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Search for the SM Higgs boson at LHC

Search for the SM Higgs boson at LHC. Patricia Lobelle Pardo (Universidad de Cantabria) On behalf of the ATLAS and CMS collaborations. XVII International Conference on Deep-Inelastic Scattering and Related Subjects DIS09, 26 th -30 th April 2009, Madrid. Higgs current limits.

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Search for the SM Higgs boson at LHC

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  1. Searchforthe SM Higgsboson at LHC Patricia Lobelle Pardo (Universidad de Cantabria) Onbehalf of the ATLAS and CMS collaborations XVII International ConferenceonDeep-InelasticScattering and RelatedSubjects DIS09, 26th-30thApril 2009, Madrid P.Lobelle (U.Cantabria)

  2. Higgscurrentlimits • Theoreticallimits • Finite and positive Higgscouplings • Experimental limits • LEP: mH > 114.4 GeV/c2 at 95% CL • Tevatron: Excludedthemassrange of 160 GeV/c2to 170 GeV/c2 at 95%CL • Indirectconstraints • Derivedfrom precise EWK measurements: • mH = 90 +36-27 GeV/c2 • (mH< 163 GeV/c2) (mH<191 GeV/c2ifincluding LEP2 results) A light Higgsisfavouredbymeasurements P.Lobelle (U.Cantabria)

  3. SM Higgsproduction at LHC • Gluon-gluonfusion: dominantmode at LHC • Vector BosonFusion • Associatedproduction P.Lobelle (U.Cantabria)

  4. SM Higgsdecays • At Lowmass ( mH< 2mZ ) • H bb: BR ~0.85 buthuge QCD background • Htt: accessiblethrough VBF • Hgg: veryimportantdespitethelow BR (~0.002 ) duetotheexcellentg/jet separation and gresolution • HWW*2l2n: accesible throughggfusion and VBF, BR~1 at mH~160 GeV/c2 • HZZ*4l :also accesible • ForHighermasses • HWW* 2l2v and HZZ*4l P.Lobelle (U.Cantabria)

  5. ATLAS and CMS P.Lobelle (U.Cantabria)

  6. State of the art • Better detector description and simulation:geometry, material budget, Geant4 • •New approaches to match parton showers and • matrix elements(ALPGEN + MLM matching, SHERPA etc.) • New (N)NLO Monte Carlos for signals & backgrounds • (MCFM, MC@NLO) • Sensitivities at NLO!! • ( NNLO maybe available before data taking , which will increase x-sections) NEW results with Full Simulation! Plus recentupdateson SM Higgschannelswith new CMSSW software FrameWork!! • More detailed, betterunderstoodreconstructionmethods(partiallybasedon test beamresults), improvedtriggersimulation, eventreconstruction and analysistools • Strategiestoestimatebackgroundsfrom data and expressAlCastreams • Improvedstatisticaltreatment(alsoincludingtreatment of systematicuncertainties) • Tevatrondata are extremelyvaluableforvalidation,work has started P.Lobelle (U.Cantabria) 6

  7. Hgg • Small branching ratio (0.002 at mH =120 GeV/c2) • Final statesproducedthrough W, top and b loops • Importantchannelfordiscovery at lowmass (mH<130 GeV/c2) • Veryclearsignature: 2 high Et isolatedphotons • Backgrounds: • Irreducible:gg,gg+jets • Reducible: g+jets, jets, DY • Requirements • Goodunderstanding of electromagneticcalorimeter performance and calibration • Excellentg/jet separation • Goodmassresolution • and Vertexreconstruction P.Lobelle (U.Cantabria)

  8. Hgg • ATLAS • Vertexdeterminedfromextrapolationusingcalorimetersamplings • Convertedphotonsusedtoimprovevertexdetermination: 57% Higgseventshave ≥1 conversion • Signaldividedintocategoriesaccordingto • hg, #jets, #convertedphotons • Inclusive analysis + searchfor di-photonswith jets • Unbinnedmaximum-likelihoodfit • CMS • Backgroundreductionusingphotonisolation and kinematicinformation • Mggobserved as a peakabovebackground • Cut-basedanalysis • Splitting in categoriesaccordingtohg and lateral showershape variable • Optimizedanalysis • Loosesorting and additionalkinematical variables • Event-by-eventkinematicalLikelihood Ratio withbackgroundpdftakenfromsidebands, signalpdffrom MC P.Lobelle (U.Cantabria)

  9. ResultsforHgg CSC 2008 • ATLAS • FormH=120 GeV/c2 and 10 fb-1 : • Inclusive analysis, usingeventcountingσ= 2.6 • Combiningthe 0,1,2 jets analysis +usinganunbinnedmaximum-likelihoodfit: • Floating (fixed) massfit : σ=2.8 (3.6) • CMS • 5σ discoverybetween LEP lowerlimit and 140 GeV/c2withlessthan 30 fb−1 of integratedluminosity • 5σ discoverywitheventbyeventestimation of the S/B ratio possible at mH=120 GeV/c2with 7-8 fb-1 CMS TDR 2006 P.Lobelle (U.Cantabria)

  10. H→WW*→2l2n • Mainsearchchannelforrange 140<mH<2mZ • -Highestbranching ratio formH >140 GeV/c2: • 95% at mH = 160 GeV/c2 Signal: 2 high pT isolated leptons, MET and no central jets • Backgrounds: • WW, tt, W+jets, Z+jets, tW, WZ, ZZ… No mass peak (undetected neutrinos) Needs a good background understanding • Twomaindiscriminants: • Anglebetweenleptons in thetransverseplane : main variable toreject WW (smallopeningangleforthesignaldueto spin correlations) • Central Jet Veto forttbarreduction P.Lobelle (U.Cantabria)

  11. Analysisstrategy H→WW*→2l2n • ATLAS( em final state, H+ 0j, H+2j analysis ) • Preselection : • 2 opposite-signisolated and identifiedleptons • Cutsonmll, MET, Δφll, Zttremoval • Central Jet veto & b-tag veto • Final selection: • 2D Fit of transversemass and HiggscandidatepT in 2 bins of di-leptonazimuthalangleΔφlltoextract S/B ratio in signalregion • CMS (ee, mm, em final states, H+0j analysis) • Preselection: • 2 opposite-signisolated and identifiedleptons • Cutsonmll, MET • Central Jet Veto • Variables usedto reduce background : • pT of theleptons, mll ,Δφll , MET • Final selection: • Massdependentcutbased & multivariateanalysis • (optimizationfor 1fb-1) • Control regions: fakeleptons, backgroundnormalization CMS PAS HIG-08-006 P.Lobelle (U.Cantabria)

  12. Results for H→WW*→2l2n • ATLAS • Combinedsignificanceabovethe5slevelfor~mH >140 GeV/c2 UpdatedCMSresult: Sensitivityto a SM Higgsimprovedusing amassdependentmultivariateanalysis ATLAS resultstobescaledby √2 and to 1 fb-1 Improvements on Lepton ID , mass dependent cuts, more data driven approaches (compared to TDR) CMS PAS HIG-08-006 CMS TDR Cutshavebeenoptimizedseparatelyfor 1fb-1maximizingtheexpectedstatisticalsignificance. P.Lobelle (U.Cantabria) 14

  13. HZZ* 4l • Signal: Verycleansignature( 2 pairs of isolatedleptons 4e, 4μ, 2e2μ)withhighbranching ratio except at mH ≈ 2 mW • Excellentmassresolution (1.5 – 2 GeV/c2for MH= 130 GeV/c2) • Powerfulanalysis in a widemassrange • Backgrounds: • Irreducible: ZZ* dominant • Reducible: Zbb, tt, ZW, Z + X • Selection of isolated (tracker + calorimeter) muon and electronpairswithoppositecharge • Isolation and ImpactParameterSignificancemain variables torejectZbb, tt • At leastoneZllonshell(Mass of theleptonspairssomediscriminationagainstthebackground) • 4-lepton massreconstruction • Peak in themassspectrum of thesignalmostdiscriminating variable against ZZ CMS PAS HIG-08-003 P.Lobelle (U.Cantabria)

  14. Results for H→ZZ*→4l • 3 channel independent • +1 combined analysis with several levels of systematic uncertainties treatment CSC 2008 CMS TDR 2006 5s • Comparable significance • Highly sensitive in the high mass region (200 GeV/c2 < mH < 400 GeV/c2) and in the 150 GeV/c2region, where the Higgs boson could be discovered with 5 fb−1 • CMS update: • Cuts optimised for 1 fb-1 and data driven methods for background optimization control • Significance of about 3σ can be reached for mH ~ 200 GeV/c2at ∫Ldt = 1 fb-1 • SM Higgs boson can be excluded for mH > 185 GeV/c2with ∫Ldt = 1 fb-1 • "look-elsewhere" effect:in real search, significance to be de-rated by ~1 sigma -1s CMS PAS HIG-08-003 P.Lobelle (U.Cantabria) 14

  15. VBF H→tt • Importantchannelforrange 115-145GeV/c2 CSC 2008 Signal: 2 leptons or t-jets in central region, MET and 2 forward jets • Backgrounds: • Zjj, tt, Z/g*->tt • 3 final states : • Lepton-lepton • lepton-hadron (CMS only this) • hadron-hadron ATLASlepton-lepton + lepton-hadron final states5s achieved in therange 115-120 GeV/c2with 30 fb-1 • Invariantmass of thetpairreconstructedviathecollinearapproximation • (breaksdownwhenthe 2 taus are back to back) • Important items: • Tau tagging (Likelihood, NN methods) • tt rejection (b-jet ID and veto for lepton-lepton) • Z+jets background, especially at low masses CMS PAS HIG-08-008 P.Lobelle (U.Cantabria) 15

  16. SM channelcombination 5fb-1 • With10 fb-1(normallyconsidered as one LHC year at lowluminosity), 5σ discoveryformH in [~120, ~500] GeV/c2 • With 5 fb-1 5σ discoveryformH in [~130, ~450] GeV/c2 • Combining results from both experiments, around half of this luminosity P.Lobelle (U.Cantabria)

  17. SM Higgssearches at √s<14TeV • LHC willstartworkingwith center of massenergylowerthan 14 TeVaround 10 TeV • ► MainEffect: crosssectionchanges • Differentenergy of LHC has twoeffects: • ● Cross sectionforsignals (and background) • goesdown • ● Signal (Higgsproduction) goesdownslightlyfaster: • Higgsismainlyproducedfromgg and backgroundsfromqq • Efficiency and Acceptance: • ●Higgsbecomesrelatively “heavier”, • i.e.decayproductsbecomerelatively more central forsmaller LHC energies P.Lobelle (U.Cantabria)

  18. CMS projection for LHC@10TeV • LHC 2009 - 2010 luminosity performance – estimate: ~200 pb-1of “good data” • Strategy: • Firstunderstanddetectors, • do SM measurements, • thensearchfortheHiggs… • Signal and bkgdyields re-scaled 14→10 TeV: lossof a factor of 1.5 in sensitivity, or a factor of ~2 in luminosity • Withroughly ~200 pb-1: Reachsensitivityfor a SM Higgswith mH~160-170 GeV/c2(comparable tothecurrentTevatronsensitivity) ( butregionjustexcludedbyTevatron) CMS AN-2009/020 P.Lobelle (U.Cantabria)

  19. Conclusions • IftheHiggsbosonisthere, ATLAS and CMS are readytofindit… • …unlessitisdiscoveredorexcludedfirst at theTevatron!!! • For a SM Higgs, with a combination of ATLAS & CMS @14TeV, between ~ 1 and 5 fb-1 are neededdependingonmassvalue. Benchmarkluminosities: • ~0.1 fb-1 exclusionlimitswillstartcarvinginto SM Higgscrosssection • ~0.5 fb-1- 1 fb-1 discoveriesstarttobecomepossible in theregionneartheoneexcludedbyTevatron ( MH~160-170 GeV/c2) • ~5 fb-1-10 fb-1 SM Higgscouldbediscovered (orexcluded) in full massrange (~110-500 GeV/c2) • LHC effortsfocusedtogetas muchluminosity as possibletogetsensitivityto a SM Higgsboson. • ATLAS and CMS are doingtheirbesttocommission and understandthedetectorstobereadywhencollisionsstart P.Lobelle (U.Cantabria)

  20. Post-discoveryquestionsthatwouldneedtobeanswered… • Isit a Standard ModelHiggs ? • What are thecouplingstotherest of particles ? • IsthereonlyoneHiggs? MSSM, othermodels? • Whatisits precise mass, width, quantum numbers ? • Doesitgenerateelectroweaksymmetrybreakingand givemasstofermions? • Higgsdiscoveryalsoraisesthe “hierarchy” problem… • Ready for Higgs discovery, but if it is not found, detectors are able to search for objects with similar signatures… LHC willanswerthese fundamental questions in thenextyears P.Lobelle (U.Cantabria)

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