Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Kurt Brendlinger
Physics with
Electrons in the
ATLAS Detector
Springer Theses
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Kurt Brendlinger
Physics with Electrons
in the ATLAS Detector
Doctoral Thesis accepted by
The University of Pennsylvania, Philadelphia, USA
123
Author Supervisor
Dr. KurtBrendlinger Prof. I.JosephKroll
Deutsches Elektronen-Synchrotron DESY Department ofPhysics
Hamburg TheUniversity of Pennsylvania
Germany Philadelphia
USA
ISSN 2190-5053 ISSN 2190-5061 (electronic)
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’
Supervisor s Foreword
The title of this dissertation summarizes concisely the importance of this research.
At hadron colliders, charged and neutral leptons with large components of
momentum transverse to the beam line originate from the decay of the weak
intermediatevectorbosons—theW(cid:1) andZ0—whicharethecornerstoneofthevast
majorityofsignaturesofelectroweakstandardmodelprocessesandphysicsbeyond
thestandardmodel(BSM).Thisdissertationdescribesthemethodsusedtoidentify
these so-called “prompt” electrons (and their antimatter counterparts positrons)
produced in the proton-proton collisions at the Large Hadron Collider (LHC) at
CERN in Geneva, Switzerland.
The research was performed using the ATLAS detector, one of two large
multipurpose experiments at the LHC. At present, approximately 3000 scientists
work on the ATLAS detector; about 1200 of these scientists are Ph.D. candidates
from182institutionslocatedin38countries fromsixcontinents.KurtBrendlinger
led the development and implementation of a new method used identify electrons,
and he made major contributions to the methods used to measure the efficiency
of the electron identification methods and to the techniques used to determine the
backgrounds from hadrons mimicking the signature of electrons. His method—
known as the “Likelihood identification”—replaced the selection-criteria-based
selection, and is now the default electron identification method used in both
real-time (or on-line) selection of the most potentially interesting proton-proton
collisions and the subsequent off-line analysis of data for physics results. In a
collaborationof3000physicists,itisstillpossibleforasingleindividualtohavea
profound impact on the scientific output of the experiment.
Electronshaveadistinctwayofinteractinginthedetector,andthecombination
ofvariousdetectorquantitiesareusedtodistinguishelectronsfrommorecommonly
producedchargedparticlessuchaspions,kaonsandprotons,collectivelyreferredto
has“hadrons.”The distributionsofsome ofthese quantitieslook verydifferent for
electronsandhadrons,somuchsothatitispossibletoexaminesuchadistribution
and draw a line or “cut:” on one side of the line the distribution is primarily
electrons and on the other side of the line the distribution is primarily hadrons.
v
vi Supervisor’sForeword
The “cut” or selection criteria can be used to distinguish electrons from hadrons.
Thenumberofhadronsproducedexceedsthenumberofpromptelectronsbymany
orders of magnitude, so a combination of several quantities must be used to ulti-
mately identify electrons. This cut-based method was the basis of the earlier
methods of electron identification with the ATLAS detector.
Therearemany detectorquantitiesfor whichthedistributionofthese quantities
isdifferentforelectronsandhadrons,butnotsodifferentthatitisstraightforwardto
draw a line that distinguishes the two distributions. The likelihood technique was
designed to address this problem. It is based on a product of the probability dis-
tributions of these various detector quantities for electrons and hadrons. More
quantities can be used in the identification, and the likelihood approach achieves a
better separation of the signal electrons from the background hadrons than the
cut-based approach. Specifically this means that for a given signal identification
efficiency, the likelihood identification achieves better background rejection than
the cut-based method, typically a factor of two better rejection.
Kurtusedthelikelihoodintwophysicsanalyses.Thefirstwasthemeasurement
of the coupling properties of the newly discovered Higgs boson in the H !ZZ(cid:3)
final state, where the Z bosons decay to either muon-antimuon pairs (l(cid:4)lþ) or
electron-positron pairs e(cid:4)eþ. To maximize the size of the statistically-limited
sample of Higgs bosons in this decay mode, the minimum transverse momentum
required for the muons and electrons was made as low as possible, where back-
grounds become problematic. Introducing the electron likelihood identification
reducedthebackgroundstothepromptelectronsbyafactoroftwo.Thisworkwas
carriedoutusingRun1datafromtheLHC,collectedatcenter-of-massenergiesof
pffiffi pffiffi
s¼7 TeV (in the year 2011) and s¼8 TeV (in the year 2012).
The second analysis used the first data from Run 2 of the LHC collected at
pffiffi
s¼13TeVintheyear2015.ThediscoveryofaHiggsbosonwithamassof125
GeV opened up several rich avenues of investigation. Perhaps the most important
of these avenues is the search for BSM physics that would explain why the Higgs
bosonhassuchasmallmass.Quantum-mechanicalcorrectionstothismassshould
push its value to be many orders of magnitude larger than 125 GeV. One of the
most popular extensions of the standard model that offers an explanation to this
“hierarchy”problemisknownas“supersymmetry.”Insupersymmetry,eachknown
boson and fermion in the standard model has a supersymmetric partner that is a
fermion or a boson, respectively. The gauge bosons and the Higgs boson have
supersymmetric partners known as “gauginos,” and one of the most important
signatures of the production of gauginos is the production of W(cid:1)Z0 boson pairs.
ThereisalsostandardmodelproductionofW(cid:1)Z0,whichformsthemostimportant
background to these particular gaugino searches.
Insufficient data was collected in 2015 to make improvements on limits on
gaugino production from Run 1 data at the LHC. However, at the new
center-of-massenergy,itwasimperativetomeasurethestandardmodelproduction
rateofW(cid:1)Z0bosonpairstoquantifytheexpectedcontributionofthisbackgroundin
searchesforsupersymmetry.Kurtwasoneofthemaincontributorstothisanalysis.
Supervisor’sForeword vii
Along with developing an expertise in electron identification, he also became a
leadingspecialistindeterminingtherateofbackgroundhadronscontributingtothe
prompt electron signature. This work led to the first publication of the standard
model W(cid:1)Z0 boson pair production cross section in proton-proton collisions at
pffiffi
s¼13 TeV.
Kurt’s dissertation includes chapters on two other major pieces of work. As I
mentioned above, he was not only the leader in developing and implementing the
likelihood identification of electrons, he was also a major player in developing
improvedmethodstomeasuretheefficienciesforidentifyingelectronsusingZ0 !
e(cid:4)eþ decays reconstructed in the data collected by the ATLAS detector. In par-
ticularheledtheimprovementofthesystematicuncertaintiesassociatedwiththese
efficiencies.Improvementsinthesesystematicsdirectlyimpacttheprecisionof,for
example, cross-section measurements that are based on prompt electrons as part
of their experimental signatures.
Finallyatthebeginningofhistenureasagraduatestudent,Kurtalsocontributed
totheoperationsoftheATLASdetectorbyworkingonthealignmentofoneofthe
three components of the ATLAS tracking detector or Inner Detector (ID).
Specifically he worked on the alignment of the Transition Radiation Tracker or
TRT, a tracking drift chamber constructed out of tubular drift-tubes (straws). The
University of Pennsylvania was a leading institution in the design, production and
commissioningofthe350,000channelsofon-detectorelectronicsthatreadoutand
process the electronic signals from the TRT. It was a natural place for us to
participate in the ID alignment.
Thealignmentreferstotheprocessbywhichthepreciselocationsofthevarious
individual components of the ID is determined using the data. It is performed in
stages.The initial stage focuses onlarge collections of detector elements, andthen
proceeds to align smaller and smaller collections of detector elements. The TRT
alignment firstadjuststhebarrel andendcapdetectors aswholerigidunits;then it
adjuststhe96modulesinthebarreland80wheelsinthetwoendcaps.Kurt’scon-
tributionwastoimplementathirdlevelofalignmentthatfocusedontheindividual
straws.Thisinvolvedabout720,000degreesoffreedom—threeordersofmagnitude
largerthanthesecondstageofTRTalignment.Theresultisanaverageknowledge
of the position of the individual straws to a level of about 100 lm, which is quite
remarkableconsideringthattheoverallscaleoftheATLASdetectoristensofmeters!
The length of this Foreword is a testament to the large number of areas where
KurtcontributedtotheATLASexperiment:detectoroperations,thebuildingblocks
of physics analysis, and two physics results that were central to the flagship mea-
surements of the ATLAS experiment and the entire LHC physics program. The
dissertationdocumentsthesecontributionsthoroughlyandatalevelthatshouldbe
accessible to beginning graduate students in particle physics.
Philadelphia, USA I. Joseph Kroll
November 2017
Abstract
This thesis presents two diboson measurements with the ATLAS experiment. The
first is a differential measurement of Higgs boson observables in the four-lepton
pffiffi
decay channel at s¼8 TeV. The second presentsa first measurementofthe WZ
pffiffi
dibosonproductioncrosssectioninthethree-leptondecaychannelat s¼13TeV.
Special emphasis is given to the identification techniques of prompt electrons,
particularly the likelihood identification first introduced in 2012 data taking, and
electron efficiency measurements in theATLASdetector.
ix
Preface
This thesis presents my work on ATLAS from July 2010 to March 2016. The
central theme of my work was physics with electrons.
AsanewmemberoftheATLAScollaborationIundertookaspecifictasktoearn
the right to be an author of publication, a process we call “qualification.” My
qualification task involved the alignment of the straws of the Transition Radiation
Tracker, one of three subdetectors that compose the ATLAS Inner Detector (ID).
TheIDisusedtoreconstructthetrajectoriesofchargedparticles.Togetherwiththe
ATLAS superconducting solenoid they form the spectrometer that is used to
determine the trajectories and momentum of charged particles produced in the
collision. The alignment of the TRT detector elements is a key ingredient for
ensuringaprecisemomentummeasurement.Thiswasanaturalplaceformetoget
involved in ATLAS since the ATLAS group at the University of Pennsylvania
played a leading role in the design, production, installation, and commissioning
of the read-out electronics for the TRT.
Theidentificationofelectrons isaprimary task ofanygeneral-purposedetector
at a particle collider. Electrons are used, for example, to identify W and Z bosons
through the decays W !e” and Z !eþe(cid:4) and to search for more massive
hypotheticalpartnersoftheseintermediatevectorbosons.ThePennATLASgroup
has played a leading role in the identification of electrons and photons, in part
because ofthe group’s involvement intheTRT, which plays a critical role in their
identification. InFall 2011,I joined Penn graduatestudents John Alison andChris
LesteronimprovingtheATLASelectronidentification.Webeganwiththegoalof
developingamultivariatestrategytoimproveontherectangularcut-basedselection
in use at the time, and settled on a likelihood construction.
Aroundthistime,hintsofaHiggsbosonsignalaround125GeVwereappearing.
ATLAS and CMS reported modest excesses of events in the 115–130 GeV mass
range in a public seminar in December 2011. By July the following year, the
discovery of the Higgs boson with a mass of around 125 GeV was announced,
energizingthefield.TherelativelylowmassoftheHiggsmeansthatitsdecaysvia
WW(cid:3) and ZZ(cid:3) feature leptons with low transverse momentum (p ), which
T
are harder to discriminate from background (which we call “fake” electrons).
xi
