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Searches for Higgs and physics beyond the Standard Model in ATLAS

Searches for Higgs and physics beyond the Standard Model in ATLAS. Cesare Bini Sapienza Università and INFN Roma on behalf of the ATLAS collaboration. Outline : ( 1 ) The ATLAS experiment at LHC Searches for a Standard Model Higgs Boson Analysis strategy A selection of channels

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Searches for Higgs and physics beyond the Standard Model in ATLAS

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  1. SearchesforHiggs and physicsbeyond the Standard Model in ATLAS Cesare Bini Sapienza Università and INFN Roma on behalfof the ATLAS collaboration

  2. Outline: • (1) The ATLAS experiment at LHC • Searchesfor a Standard ModelHiggsBoson • Analysis strategy • A selection of channels • Combined results • Searchesforphysicsbeyond the Standard Model • Measurement of Standard Model processes • Searches for new particles • Supersymmetry • Conclusions

  3. The ATLAS Experiment at LHC - I Multi-purpose, high resolution and highly hermetic detector Magnets  1 Central Solenoid + 3 air-core toroids Tracking Silicon+Transitionradiation tracker EM caloSampling LArcalo HAD caloPlastic scintillator (barrel) + LArtechnology (endcap) MuonTrigger chambers (RPC and TGC) + Precision chambers (MDT and CSC) EM Calorimeter • Reconstructed Objects: • - leptons • - electrons • - muons • - taus • photons • jets • missingenergy • b-jets • Kinematicvariables: • pT=|p|sinq • h = -logtan(q/2) Muon Spectrometer Hadronic calorimeter Inner Detector

  4. The ATLAS Experiment at LHC - II • LHC reached a peak luminosity of ~ 3.3 ×1033 cm-2 s-1 with 50 ns bunch trains •  Data taking started in March with √s = 7 TeV and going on up to the end of 2012; • 95% of LHC delivered luminosity (3.9 fb-1 up to date) is recorder by ATLAS; • All subsystems are highly efficient (between 90% and 100%), performance are • continuosly monitored The Pile-Up effect is a “challenge” forextracting physics ! <#evts/bunch crossing> ~ 6 and depending on LHC conditions. Much progress understanding impact on performance, with data & simulation Exampleofcollisionwith 11 events: in yellow Zm+m- Resultspresentedhere: up to August 2011 Lint < 2.2 fb-1 Ongoingruns (Sept.2011)with high luminosity: <#evts/bunchcrossing> ~ 15

  5. Searchesfor a Standard ModelHiggsBoson First item of the ATLAS physicsprogram in 2011-2012 run: Discovery or exclusionof the Standard ModelHiggsBoson in the full allowed mass range Higgs production in pp collisions at √s = 7 TeV Gluon Fusion ppH Associated Production: ppWH, ZH, ttH Vector Boson Fusion: ppqqH gluon-gluonfusionis the most abundant production channel, butVBF and associate production can beexperimentally more favourable in specificcases.

  6. Higgsdecaychannels: BRsdepend on Higgs mass Depending on Higgs mass differentchannelsbecomerelevant A “multi –channel” combinedanalysisisrequired In any case the expectedcross-sections are below the “fewpb” level integrated luminosity and background rejections are the two important issues Additionalchannels are possible and willbeaddedwithlargerluminosities

  7. Higgsbosonsearches: analysisstrategy • “Cut & Count” basedanalyses • Trigger (changingalong data taking) • EventSelection • Objectdefinition (lepton, jets, missing ET,…); • Collisioneventselection (common toallchannels); • Specificeventselection and acceptancedefinition • Background evaluation • Mostly, data-drivenmethods: • (1) countevents in background enhancedcontrolregions • (2) extrapolationtosignalregionbased on MC or data • Foreachvalueof MH likelihoodfitof data in one or more variables • Confidenceintervalbased on CLsmethod on Then: channel “combination”  Confidence intervals for m vs. MH

  8. Hgg: the low mass “golden channel” Low cross-section ( < 0.1 pb) BUT verycleansignaturewithlimited background. Signal Trigger: 2 photons with ET>20 GeV (≈99% efficient) Selection: Object “photon”: measureenergy and direction 2isolated “photons” (ET1>40 GeV, ET2>25 GeV) Di-”photon” Invariant Mass range 100 ÷160 GeV Backgrounds: Di-photon (irreducible) (72%) photon + jet (rej. needed ≈104) Di-jet (rej. needed ≈107) Discriminantvariable mgg , resolution ≈ 1.7 GeV Fit exponential (background) CrystalBallresol. function(signal) Irreducible background: ppgg + X No signalfound  Set upper limits on mfor 110<MH<150 GeV

  9. H WW(*)lnln: the best channel at intermediate masses - I Highestsensitivityfor 130<MH<200 GeV and goodsensitivityfor 110<MH<300 GeV Signature: twoleptons, Missingenergy and limited “jet activity” No possibilitytoreconstructHiggs mass. Trigger: Single lepton: pT(electron)>20÷22 GeV OR pT(muon)>18 GeV Selection: 2oppositesignisoltedhigh-pT(*) leptons LargeETmiss > 40 (25) GeV Topologicalcuts on two-lepton system (mll, pTll, Dfll) Transverse mass mT (**) cut (MH); 0.75×MH<mT<MH Backgrounds: WW production (irreducible) top-antitop single top Z+jets Counteventsin signalregion, estimate backgrounds (*) cutsdepend on leptonsbeingee, mm or em (**)

  10. H WW(*)lnln: the best channel at intermediate masses - II Afterselection the events are divided in twocategories: 0-jet 1-jet (pT(jet)> 25 GeV, |h|(jet)<4.5, b-tag veto) to exploit the different background composition in the twobins. 0-jet 1-jet • No signalfound • Set upper limits on mfor • 110<MH<300 GeV

  11. H ZZ(*)llll: the “golden” channel Verycleansignature (4m, 4e ,2m2e) withgoodsensitivity in the full mass range • Trigger: • Single lepton: pT(electron)>20÷22 GeV OR pT(muon)>18 GeV • Selection: • Fourisolatedleptons: 2withpT>20 GeV, 2withpT>7GeV • Twopairsofsameflavouroppositesignleptons • M12within MZ ±15 GeV; M34> 15 ÷ 60 GeV(depending on MH) • Backgrounds: • ZZ (irreduciblebutdominant) • Z+jets (for electron channel) • Zbb (formuonchannel) • top-antitop • Discriminantvariable: • 4l invariant mass m4l. • s(m4l) = 4.5 ÷ 6.5 GeV (low MH) 15 GeV (high MH) errorbars = 68.3% centralconfidenceintervals • No signalfound • Set upper limits on mfor 110<MH<600 GeV

  12. 2μ2e candidate: mlead: 85.9 GeV msubl: 85.5 GeV m4l: 210 GeV

  13. Higgsbosonsearches: summaryof single limits 95% C.L. limit (frequentistCLsmethod) on mforallchannels:  solid colored lines: observed limit  dashed/dotted colored lines: expected limit based on MC pseudo-experiments Observed > Expected  data “over-fluctuate” Observed < Expected  data “under-fluctuate”

  14. Higgsbosonsearches: combinedlimits ATLAS excludes @ 95% C.L. a SM HiggsBosonwithmasses in threeranges: 146<MH<232 GeV, 256<MH<282 GeV, 296<MH<466 (expectedexclusion131<MH<447 GeV) Poorconsistencywith “background onlyhypothesys” in the region 130<MH<170 GeV

  15. Higgsbosonsearches: conclusions • ATLAS has performed a Higgs boson search on pp data corresponding to an integrated luminosity between 1 and more than 2 fb-1 using several channels • No significant excess is found in the mass range 110-600 GeV. • Exclusion limits at 95% C.L. are set in the mass regions: • 146 < mH< 232 GeV • 256 < mH< 282 GeV • 296 < mH< 466 GeV • These are exclusions of SM HIGGS BOSON, so that searches in the excluded regions are going on (a Higgs with different properties could emerge) • By end of 2012 with O(10 fb-1) a conclusive answer on the Standard Model Higgs should be obtained.

  16. Searchesforphysicsbeyond the Standard Model A hugeamountofsearches in a wide rangeofphysics

  17. Itisnotpossibletodescribeallsearches. Move in threesteps: • Cross-section (and otherobservable) measurements • of Standard Model processes at √s = 7 TeV are compared • totheory predictions and extrapolations; • Searchfornewparticlesappearingas “peaks” in mass • distributions: the newenergyfrontieropensnew “horizons”; • Searchforspecificsignaturescorrespondingtowell • definedmodelsof BSM physics: • Mychoice: mainresultsof the searchesforSupersymmetry.

  18. Step-(1) Cross-sectionmeasurementscomparedto SM predictions  Predictions are evaluated at NLO and more;  PDF extrapolation uncertainty is included  Good agreement over 4 orders of magnitude  SM is able to predict cross-sections once we increase energy

  19. Step-(2) Increaseofenergyopensnewhorizons. • Searchfor a Z’ boson in dileptons (mm or ee) • “conceptually” simpleanalysis: searchfor a peak in a mass spectrum. e+e-channel • Modelindependent upper limits on s×B.R. • as a functionof the mass of the newvectorboson; • Model dependent lower limits on the Z’ mass: • SSM: m(Z’) >1.83 TeV @ 95% C.L. • RS graviton: m(G*)>1.64 TeV @ 95% C.L. m+m-channel

  20. Step-(2) Increaseofenergyopensnewhorizons. • Searchfor a W’ bosondecaying in lepton+neutrino • searchfor a “jacobian” peak in a transverse mass spectrum. enchannel • Modelindependent upper limits on s×B.R. • as a functionof the mass of the newvectorboson; • Model dependent lower limits on the Z’ mass: • SSM: m(W’) >2.15 TeV @ 95% C.L. mnchannel

  21. Step-(2) Increaseofenergyopensnewhorizons. • Searchfor jet-jet resonances in Dijetevents • searchfor a peak in a dijet mass spectrum. Reconstructed dijet mass distribution fitted with a smooth functional form describing the QCD background. Vertical lines show the most significant excess found by the “BumpHunter” algorithm. Modeldependent Mass lowerlimit: Excited Quark q*: M(q*)>2.99 TeV Axigluon: M(A) > 3.32 TeV Colour Octet Scalar: M(S) > 1.92 TeV

  22. Step-(3) SUSY selecteventtopologieswithlargeETmiss and multi-jet/leptonsactivity SUSY particles production in ppcollisions: cascade toward a LSP Generalanalysisstrategy: cut-basedselection background estimate throughcontrolregions countevents at end ofselection and compare to background predictions upper limit on cross-sections exclusionareas in SUSY planes (manypossiblechoises, mainlymSUGRA/MSSM)

  23. Examples: • 0leptons + jets + ETmissanalyses: • 2÷4 jet analysis • > 6 jet analysis • Discriminantvariables are meff(a) or ETmiss/HT(b) (a) (b) Data agreewellwith background estimates (SM processes) and exclude SUSY points SU(660,240,0,10) means (m0,m1/2,A,tgb)

  24. ATLAS SUSY exclusionplotsfrom 0leptons + jets + Etmissanalysis: IfM(squark)=M(gluino)=M M> 980 (1075) GeV Stilllargespacefor SUSY, butconstraints start tobesignificant (see “Implicationsof LHC resultsforTeV-scalephysics” CERN workshop 29/8-2/9) Higgs mass > 130 GeV SUSY experimentally unaccessible

  25. Conclusions • The LHC and ATLAS are workingwell: continuous monitor • and improvementof performance; • Standard Modelalsowell at work: allpredictions are up to • nowwellmatchedby data, remarkably accurate • descriptionof “background processes” fornewphysics • searchesisavailable; • Searchesfor the Higgsbosonare in a veryexciting status, • the answerto the questfor SM Higgsisbehind the corner, • 2011-2012 runiscrucial; • ManyBSM searchesare in progress with a wide and open • rangeofpossibilities: no signalfoundyet, however • stilllargespacefordiscoveries.

  26. Backup

  27. Higgscross-sections

  28. Searchfor low mass Higgs in WH, Hbb and in ZH, Hbb Searchfor low mass Higgs in Httlthad3n (left) and H ttll4n (right)

  29. Searchfor high mass Higgs in HZZlljj: untagged jets (left) and b-tagged jets (right) Searchfor high mass Higgs in HZZllnn The most sensitive high mass Higgsbosonchannel. With1 fb-1, the llnnanalysis alone excludes the Standard ModelHiggs mass range 350<MH<450 GeV

  30. Limitsetting: a frequentist procedure based on the CLsconcept(A.Read, J.Phys.G28 (2002) 2693)  First write down the expected number of events for a given channel i • = signalstrength • si(q) = expectedsignalforchanneli at the end of the selection • (includescross-section, luminosity, efficiency…) • bi(q) = sum of the expectedbackgrounds at the end of the selection • (depends on controlregion, MC, cross-sections,…) • q= set of “nuisanceparameters” From the profilelikelihoodratioget the observedvalueof the test statistic thatdepends on observed and expectednumberofeventsforanygivenm. Thenfindthatvalueofmforwhich the probabilityratiosatisfies For the combinedresultuse the sametechniquewith a profilelikelihoodbased on the productof the single channelslikelihoods

  31. The combinationmethodnaturallytakesinto account alluncertainties. Correlateduncertainties are due toexperimental common conditions (seetablebelow) Uncorrelateduncertainties are due tochannelspecificproblems (controlregions, background extrapolation,..) Theoreticaluncertainties are eithercorrelated (PDFs, cross-sections) and uncorrelated

  32. Another “view” of the exclusion plot. The value of the combined CLs for μ = 1 (testing the Standard Model Higgs boson hypothesis) as a function of MH in the full mass range. By definition, the regions with CLs < α are considered excluded at the (1 − α ) CL or stronger.

  33. Anotherviewof the exclusion plot: The consistency of the observed results with the background-only hypothesis is shown in the full mass range. The dashed line shows the median expected significance in the hypothesis of a Standard Model Higgs boson production signal.

  34. The combined 95% CL limit on the Higgs boson production cross section in the framework of a Standard Model with the addition of a heavy fourth generation of fermions as a function of MH. All the range is clearly excluded.

  35. Searchfor the HiggsBosons in the contextof MSSM Higgssector in MSSM 5physicalstates: h/H/A (neutrals scalar/pseudoscalars) H± (charged scalar) Masses and propertiesdepending on two parametersonly : mA and tanb Production mechanism in ppcollisions: ggh/H/A (gluon-gluon fusion) bbh/H/A (associate production) tH+b (from top decay, light H) Main decay channels (in large regions of parameter space): h/H/At+t- H±tn, cs Expected and observed exclusion limits based on CLs in the mA − tan βplane of the MSSM derived from the combination of the analyses for the eμ, lτhad and τhadτhadfinal states. The dark green and yellow) bands correspond to the ±1σ and ±2σ error bands, respectively. ATLAS analyses: h/H/At+t- (see limit) tH+bcsbarb H+τν

  36. MSSM Higgs bosons • Five Higgs bosons: h, H, A, H+/- • Higgs sector is defined at tree level by MA, tanb • rad. corrections introduce dependence on other model parameters • Upper limit on mh ~ 135 GeV

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