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The Search for the Higgs Boson at CMS PDF

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18 TESI THESES tesidiperfezionamentoinFisicasostenutail22gennaio2013 COMMISSIONEGIUDICATRICE RiccardoBarbieri,Presidente MarumiKado MichaelJosefMorello IvicaPuljak LuigiRolandi YvesSirois EnricoTrincherini GiovanniPetrucciani CERN Geneva Switzerland TheSearchfortheHiggsBosonatCMS Giovanni Petrucciani The Search for the Higgs Boson at CMS (cid:2)c 2013ScuolaNormaleSuperiorePisa ISBN978-88-7642-481-6 ISBN978-88-7642-482-3(eBook) Contents Introduction ix ThequestforthestandardmodelHiggsboson . . . . . . ix SearchesforaHiggsbosonattheLHC . . . . . . . . . . x Outlineofthisdocument . . . . . . . . . . . . . . . . . xi ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . xii 1 Thestandardmodelofelementaryparticles 1 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2. Thestandardmodelasgaugetheory . . . . . . . . . . . 2 1.2.1. Thestandardmodelgaugegroup . . . . . . . . . 3 1.3. SpontaneoussymmetrybreakingandtheHiggsboson . . 5 1.3.1. Phenomenologyofspontaneoussymmetry breaking. . . . . . . . . . . . . . . . . . . . . . 6 1.3.2. Electroweak symmetry breaking in the standard model . . . . . . . . . . . . . . . . . . . . . . . 7 1.4. StandardmodelHiggsbosonproductionmechanisms . . 8 1.5. StandardmodelHiggsbosondecays . . . . . . . . . . . 12 2 TheCMSexperimentattheCERNLHC 15 2.1. TheLargeHadronCollider . . . . . . . . . . . . . . . . 15 2.2. OverviewoftheCMSexperiment . . . . . . . . . . . . 17 2.3. Trackandvertexreconstruction. . . . . . . . . . . . . . 19 2.3.1. Innerdetectorlayout . . . . . . . . . . . . . . . 20 2.3.2. Trackreconstructionalgorithm . . . . . . . . . . 21 2.3.3. Trackreconstructionefficiencyandfakerate . . 23 2.3.4. Momentumandimpactparameterresolution . . 26 2.3.5. Primaryvertexreconstruction . . . . . . . . . . 28 2.3.6. Heavyflavourtagging . . . . . . . . . . . . . . 29 2.4. Muonreconstructionandidentification . . . . . . . . . . 31 2.4.1. Muondetectorlayout . . . . . . . . . . . . . . . 31 vi GiovanniPetrucciani 2.4.2. Standalonemuonreconstruction . . . . . . . . . 32 2.4.3. Muonidentification . . . . . . . . . . . . . . . . 33 2.4.4. Muonisolation . . . . . . . . . . . . . . . . . . 37 2.4.5. Muontrigger . . . . . . . . . . . . . . . . . . . 40 2.5. Electronandphotonreconstructionandidentification . . 43 2.5.1. Electromagneticcalorimeter . . . . . . . . . . . 44 2.5.2. Electronandphotonreconstruction . . . . . . . 44 2.5.3. Electromagneticenergycalibration . . . . . . . 46 2.5.4. Electronandphotonidentification . . . . . . . . 47 2.5.5. Isolation . . . . . . . . . . . . . . . . . . . . . 48 2.5.6. Triggersforelectronsandphotons . . . . . . . . 49 2.6. Particleflowreconstruction . . . . . . . . . . . . . . . . 50 2.6.1. Jetand Emiss missingtransverseenergy T reconstruction. . . . . . . . . . . . . . . . . . . 52 2.6.2. Taureconstructionandidentification . . . . . . . 56 3 SearchforaHiggsbosonintheH→WW→2(cid:2)2ν channel 59 3.1. Triggers . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.2. Simulateddatasets . . . . . . . . . . . . . . . . . . . . 61 3.3. Physicsobjectreconstruction . . . . . . . . . . . . . . . 62 3.4. WWeventselection . . . . . . . . . . . . . . . . . . . . 66 3.5. Signalextraction . . . . . . . . . . . . . . . . . . . . . 70 3.6. Signalmodellingandsystematicaluncertainties . . . . . 73 3.6.1. Experimentaluncertainties . . . . . . . . . . . . 73 3.6.2. Theoreticaluncertainties . . . . . . . . . . . . . 74 3.7. Backgroundpredictions . . . . . . . . . . . . . . . . . . 76 3.7.1. W+jetsandQCDmultijetbackgrounds . . . . . 77 3.7.2. Wγ(∗) background . . . . . . . . . . . . . . . . 80 3.7.3. Z+jetsbackground . . . . . . . . . . . . . . . 81 3.7.4. Z→τ+τ− background . . . . . . . . . . . . . 85 3.7.5. Topquarkbackground . . . . . . . . . . . . . . 86 3.7.6. ElectroweakWWbackground . . . . . . . . . . 90 3.7.7. WZandZZbackground . . . . . . . . . . . . . 91 3.8. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4 SearchesforaHiggsbosonintheH→ZZ→4(cid:2)decay channels 95 4.1. Eventselection . . . . . . . . . . . . . . . . . . . . . . 95 4.1.1. Leptonselection . . . . . . . . . . . . . . . . . 96 4.1.2. Finalstateradiationrecovery . . . . . . . . . . . 99 4.1.3. Combinatorialreconstruction. . . . . . . . . . . 101 4.2. Matrixelementlikelihoodanalysis . . . . . . . . . . . . 103 (cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)(cid:3)vii(cid:0)(cid:0)(cid:52)(cid:72)(cid:69)(cid:0)(cid:51)(cid:69)(cid:65)(cid:82)(cid:67)(cid:72)(cid:0)(cid:70)(cid:79)(cid:82)(cid:0)(cid:84)(cid:72)(cid:69)(cid:0)(cid:40)(cid:73)(cid:71)(cid:71)(cid:83)(cid:0)(cid:34)(cid:79)(cid:83)(cid:79)(cid:78)(cid:0)(cid:65)(cid:84)(cid:0)(cid:35)(cid:45)(cid:51) 4.3. Signallineshapemodelling . . . . . . . . . . . . . . . . 105 4.4. Backgroundestimationandmodelling . . . . . . . . . . 107 4.4.1. ZZbackground . . . . . . . . . . . . . . . . . . 107 4.4.2. Reduciblebackgrounds . . . . . . . . . . . . . . 109 4.5. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.6. SearchesintheH→ZZ→2(cid:4)2τ decaychannels . . . . 115 4.6.1. EventSelection . . . . . . . . . . . . . . . . . . 115 4.6.2. Backgroundestimation . . . . . . . . . . . . . . 117 4.6.3. Results . . . . . . . . . . . . . . . . . . . . . . 118 5 SearchesforalightHiggsbosonintheγγ andττ final states 119 5.1. H→γγ channel . . . . . . . . . . . . . . . . . . . . . 119 5.1.1. Primaryvertexidentification . . . . . . . . . . . 119 5.1.2. Inclusiveeventselection . . . . . . . . . . . . . 121 5.1.3. SelectionofeventswiththeVBFtopology . . . 124 5.1.4. Signalextraction . . . . . . . . . . . . . . . . . 125 5.2. H→ττ channel . . . . . . . . . . . . . . . . . . . . . 127 5.2.1. Eventselection . . . . . . . . . . . . . . . . . . 128 5.2.2. Eventclassification . . . . . . . . . . . . . . . . 131 5.2.3. Backgroundestimation . . . . . . . . . . . . . . 132 5.2.4. Signalextraction . . . . . . . . . . . . . . . . . 133 6 SearchesforVHandttHassociatedproduction 135 6.1. VH,H→bbchannel. . . . . . . . . . . . . . . . . . . 135 6.1.1. Eventselection . . . . . . . . . . . . . . . . . . 135 6.1.2. Backgroundestimationandsystematical uncertainties . . . . . . . . . . . . . . . . . . . 137 6.1.3. Signalextraction . . . . . . . . . . . . . . . . . 138 6.2. WH→3(cid:4)3ν channel . . . . . . . . . . . . . . . . . . . 140 6.3. WH→τ 2(cid:4)channel . . . . . . . . . . . . . . . . . . . 142 h 6.4. ttH,H→bb . . . . . . . . . . . . . . . . . . . . . . . . 143 7 SearchesforaheavyHiggsbosondecayingintoWW,ZZ infinalstateswithhadronicjetsandneutrinos 147 7.1. H→ZZ→2(cid:4)2qchannel . . . . . . . . . . . . . . . . 147 7.1.1. Eventselectioninthehigh-massregion . . . . . 147 7.1.2. Eventselectioninthelow-massregion. . . . . . 151 7.1.3. Signalandbackgroundmodelling . . . . . . . . 152 7.2. H→ZZ→2(cid:4)2ν channel . . . . . . . . . . . . . . . . 153 7.2.1. Eventselection . . . . . . . . . . . . . . . . . . 155 7.2.2. Backgroundestimation . . . . . . . . . . . . . . 156 viii GiovanniPetrucciani 7.2.3. Signalextractionandsystematicaluncertainties . 158 7.3. H→WW→(cid:4)νqqchannel . . . . . . . . . . . . . . . 159 7.3.1. Eventreconstructionandselection . . . . . . . . 160 7.3.2. Signalextraction . . . . . . . . . . . . . . . . . 161 8 StatisticalanalysisforHiggsbosonsearches 165 8.1. Signalmodel . . . . . . . . . . . . . . . . . . . . . . . 165 8.2. Systematicaluncertainties . . . . . . . . . . . . . . . . 166 8.3. Likelihoodfunctions . . . . . . . . . . . . . . . . . . . 168 8.4. Statisticalinference . . . . . . . . . . . . . . . . . . . . 170 8.5. Quantifyingalocalexcess . . . . . . . . . . . . . . . . 172 8.6. Thelook-elsewhereeffect . . . . . . . . . . . . . . . . . 174 8.7. Modifiedfrequentistupperlimits . . . . . . . . . . . . . 176 8.8. Bayesianupperlimits . . . . . . . . . . . . . . . . . . . 178 8.9. Parameterestimation . . . . . . . . . . . . . . . . . . . 179 9 ResultsoftheCMSsearchesforastandardmodelHiggs boson 183 9.1. Resultsfromindividualsearches . . . . . . . . . . . . . 185 9.1.1. H→γγ searches . . . . . . . . . . . . . . . . 185 9.1.2. H→bbandH→ττ searches . . . . . . . . . . 187 9.1.3. H→WWsearches . . . . . . . . . . . . . . . . 188 9.1.4. H→ZZsearches. . . . . . . . . . . . . . . . . 189 9.2. ResultsfromthecombinationofallCMSsearches. . . . 191 9.2.1. Exclusionlimits . . . . . . . . . . . . . . . . . 191 9.2.2. Statisticalsignificanceoftheobservedexcess . . 193 9.2.3. CompatibilitywiththeSMHiggshypothesis . . 196 9.3. Measurementofthemass . . . . . . . . . . . . . . . . . 199 9.4. Probingthecouplingsoftheobservedboson . . . . . . . 200 9.4.1. Testofcustodialsymmetry . . . . . . . . . . . . 200 9.4.2. Probingcouplingstofermionsandbosons . . . . 202 Conclusions 207 References 209 Introduction ThequestforthestandardmodelHiggsboson Sinceitsfirstformulationsinthe1960s,thestandardmodel[1,2,3]has been extremely successful in describing the physics of all known ele- mentaryparticles,andhasbeentestedtoremarkableaccuracyinseveral generationsofexperiments. Thefoundationofthestandardmodelisthe existence of a hidden symmetry of the electromagnetic and weak inter- actions,whichisspontaneouslybrokenbythepresenceinthevacuumof a non-vanishing Higgs boson field [4, 5, 6, 7, 8, 9]. As a consequence oftheelectroweaksymmetrybreaking,theweakgaugebosonsWandZ acquiremassesthroughtheirinteractionwiththeHiggsbosonfield, and theirdynamicchanges;themassesofquarksandleptonsalsoarisefrom thesamemechanism. After the symmetry breaking, the Higgs boson field manifests itself as a massive neutral scalar particle H, the dynamics of which are com- pletely predicted by the theory as function of the unknown Higgs bo- son mass m , and of the known masses and couplings to fermions and H gaugebosons[10,11]. Theexperimentalsearchesforthisparticleatthe Large Electron Positron collider have yielded negative results, thereby excluding the whole kinematic range m < 114.4GeV/c2 at 95% con- H fidence level [12]. Likewise, no evidence of the Higgs boson has been found at the Tevatron collider, and recent analyses of the data collected by the CDF and D0 experiments have ruled out the narrow m region H 147–179GeV/c2 at 95% confidence level(cid:3)[13, 14]. In addition to the dir- ect searches, phenomenological considerations bound the Higgs boson mass to be below about 1TeV, and indirect constraints from precision electroweakmeasurementsfavourthehypothesisofalightHiggsboson, m <158GeV/c2 at95%confidencelevel[15]. H OneofthemaingoalsofthephysicsprogrammeattheLargeHadron Collider(LHC)[16]istoprovideadefinitiveanswerabouttheexistence ofthestandardmodelHiggsboson,andmoreingeneralaboutthemech-

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