Markus Schumacher (Albert-Ludwigs-Universität Freiburg) on behalf of the ATLAS Collaboration - PowerPoint PPT Presentation

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Markus Schumacher (Albert-Ludwigs-Universität Freiburg) on behalf of the ATLAS Collaboration

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  1. Higgs Boson Searches with ATLAS Based on Data Collected in 2010 Markus Schumacher (Albert-Ludwigs-Universität Freiburg) on behalf of the ATLAS Collaboration Rencontres de Moriond 2011 Electroweak Session bmb+f • Prologue • Hgg • HWW  lnln • HZZllnn/llqq • Htt in MSSM • Amm at low mass • Epilogue

  2. Signal Rates and Limit Derivation Technique • signal rates and their theoretical uncertainties (mf, mr, PDF and as) based on • „Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables.“ arXiv:1101.0593 [hep-ph] • frequentist limit derivation based on: • - profile likelihood method (nuisance parameters for systematic uncertainties) • - power constraint CLS+B (impose CLB >16%) i.e. if observed cross section limit • smaller than median expected – 1s, then quote median expected – 1s • - for comparison also CLS used at LEP and TEVATRON shown M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 1

  3. The Challenge: Tiny Signal-to-Background Ratio measurements of “EW“ backgrounds multijet background: estimation from data and/or found to be negligible W+,W- Z/g* top pairs < 1 detectable Higgs boson per 1012 collisions M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 2

  4. H 2 Photons with 37.6 pb-1 q g q g q g g g g q p0 • main backgrounds: • 2 isolated photons with pT > 40 / 25 GeV reducible: g-jet and jet-jet  discriminate photon from jet irreducible: gg  excellent reconstruction of Mgg MC event comparison of MC prediction and estimation from data • check of backgrounds from „sideband“ B loose g ID D A C tight g ID isolated non isolated iterative „A=B*C/D“ method M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 3

  5. H 2 Photons: Exclusion Limit • signal mass shape described by • Crystal Ball + Gaussian • signal systematic uncertainties FWHM = 4.4 GeV MH=120 GeV • exclusion limit • background extraction via exponential • shape with two nuisance parameters comparable to Tevatron 2010 result M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 4

  6. H WWlnln with 35 pb-1 • production via gluon fusion + VBF considered • backgrounds: diboson qq,ggWW,ZZ,WZ • W+jets, Z+jets, tt, single top, ... - - • preselection: • - 2 leptons (e,m), pT>20(15) GeV • - Mll >15, |MZ-Mll|>10GeV (ee,mm) - MET>30 GeV - Dfll <1.3 (1.8) for MH<170 (>=170GeV) • (see ATLAS-CONF-2011-005 for details) • optimise sensitivity by • branching in 0, 1 and 2 jets • and additional topological cuts • uncertainty on signal events • 0 / 1 / 2 jets: 10 / 6 / 35 % • evaluated by variation of • mf, mr, PDF, as • in NNLO+NNLL calculation M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 5

  7. HWWlnln: MT Distributions and Background Estimate • final cut on transverse mass 0.75 MH< <MH 0 jets 1jet • estimation of all major backgrounds from control regions in data • sys. uncertainties on a and b • - experimental: • E scales and resolutions, • tagging efficiencies, ... • - theo. uncertainties • renormalisation scales, .. M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  8. HWWlnln: Exclusion Limits • absolute limit on cross section • limit w.r.t. SM prediction 2010 cross section limits: 54 (11,71) pb at MH=120 (160,200) GeV sensitivity dominated by gluon fusion 1.2 x SM cross section sSM excluded at MH = 160 GeV M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 7

  9. H  ZZ llqq and llnn with 35 pb-1 • improves sensitivity for large MH where HZZ4 leptons rates gets small • 2 leptons (e,m) with pT>20GeV 76 GeV < Mll <106 GeV • llqq:final discriminant MlljjMET<50 GeV , 70< Mjj<105 GeV, MH>360 GeV: pT(jets)>50GeV, Df ll ,Df jj < p/2 • llnn:final discriminant : transverse mass b-jet veto, MET>66/82GeV,Df ll <2.64/2.25forMH</≥260GeV • background estimation: ZZ/WW/WZ from MC (uncertainty 15%) • other backgrounds confirmed from sideband in Mjj and Mll and reversed cuts • s(Z) = 5% s(tt) = 25% s(W) = 50% s(multijet) = 50% M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 8

  10. H  ZZ llqq and llnn: Exclusion Limits • combination of both channels: • expected 6.5 to 24.5 x sSM • observed 3.5 to 39 x sSM • signal uncertainties: • theoretical : 17% • experimental : 20 to 25% • + shape uncertainties M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 9

  11. MSSM: H  tt  e/m + thad with 36 pb-1 • MSSM Higgs sector: 5 bosons h,H,A,H+,H- • governed by 2 parameters MA, tanb at Born level • signal: b(b)H and gluon fusion production • backgrounds: - irreducible: Ztt, ttlthad X  visible mass reconstruction • - reducible: W+jets,tt,Zll, ...  tau and lepton ID, MET • event yields: data versus MC • selection • - e (pT>20 GeV) or m (pT>15 GeV) + 1 thad(pT>20 GeV) • - MET > 20 GeV • - < 30 GeV but use of data-driven techniques to predict spectrum for Mtt (visible) M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 10

  12. H  tt  e/m thad: Background Estimation • Ztt from embedding: select Zmm in • data and replace m by t from simulation • multijet and W+jets: • same sign (SS) mass spectrum • and opposite sign OS/SS ratios • from control regions • dominant systematic uncertainties: • exp. : jet and tau energy scale (5%) signal +17/-14% Ztt +32/-21% • theo: cross section signal 10 to 20% Ztt 4% • total uncertainty on signal efficiency: 24.2% (15.4%) for MHiggs= 120 (200) GeV M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 11

  13. H  tt  e/m thad: Mass Spectrum and Exclusion Limit • limit in „MA versus tanb“ plane • in „mh max“ scenario (m>0) • visible mass pectrum after all cuts 4 flavour calculation used as yields more conservative limit than 5 flavour calculation background prediction from data-driven methods M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 12

  14. A  mm at low mass with 35 pb-1 • search for low mass scalar A in range [6,9] and [11,12] GeV e.g. inspired by NMSSM • two muons with pT>4 GeV (blue line) • likelihood ratio selection (black markers) • with PDFs derived from data itself • signal : 9 to 11 GeV (Us and A agree) • background: 4.5 to 5.5, 12.5 to 14.5 GeV • PDFs for likelihood ratio inputs: primary vertex c2/ndf and calo. isolation of muons M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 13

  15. A  mm: Exclusion Limit • limit setting: normalisations floating • - signal A: double gaussian with width and • fractions of 2 gaussian related to U • - U(1/2/3s) double gaussian: • widths and fractions free parameters, • masses fixed to PDG values • - continuum BG: 4th order Chebyshev • polynom with all parameters free • limit on s(ggA)x BR(Amm) • signal efficiency uncertainties M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 14

  16. Conclusions and Prospects • ATLAS detector performs very well, but no hint for Higgs @ LHC yet • so far: inclusive and simple cut-based selections • dominant background contributions determined with data-driven techniques • sensitivity in H2 photons comparable to Tevatron 2010 findings • close to SM sensitivity in HWWlnln (1.2 x SM) already with 35 pb-1 • HZZllnn/llqq best limits in high mass regime • h/H/Att in MSSM supersedes published Tevatron 2010 results projection of sensitivity with <4fb-1 we could exclude down to LEP limit, but hopefully we will not do so! M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 15

  17. Documentation • https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ • H  2 photons ATLAS-CONF-2011-025 • HWWlnln ATLAS-CONF-2011-005 • HZZllnn/llqq ATLAS-CONF-2011-026 • Htt ATLAS-CONF-2011-024 • Amm ATLAS-CONF-2011-020 M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  18. Backup M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  19. H 2 photons: Comparison of Limits • power constraint CLS+B limit • CLS limit • exclusion limits in numbers: M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  20. H WWlnln: Details of Limits • power constraint CLS+B limit M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  21. H WWlnln: Analysis Details • signal production via gluon fusion and VBF considered • backgrounds: diboson qqbar,ggWW,ZZ,WZ, W+jets, Z+jets, ttbar, single top • preselection: • - 2 pleptons (e,m), pt >20(15) GeV - Mll >15, |MZ-Mll|>10GeV (ee,mm) - MET>30 GeV: em (ee,mm) - Dfll <1.3 (1.8) MH<170 (>=170GeV) • final cut on transverse mass 0.75 MH< <MH M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  22. HWWlnln: Branching in Jet Multiplicities • branch analysis in 0, 1 and 2 jets top optimise sensitivity • - jets reconstructed with anti-kt R=0.4 algorithm, pt>25 GeV, |h|<4.5 • - associated to primary vertex PV by requiring fraction of momentum from • tracks from PV to total jet energy > 0.75 • uncertainty on fraction of signal events in multiplicity bins • 0 jet: 10% 1 jet: 6% 2 jet:35% • evaluated by variation of renorm., factor. scales, PDFs, as in HNNLO • additional cuts for each jet multiplcity branch: • 0jet: <30 GeV • 1jet: veto on b-jet , <30GeV, Ztt veto, • 2 jet: veto on b-jet, <30 GeV, Ztt veto, <80GeV • plus VBF cuts: mjj>500GeV, Dhjj>3.8, no 3rd central jet M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  23. HWWlnln : Section Results H+2 jets: H+0 jets: H+1jet: M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  24. BG Estimate e.g.: ttbar and WW BG in H+1 Jet Analysis • ttbar control region: replace b-veto by b-tag and drop cuts on mll, MT, Dfll  att + systematic uncertainty • WW control region: mll >100 GeV and drop cuts on MT, Dfll aww + systematic uncertainty combining both tables  btt (pollution in WW control region) • similar methods for all backgrounds in all jet mutiplicity bins M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  25. HWWlnln: Systematic Uncertainties Experimental uncertainties Background extraction M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  26. MSSM: H  tt  e/m + hadronic tau decay thad • signal: b(b)H and gluon fusion production • signature: e or m , thad, MET from 3 n • backgrounds: • - irreducible: Ztt, ttlthad X • mass reconstruction • - reducible: W+jets,tt,Zll, ... • tau and lepton ID,MET • e (pT>20 GeV) or m (pT>15 GeV) and 1 thad (pT>20 GeV) • MET > 20 GeV • Transverse mass < 30 GeV observed data and MC prediction after MET cut M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  27. H  tt  e/m thad: Background Estimation • estimation of background from same sign lepton + had. tau events • assumptions verified in simulated MC events: shape of visible mass distribution is the same for OS and SS events ratio r between OS and SS events is the same in signal and control region ratiofor multijet production rQCD= 1 • rQCD from control region CR with MET<15 GeV: 1.16+/- 0.04+/-0.09 • kW+jets =r W+jets-1 from CR with MT>50GeV: 1.43+/- 0.15 • shape of W+jets and shape + normalisation of other backgrounds from MC • MC prediction w/o QCD: 207 ± 6 • QCD estimate from data: 12 ± 7 • Daten driven estimate: 195 ± 33 • Data: 206 M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  28. H  tt  e/m thad: Background Estimation • good agreement between background prediction from data and data in signal region • details of • systematic • uncertainties M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  29. H  tt  e/m thad: Verification of Ztt Description • use jjZmm to modeljjZttXas they have same topology • Zmm: signal free and high purity control sample selectable • apart from m and tX energy deposits same detector response • (including pileup, underlying event, noise, …) • Methodology: • 1) select Zmm event in collision data • 2) use 4-momenta of m as input for t decays • 3) simulate ZttXY decay • 4) replace cones around m in data event • by cones in simulated Ztt decay • on calorimeter cell level •  „embedding“ • 5) re-reconstruct merged hybrid event • 6) apply standard selection M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  30. H  tt  e/m thad: Cross-Sections • MHMAX scenario: • 5 flavour versus • 4 flavour calculation • comparison of • production cross section from • gluon fusion and • in association with b-quarks M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011

  31. H  ZZ  llqq: Analysis Details • event yields • including • systematic • uncertainties M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  32. H  ZZ llqq: Background Estimate • Z+jets: control region from Mjj sideband 40 to 70 and 105 to 150 GeV • tt background: control region from Mll sideband 60 to 76 and 106 to 160 GeV • + reversed MET>50GeV cut M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  33. H  ZZ  llnn : Analysis Details • event yields • including • systematic • uncertainties M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  34. H  ZZ llnn: Background Estimate • W+jets ++ tt : control region from Mll sideband 60 to 76 and 106 to 160 GeV • tt: • MET>20 GeV, • one b-tagged jet • W+jets: • MET>36 GeV, • no b-tagged jet • tt b-tagged control region: data 17 events MC prediction 18.6 ± 0.5 events • W+jets b-vetoed control region: data 27 events MC prediction 25.8 ± 2.0 events M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  35. H  ZZ lljj and llnn : Details on Limit • combined limit • limits in numbers M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  36. H  ZZ lljj/llnn: Comparison of PCLS+B and CLS • power constraint CLS+B limit • CLS limit M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  37. H  ZZ llqq: Comparison of PCLS+B and CLS • power constraint CLS+B limit • CLS limit M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6

  38. H  ZZ  llnn: Comparison of PCLS+B and CLS • power constraint CLS+B limit • CLS limit M. Schumacher Higgs Boson Searches with ATLAS Based on 2010 Data Moriond EW, March 2011 6