-150 GeV with a 5 GeV step. • The shape is described by : • Crystal-ball (CB) + Gaussian • For 120 GeV, resolution of CB is from 1.4 to 2.3 for different categories with inclusive 1.7. • Simultaneous fit is applied for all 9 categories. • For those mass points not available, derived from parameterization. • Signal events passed the inclusive selection on previous page ~70 events for mH = 110-125 GeV

  17. Background modeling • Background shape is determined by a fit with single-exponential in the mass range from 100 to 160 GeV. • The mis-modeling is treated as systematic uncertainty on number of signal events (“spurious” signal). • Simultaneous fit on all categories with the same mass. inclusive 9 Categories

  18. Systematics 20% 14%

  19. Exclusion limit w.r.t SM prediction 95% CL→ If there is Higgs, there is 5% chance one will make a claim of exclusion by mistake. Expected limit (110-150 GeV): (1.61-2.87)XSM Observed new exclusion (mH): 114-115 GeV, 135-136 GeV

  20. Excess around 126 GeV Observed excess at mH = 126 GeV Significance w/o look-else-where effect (LEE) : 2.8 σ • p0 : If there is no Higgs, one could make a wrong claim (there is Higgs) with a probability p0 . It has to be very small because nobody wants to make a wrong claim of a discovery by mistake (5 s is regarded as a safe one). • In the other words, p0 tells how much possibility an upward fluctuation of the background can be as large as (or larger than) the signal.

  21. Comparison with CMS results CMS shows similar excess : 3.2 σ w/o LEE. The mass is 124 GeV. So before making this claim, one has to scratch her/his hair and really thinks hard.

  22. H→ZZ channel Phys. Lett. B 710(2012) 383-402 Phys. Lett. B 707(2012) 27-45 Phys. Rev. Lett. 107(2011) 221802 ATLAS-CONF-2012-017 ATLAS-CONF-2012-016 ATLAS-CONF-2011-162 ATLAS-CONF-2011-150 ATLAS-CONF-2011-148 ATLAS-CONF-2011-131 ATLAS-CONF-2011-048 ATLAS-CONF-2011-026 4-μ events

  23. H→ZZ*→4l • “Golden Channel” : • low cross section : expect 10-25 signal events with 5 fb-1, clean (only leptons (e or m) in final state). • narrow peak. • but constrained by natural H width for mH>>200 GeV. • Simple and loosen selections: • 4 leptons: pT1,2,3,4 > 20,20,7,7 GeV; m12 = mZ ± 15 GeV; m34 > 15-60 GeV (depending on mH) • Backgrounds: • ZZ(*) (irreducible) • Z+jet (in particular bb), tt (Prompt lepton requirements : isolation and impact parameter). • Challenge of the analysis : • Good reconstruction and identification of low pt lepton. • Reducible backgrounds have to be estimated from data. • Low statistics with current luminosity ~ 5 fb-1.

  24. Signal reconstruction The resolution of the mH is fairly good.

  25. Control reducible backgrounds • tt contribution: • use em channel as a control region. • Zjet, ttbar, ZZ estimation • ZZ,WZ from MC • No isolation, impact parameter, charge requirement on second lepton pair. Z→μμ Z→ee

  26. Exclusion limit w.r.t Standard Model prediction Main systematic uncertainties Higgs cross-section : ~ 15% Electron efficiency : ~ 2-8% ZZ* background : ~ 15% Zbb, +jets backgrounds : ~ 40% Observed exclusions : 135-156, 181-234, 255-415 GeV Expected exclusions : 136-158, 182-400 GeV

  27. The distribution of M4l and p0 (for background only hypothesis)

  28. H→ZZ→llνv two regions of selections • H→ZZ→llvv is more sensitive at • high mass region (both Z on shell). • High pile-up and low pile-up analysis are separated. Most sensitive for high mass Observed exclusions : 320-560 GeV Expected exclusions : 260-490 GeV Z mass window, EmissT and ΔΦll selections

  29. H→ZZ→llqq Observed exclusions : 300-310 , 360-400 GeV Expected exclusions : 360-400 GeV • Highest rate among ZZ decaying with leptons. • on-shell is focused here (ZZ : 200-600 GeV). • Backgrounds : • Z+jets (largest), top estimated from sideband • ZZ,WZ (MC) • Selection : • Two leptons with 83<mll<99 GeV • Two jets with 70<mjj<105 GeV • ETmiss<50 GeV • More selection for mH>300 GeV • Divide into two categories : b-tagged and untagged

  30. H→WW→lvlv/lvqq channelH→ tt channelH→bb channel Phys. Rev. Lett. 108, 11802(2012) Phys. Rev. Lett. 107, 231801(2011) ATLAS-CONF-2012-018 ATLAS-CONF-2011-134 ATLAS-CONF-2012-012 ATLAS-CONF-2012-015 ATLAS-CONF-2012-015 ATLAS-CONF-2012-014 ATLAS-CONF-2010-092

  31. Limits for H→WW→lvlv/lvqq For H→WW→lvlv sub-channel : For H→WW→lvqq sub-channel, exclusion of 1XSM Higgs prediction is not achieved yet. No significant excess is observed in H→WW channels. Expected exclusions : 130-260 GeV Observed exclusions : 127-234 GeV

  32. H→bb and H→ tt • For H→bb, only W/ZH model (with leptonic decay of W/Z) is considered due to significant QCD background. • For H → tt ,events are divided into three categories : leptonic, hadronic, leptonic-hadronic decays Not exclude 1XSM Higgs prediction yet.

  33. Combined limit and local p0 ATLAS-CONF-2012-019 • The expected excluded region covers from 120 to 555 GeV (including 126 GeV). • The observed excluded regions are :110.0 GeV -117.5 GeV, 118.5 GeV- • 122.5 GeV and from 129 GeV to 539 GeV. • With full 2011 data, we observed an excess from both H→γγ and H→ZZ*→4l • channels. The largest excess from the combination appears at mH=126 GeV.

  34. Conclusion and Outlook for 2012 • With the full data of 2011, wide mass range of SM Higgs boson mass has been excluded by the ATLAS detector. • 110.0 GeV -117.5 GeV, • 118.5 GeV- 122.5 GeV, • 129 GeV -539 GeV. • An excess has been observed around 126 GeV. • With expected 2012 data (15-20 fb-1), ATLAS expects to exclude the whole mass region ( May 26th: 3.1 fb-1). • It is also possible to have a 5σ discovery with 2012 data. (Of course, we prefer the latter). Thank you

  35. backup slides

  36. Masses of the gauge bosons through symmetry breaking No mass prediction for Higgs . It tends to smaller than a few hundred GeV from a meanful perturbation expansion.

  37. Shower shape variables for photon and jet

  38. The ATLAS Collaboration 3000 scientists including 1000 graduate students 38 countries 174 universities and research labs 38

  39. Physics Analysis Performance of the Reconstruction Event Generation & Simulation Event Reconstruction & Calibration Detectors Construction & Commissioning Trigger &Data Acquisition LHC

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