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Search for SM Higgs with the ATLAS detector

Search for SM Higgs with the ATLAS detector. 方亚泉 威士康辛大学麦迪逊分校 欧洲核子研究中心 University of Wisconsin, Madison CERN yaquan.fang@cern.ch 前沿物理工作月 北京-上海-武汉. August 29th, 2012. Outline. LHC and ATLAS detector. Standard Model, Higgs Mechanism and its cross-section and branching ratio.

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Search for SM Higgs with the ATLAS detector

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  1. Search for SM Higgswith the ATLAS detector 方亚泉 威士康辛大学麦迪逊分校 欧洲核子研究中心 University of Wisconsin, Madison CERN yaquan.fang@cern.ch 前沿物理工作月 北京-上海-武汉 August 29th, 2012

  2. Outline • LHC and ATLAS detector. • Standard Model, Higgs Mechanism and its cross-section and branching ratio. • Show the most updated results of 2011+2012 • H→γγ • H→ZZ→4l • H→WW • Combination of 2011+2012 • Conclusion

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

  4. ATLAS detector • Long: 44 meters，12.5 meters in radius, ~7000 tons. • (One Eiffel tower ,~100 jet 747). • components built within 35 countries : • Muon Spectrometer, Hadronic Calorimeter, • Electromagnetic (EM) Calorimeter ，Inner Detector,

  5. The ATLAS Collaboration 3000 scientists including 1000 graduate students 38 countries 174 universities and research labs 5

  6. 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…..

  7. 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)

  8. V(f) f Higgs Mechanism Peter Higgs in 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.

  9. SM Higgs Production and decay for LHC Associated (small cross-sections) Vector Boson Fusion (VBF): Second largest ggFusion : dominant • Most sensitive channels are : <130 GeV: γγ; 125-300 GeV: ZZ*→4l; • 300-600 GeV: ZZ→llvv, 125-180 GeV: WW*→lvlv. • H→tt, H→ are significantly affected by QCD backgrounds. Try associated/VBF mode 9

  10. 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.

  11. Data taken with ATLAS detector in 2011-2012 high lum. low lum. • In 2011-2012, LHC operated successfully with high luminosity. • peak lumi : 3.65X1033 /cm2/s (2011), 6.8X1033/cm2/s (2012). • present buch space 50 ns, 30 collision/bunch crossing • Precise understanding pile-up effect is crucial for the analyses. • Especially for analyses related with ETmiss and jets.

  12. Tag jet Forward jets Tag jet f h Higgs Decay Analysis strategy : multi-jet analysis Slicing phase space in regions with different S/B seems more optimal when inclusive analysis has little S/B H+1jet H+2jet Inclusive (H+0jet) Tag jet Not tagged Tag jet Not Tagged Not tagged Analyses in TDR were mostly inclusive Applied to H,,WW(*)

  13. Higgs Analysis with individual channels from 2011 and 2012 data

  14. 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 ATLAS-CONF-2012-079 PRL cover

  15. Signal and backgrounds for H→γγ • Signal : • Higgs decays to diphoton via top/W triangle • Small branching ratio as page 9 shows. • Expect ~400 events (120 GeV) with 10 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

  16. 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%

  17. 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) .

  18. Analysis strategy and selections + VBF • Selection : Two photons passing trigger, identification, isolation with pTγ1,γ,2 >40, 30 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 .

  19. 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. • For those mass points not available, derived from parameterization. • Signal events passed the inclusive selection ： ~80 events with mH = 110-125 GeV for 7 TeV/4.8 fb-1 ~110 events for 8 TeV/5.9 fb-1.

  20. Background modeling • Background shape is determined by a fit with single-exponential like 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

  21. Excess around 126 GeV • p0 : If there is no Higgs, one could make a wrong claim (there is Higgs) with a probability p0 . Observed excess at mH = 126.5 GeV significance w/o look-else-where effect (LEE) : 4.5 σ The difference between black (separate VBF) and red curves shows that VBF is crucial for July 4th discovery. The observation of H→γγ disfavors spin 1 particle (Landau Yang theorem)

  22. Comparison with CMS results CMS shows similar excess : 4.1 σ w/o LEE. The mass is 125 GeV.

  23. 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-2012-092 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

  24. H→ZZ*→4l • “Golden Channel” : • low cross section : expect 20-50 signal events with 10 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 for 7 TeV. • 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 ~ 5-10 fb-1.

  25. Signal and the estimation of different backgrounds • The resolution of mH for signal is fairly good. • The case of 8 TeV is slightly worse than that of 7 TeV. • tt contribution: • use em channel as a control region. • Zjet, ZZ/WZ estimation • ZZ,WZ from MC • Normalization of Zjet: No isolation, impact parameter, charge requirements on the second lepton pair.

  26. The distribution of M4l and p0 (for background only hypothesis) In 120<mH<130 GeV, Expected Background : 5.1±0.6, Expected signal 5.3±0.8. Observed events : 13 . Observed significance around 125 GeV is 3.4σ (expected 2.6σ).

  27. H→WW→lvlv channel Event signature : 2 leptons + missing ET (can’t reconstruct the Higgs mass: challenging) where Instead, use Phys. Rev. Lett. 108, 11802(2012) Phys. Rev. Lett. 107, 231801(2011) ATLAS-CONF-2012-098 ATLAS-CONF-2012-060 ATLAS-CONF-2012-018 ATLAS-CONF-2011-134 ATLAS-CONF-2011-111 ATLAS-CONF-2011-005 ATLAS-CONF-2010-092

  28. the estimation of backgrounds from data • The fake rate of wjet estimated by fakeable obj. • using di-jet events (with loose ID) . • ttbar survival probability (for 0-jet) with quasi data-driven method by tagging one b-jet: • Z+jets : “ABCD” sideband. • WW : MC based but: (in a high mll control region)

  29. The distribution of MT (8 TeV) and p0

  30. Higgs Combination using results from 2011 and 2012 data Submitted to Physics Letters B

  31. What Goes into the Combination

  32. On July 4th, 5 sigma was achieved. With the addition of WW a ~ 6 sigma effect is reached

  33. Signal Strength and mass • The combination provides the signal strength with 1.4±0.3 SM Higgs prediction. • The mass of the discovered particle is 126.0±0.4(stat)±0.4(sys.) GeV. • The confidence intervals in the (μ,mH) plane indicates the consistency from different • channels.

  34. conclusion and the impact on the future in HEP • With dedicated work of ~10K scientists for ~20 years, we eventually found Higgs-like particle with more than 5 sigma. • H→γγ (4.5σ), H→ZZ→4l (3.6σ), H→WW→lvlv(2.8σ). • It is the victory of the Standard Model or it may open a door to the world of new particles. • CERN has extended the running of machine towards the end of the year. • We are working hard to measure the properties of the new particles such as spin, coupling, etc. • Hopefully, we can draw an conclusion whether it is the Standard Model of Higgs soon. • The accomplishment of LHC in searching for Higgs definitely encourages building new colliders : International Linear Collider (ILC), Compact Linear Collider (CLIC). • The former is very possibly hosted by Japan. If so, China will for sure invest much more than 1% . The question is : are we ready for that ?

  35. We found Higgs Right after the seminar on July 4th, Sau Lan Wu walked towards Peter Higgs and said : We have worked for many years looking for you. You found me.

  36. Thank you

  37. backup slides

  38. 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.

  39. Shower shape variables for photon and jet

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

  41. 2011 Data 2012 Data 2011+2012 Data

  42. Systematics 20% 14%

  43. Strength for different categories

  44. 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. Observed exclusion (mH): 112-122.5 GeV, 132-143 GeV

  45. 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

  46. H→ZZ→llνv two regions of selections • H→ZZ→llvv is more sensitive at • high mass region (both Z on shell). • zjets significantly suppressed at high mass region. • 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

  47. 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

  48. Higgs decay to Z0Z0 Reducible 4l backgrounds Irreducible Z0Z0 backgrounds Z Z

  49. Higgs decay to W+W- Two leptons + neutrinos No mass peak Event counting experiment W+W- backgrounds

  50. Flow chart for Back. Extraction Complete propagation of systematic errors

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