The Road to Discovery: Detector Alignment, Electron Identification, Particle Misidentification, WW Physics, and the Discovery of the Higgs Boson

314 Pages·2015·15.64 MB·English

by  John Alison (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
John Alison
The Road
to Discovery
Detector Alignment,
Electron Identification,
Particle Misidentification,
WW Physics, and the
Discovery of the Higgs Boson
Springer Theses
Recognizing Outstanding Ph.D. Research
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John Alison
The Road to Discovery
Detector Alignment, Electron Identification,
Particle Misidentification, WW Physics,
and the Discovery of the Higgs Boson
Doctoral Thesis accepted by
the University of Pennsylvania, USA
123
Author Supervisor
Dr. JohnAlison Prof. I.JosephKroll
Enrico Fermi Institute Department of Physicsand Astronomy
Universityof Chicago Universityof Pennsylvania
Chicago, IL Philadelphia, PA
USA USA
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-10343-3 ISBN 978-3-319-10344-0 (eBook)
DOI 10.1007/978-3-319-10344-0
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To the Penn Army
To Brig, Elliot, Evelyn and Joe
To my family and friends
To Steph
’
Supervisor s Foreword
OnJuly4,2014,theATLASandCMSCollaborationsannouncedthediscoveryof
anewbosonwithamassof125GeVusingdatafromtheLargeHadronColliderat
CERN,locatedinGeneva,Switzerland.Theworkdescribedinthisthesiswasapart
of that discovery. The new boson had the expected properties of the long-sought
HiggsBoson,ascalarparticlethatwouldexplainelectro-weaksymmetrybreaking,
thatis,whythecarrieroftheelectromagneticforce,thephoton,ismass-less,butthe
carriers of the weak force, the intermediate vector bosons, are massive. This dis-
covery was a crucial milestone in an experimental search that had been prompted
almost 50 years earlier with the invention of the Brout-Englert-Higgs mechanism.
The Higgs boson is unstable and is expected to decay into many different final
states.Amassof125GeVisfortuitousinthatmanydecaymodesareimportantat
that mass. The discovery was made by observing the Higgs boson decaying into
pairs of vector bosons, either two photons (γγ), two neutral intermediate vector
bosons (Z0Z0)or apair ofcharged intermediate vector bosons (W+W−).This thesis
reports on the search for the production and decay of the Higgs boson into this
W+W− final state with the ATLAS detector.
The W bosons themselves are unstable. The most favorable signal-to-back-
ground in this decay channel is achieved when both W bosons decay leptonically,
that is, to a charged lepton, either an electron or a muon, and the corresponding
neutrino. ATLAS was designed to have excellent capabilities for charged lepton
detection. Because they are only weakly interacting, neutrinos are not detected
directly. Instead the production of neutrinos is inferred by reconstructing an
imbalance in the observed momentum of the final state. The final state studied in
this thesis is two charged leptons and “missing momentum” due to the two
neutrinos.
There are many backgrounds to this Higgs-boson signature: the largest is the
nonresonant production of W+W− pairs, which is well understood. The most
nefarious background is the production of a W boson in association with jets from
quarksorgluons.TheWbosondecaysleptonicallyproducingachargedleptonand
missing momentum; a jet is misidentified as a charged lepton. This process has a
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viii Supervisor’sForeword
cross section that is many orders of magnitude larger than the production cross
section for the Higgs boson. As a consequence, even though it is very rare to
misidentifyajetasanelectronoramuonfromWbosondecay,thisbackgroundis
significant. It is also notoriously difficult to predict. John’s thesis focuses on this
background, and in particular on electron identification—a second major theme in
this dissertation—and methods to reduce and predict reliably this background.
Withoutthiswork,thesensitivitytodetectingtheHiggsbosonintheW+W−decay
mode would have been compromised.
The third topic covered in this thesis is the alignment of the transition radiation
detector (TRT). The TRT is a straw-tube tracker that forms a part of the ATLAS
Inner Detector (ID) used to reconstruct the trajectories of charged particles in the
ATLAS spectrometer. The alignment refers to the process of using data to deter-
mine the actual positions of the individual straws, which is necessary in order to
obtainthemostprecisemeasurementsofthetrajectories.Themethodusedtoalign
theTRTwaseventuallytransferredtothetwoothersub-componentsoftheATLAS
ID: the silicon-pixel and silicon-strip based detectors; John ultimately became one
of the experts in ATLAS on the alignment of the entire ATLAS ID.
The alignment was John’s first project on ATLAS, and although it appears
unrelated to electron identification and the search for the Higgs boson, it was his
expertise in charged-particle tracking that led to John’s involvement in electron
identification with the first data taken in 2010. This effort grew, and John became
one of the experts in ATLAS in cut-based electron identification, both for offline
analysis and for the real-time selection of collisions (the so-called trigger). Even-
tuallyheinitiatedalikelihood-basedelectronidentification,whichhasbeenusedin
thelatestresultsontheHiggsbosonandwhichwillbeusedinthenextdatataking
period of the LHC that begins in 2015. John’s efforts in electron identification on
ATLASareanexcellentexampleofhowtheeffortsofasinglestudentcanstillhave
aprofoundimpactonanexperimentalcollaborationconsistingof3,000physicists,
andultimatelycontributetooneofthemostimportantintellectualachievementsof
mankind.
Philadelphia, August 2014 Prof. I. Joseph Kroll
Abstract
TheStandardModelofparticlephysicshasbeentestedbymanyexperimentsand
describes all observed phenomena up to the highest particle interaction energies.
The existence of a scalar particle, the Higgs boson, is central to the theory. The
Higgsbosonwastheonlyfundamentalparticlethathadnotbeenobservedpriorto
the turn-on of the Large Hadron Collider (LHC). This thesis describes a
progression of research that builds to a search for the Higgs boson using the
ATLAS detector at the LHC. The search uses the signature of the Higgs boson
decaying to a pair of W bosons (WW). Both W bosons are required to decay
leptonicallyintoachargedleptonandaneutrino.Thissignaturesuffersfrommany
sources of background; the most important are continuum electroweak WW
production and the production of single W bosons accompanied by a jet
misidentified as a lepton (W+jet background). To understand and quantify these
backgrounds, a measurement of the WW cross section has been performed, and
analysis techniques have been developed to model the W+jet background. This
pffiffi
thesispresentsthemeasurementoftheWWcrosssectionusing1.02fb−1of s¼
7 TeV collision data and documents the method for modeling the W+jet
background. Understanding the detector is a crucial first step in these analyses.
Two commissioning activities are described: detector alignment and prompt
electron identification. Detector alignment is needed to accurately reconstruct the
trajectory of charged particles in the ATLAS Inner Detector (ID). This thesis
documentsthealignmentoftheTransitionRadiationTracker,akeycomponentof
theID.Chargedleptons(electronsandmuons)aresignaturesofmanyofthemost
interesting physics processes at hadron colliders, and the efficient and reliable
identification of charged leptons are critical to the physics program at ATLAS.
This thesis describes work on electron identification used both for real-time
selection of interesting events and for physics analysis. Finally, the search for the
Higgs boson in the H !WWð(cid:2)Þ !lνlν channel is presented using 4.7 fb−1 of
pffiffi pffiffi
s¼7 TeV collision data and 5.8 fb−1 of s¼8 TeV collision data.
ix
Preface
The Standard Model of particle physics has been tested by many experiments and
has been shown to accurately describe high energy particle interactions. The
existence of a scalar particle, known as the Higgs boson, is central to the theory.
The Higgs boson breaks electro-weak symmetry and provides mass to the ele-
mentary particles in a consistent way. The Higgs boson was the only fundamental
particleintheStandardModelthathadnotbeenobservedpriortotheturn-onofthe
Large Hadron Collider. The ultimate motivation of the work in this thesis is the
Higgs;thegoalofthisworkwastodiscoverorexcludethepresenceoftheStandard
Model Higgs boson.
The mass of the Higgs boson is not predicted by the Standard Model. Experi-
mentsatLEPhaveexcludedHiggsbosonmassesbelow115GeV.Fitstoprecision
electro-weak data disfavor a Higgs mass above 200 GeV. Between these masses,
thesearch forthe H !WWð(cid:2)Þ !lνlνdecay isoneofthemost sensitivechannels.
This decay occurs with a relatively high rate, and can be efficiently observed
experimentally. The work presented in this thesis builds to a search for
H !WWð(cid:2)Þ !lνlν, performed with the ATLAS detector at the LHC.
TheH !WWð(cid:2)Þ !lνlνanalysissuffersfrommanysourcesofbackground,the
most important being: continuum Standard Model WW !lνlν production and
events in which a W boson is produced in association with a particle that is misi-
dentified as a lepton, referred to as W+jet background. To understand these back-
grounds,inpreparationfortheH !WWð(cid:2)Þ !lνlνanalysis,ameasurement ofthe
StandardModelWWcrosssectionhasbeenperformed.Thismeasurementallowed
forthedevelopmentofanalysistechniquescarriedoverdirectlytotheHiggssearch.
The most important example is the development of a data-driven procedure for
measuring W+jet background arising from particle misidentification.
pffiffi
SearchesforH !WWð(cid:2)Þ !lνlνusing s¼7TeVdatacollectedin2011,and
pffiffi
using s¼8TeVdatacollectedin2012,havebeenperformed.Anexcessofevents
overtheexpectedbackground,consistentwiththeproductionoftheStandardModel
Higgs boson, is observed. These analyses are combined with other ATLAS Higgs
xi