Table Of ContentSpringer Tracts in Modern Physics 267
Matthias U. Mozer
Electroweak Physics
at the LHC
Springer Tracts in Modern Physics
Volume 267
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Matthias U. Mozer
Electroweak Physics
at the LHC
123
Matthias U.Mozer
Institut für Experimentelle Kernphysik
KIT
Karlsruhe, Baden-Württemberg
Germany
ISSN 0081-3869 ISSN 1615-0430 (electronic)
SpringerTracts inModern Physics
ISBN978-3-319-30380-2 ISBN978-3-319-30381-9 (eBook)
DOI 10.1007/978-3-319-30381-9
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Acknowledgements
First and foremost I would like to thank Thomas Müller, who has made this book
possibleinthefirst place. Withouthis continuedsupportmy work would nothave
been possible.
I’d also like to extend my gratitude to Pietro Govoni, Maurizio Pierini and
Gautier Hamel de Monchenault for reviews of the draft and useful discussions.
Additionally, I’d like to thank Matthew Herndon for his assistance in the prepa-
ration of summary plots.
The book would have been impossible without the hard work of the people at
CERN, who built and operated the LHC with immense success. Similarly, I’d like
to thank my colleagues in the CMS and ATLAS collaboration for building and
operatingthedetectorsaswellasfortheiruntiringeffortstousethecollecteddatain
meaningful physics measurements, which I had the honour to summarise in this
book.
I’dliketothankCERNaswellastheCMSandATLAScollaborationsfortheir
permissions to reuse their figures. Similarly I am indebted to the LEP electroweak
working group, the LHC Higgs cross section working group, the NNPDF collab-
oration, the GFitter collaboration and the particle data group as well as Graeme
Watt, who allowed me to use their material.
Furthermore,mythanksgotoUteHeuser,whoorganisedthepublicationofthis
book at Springer, for her helpful advice on organisational matters.
v
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Historical Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 The Large Hadron Collider. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 The Compact Muon Solenoid. . . . . . . . . . . . . . . . . . . . . 4
1.2.2 The ATLAS Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Particle Reconstruction and Identification . . . . . . . . . . . . . . . . . . 11
1.3.1 Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.2 Muons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.3 Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.4 Missing Transverse Energy. . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Theory Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1 The Standard Model Lagrangian . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 Electroweak Symmetry Breaking and the Higgs
Mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.2 Fermion Masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Predictions in Hadron Collisions . . . . . . . . . . . . . . . . . . . . . . . . 24
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 Experimental Signatures of EWK Bosons . . . . . . . . . . . . . . . . . . . . 27
3.1 Leptonic Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Hadronic Decays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.1 Decay to Dijets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.2 Boosted Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4 EWK Bosons and QCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.1 Inclusive Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.2 Differential Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
vii
viii Contents
4.3 EWK Bosons in Association with Jets . . . . . . . . . . . . . . . . . . . . 44
4.3.1 Selection Outline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.2 Signal Extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.3.3 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3.4 Results with Heavy Quarks. . . . . . . . . . . . . . . . . . . . . . . 48
4.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5 Electroweak Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1 Effective Weak Mixing Angle. . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.2 W Boson Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.3 Global Fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6 EWK Bosons and the Higgs Boson. . . . . . . . . . . . . . . . . . . . . . . . . 63
6.1 Higgs Searches in Di-photon Decays . . . . . . . . . . . . . . . . . . . . . 64
6.2 Higgs Searches in WW Decays. . . . . . . . . . . . . . . . . . . . . . . . . 66
6.3 Higgs Searches in ZZ Decays . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.3.1 Search in the High Mass Regime . . . . . . . . . . . . . . . . . . 69
6.3.2 Search in the Low Mass Regime. . . . . . . . . . . . . . . . . . . 72
6.4 Current Status of Higgs Searches. . . . . . . . . . . . . . . . . . . . . . . . 73
6.5 Search for Exotic Scalar Bosons . . . . . . . . . . . . . . . . . . . . . . . . 75
6.5.1 Boosted Topologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.5.2 VBF Tagging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6.6 Higgs Boson Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.7 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7 Diboson Resonances. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.1 Searches with Fully Leptonic Decays. . . . . . . . . . . . . . . . . . . . . 86
7.2 Searches with Fully Hadronic Decays. . . . . . . . . . . . . . . . . . . . . 87
7.3 Searches with Semi-Leptonic Decays. . . . . . . . . . . . . . . . . . . . . 88
7.3.1 Graviton Search with Resolved Jets. . . . . . . . . . . . . . . . . 89
7.3.2 Generic Search with Boosted Hadronic Decays. . . . . . . . . 91
7.4 Searches Involving the Higgs Boson . . . . . . . . . . . . . . . . . . . . . 94
7.5 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8 Nonresonant Multi-Boson Production. . . . . . . . . . . . . . . . . . . . . . . 99
8.1 aTGCs and Dibosons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.2 Quartic Couplings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
8.2.1 Results of Triple Boson Production. . . . . . . . . . . . . . . . . 105
8.2.2 Vector Boson Scattering Results . . . . . . . . . . . . . . . . . . . 106
Contents ix
8.3 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
9 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Chapter 1
Introduction
Thestandardmodelofparticlephysics(SM)describesalargevarietyofresultsfrom
acceleratorexperimentsaswellascosmicrayobservations.WhiletheLargeHadron
Collider (LHC) at CERN was build with the goals of discovering the remaining
missingparticleoftheSM,theHiggsboson,aswellassearchingforphysicsbeyond
theSM,itservesalsotogainadeeperunderstandingoftheSManditsparticles.
AttheendofitsinitialrunningperiodmoreWandZbosonshavebeenproduced
inLHCcollisionsthanany previous accelerator. Thelargenumber ofelectroweak
bosonsproducedmakestheLHCthepreeminentlaboratoryforelectroweakphysics
incurrenttimes.
WhileprecisionmeasurementsofparametersoftheSMarepossibleattheLHC,
their accuracy is limited by the complicated initial and final states of the proton–
protoncollisions,aswellasthelargenumberofsimultaneouscollisions(“pile-up”).
Inmanycases,previousmeasurementsfromleptoncollidersaremoreprecisewith
LHC results adding additional knowledge about the scale dependence of the para-
meters,asthehighcenter-of-massenergyallowsforalargerreach.Themostnotable
exceptionistheW-bosonmass.Alreadythecurrentlymostprecisemeasurementof
thisquantityoriginatesfromahadroncolliderandmeasurementsattheLHCpromise
toimprovethisevenfurther,thoughnoresultispublicatthistime.
The theoretically tractable electroweak bosons may serve as a clean probe into
the QCD effects that form the proton into the complex object it is. The kinematic
distributionsofvectorbosonproductionprovidesinformationaboutthepartondistri-
butionfunctions(PDFs)oftheproton.Measurementswithelectroweakbosonsand
heavyquarksprovideimportantconstraintsontheflavorcontent,withthedifferent
couplingsoftheWandZbosonscomplementingeachother.Especiallyusefulinthis
respectareseparatemeasurementsforpositivelyandnegativelychargedWbosons,
whicharesensitivetothedifferencebetweenquark-andantiquark-densitiesinthe
proton.
Oneofthemostinterestingfeaturesofthestandardmodelisthemechanismof
electroweaksymmetrybreaking,whichimpartsmassontheelectroweakbosonsvia
the Higgs mechanism. Naturally, studies of the symmetry breaking often involve
electroweak bosons. This may take the form of searches for the Higgs boson, but
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inModernPhysics267,DOI10.1007/978-3-319-30381-9_1
