The Search for the Higgs Boson at CMS

232 Pages·2013·9.627 MB·English

by  Giovanni Petrucciani (auth.)

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
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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
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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-