Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Daniel Salerno
The Higgs Boson
Produced With
Top Quarks in Fully
Hadronic Signatures
Springer Theses
Recognizing Outstanding Ph.D. Research
Aims and Scope
The series “Springer Theses” brings together a selection of the very best Ph.D.
theses from around the world and across the physical sciences. Nominated and
endorsed by two recognized specialists, each published volume has been selected
foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield
of research. For greater accessibility to non-specialists, the published versions
includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor
explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill
provide a valuable resource both for newcomers to the research fields described,
and for other scientists seeking detailed background information on special
questions. Finally, it provides an accredited documentation of the valuable
contributions made by today’s younger generation of scientists.
Theses are accepted into the series by invited nomination only
and must fulfill all of the following criteria
(cid:129) They must be written in good English.
(cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences,
Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience,
Chemical Engineering, Complex Systems and Biophysics.
(cid:129) The work reported in the thesis must represent a significant scientific advance.
(cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis
must be gained from the respective copyright holder.
(cid:129) They must have been examined and passed during the 12 months prior to
nomination.
(cid:129) Each thesis should include a foreword by the supervisor outlining the signifi-
cance of its content.
(cid:129) The theses should have a clearly defined structure including an introduction
accessible to scientists not expert in that particular field.
More information about this series at http://www.springer.com/series/8790
Daniel Salerno
The Higgs Boson Produced
With Top Quarks in Fully
Hadronic Signatures
Doctoral Thesis accepted by
ü
the University of Zurich, Z rich, Switzerland
123
Author Supervisor
Dr. DanielSalerno Prof. Dr. FlorenciaCaneli
Department ofPhysics Department ofPhysics
University of Zurich University of Zurich
Zürich,Switzerland Zürich,Switzerland
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-31256-5 ISBN978-3-030-31257-2 (eBook)
https://doi.org/10.1007/978-3-030-31257-2
©SpringerNatureSwitzerlandAG2019
Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart
of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,
recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission
orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar
methodologynowknownorhereafterdeveloped.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this
publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom
therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse.
The publisher, the authors and the editors are safe to assume that the advice and information in this
book are believed to be true and accurate at the date of publication. Neither the publisher nor the
authors or the editors give a warranty, expressed or implied, with respect to the material contained
hereinorforanyerrorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregard
tojurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations.
ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG
Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland
’
Supervisor s Foreword
The discovery of the Higgs boson by the CMS and ATLAS collaborations at the
LargeHadronCollider(LHC)hasopened uptheway forexploringitsinteractions
with other particles. Thanks to an unprecedented luminosity at the LHC, enough
Higgsbosonshavebeenproduced,andallofitsmainproductionanddecaymodes
havenowbeenobserved.Nonetheless,somepropertiesoftheHiggsbosonhavenot
yetbeenestablished.ThemaingoalofthisPh.D.thesishasbeentheelucidationof
one of these properties, the tree-level coupling of Higgs bosons to top quarks.
In the Standard Model, the Higgs boson couples to fermions in a manner
describedby theYukawa sector, with a coupling strength proportional tothe mass
of the fermions. For the heaviest known fundamental fermion, the top quark, the
Yukawa coupling can be measured in a model-independent way in the associated
(cid:1)
productionofaHiggsbosonandatopquark-antiquark(tt)pair,inaprocesscalled
“t(cid:1)tH production”. This measurement is usually pursued in the different decay
(cid:1)
channels of the Higgs boson and of the tt pair. In particular, the fully hadronic
channel, is the main topic of Daniel Salerno’s Ph.D. thesis. In this signature the
Higgsbosondecaysintoabottomquark-antiquarkpairandthetopquarkpairdecays
onlytojets,leavingatotaloffourjetswhichcontainthedecayproductsofBhadrons
(cid:1)
(b jets). Eventhough this isthemost commondecaysignature for the ttH process,
the large number of jets from the decays makes the process challenging to
unequivocallyreconstructandthehugecontaminationof“irreducible”backgrounds
with signatures very similar to the t(cid:1)tH signal makes it difficult to extract from the
data.
In Chap. 1 the author provides a summary of the Standard Model (SM) of
particle physics and the theoretical background which includes a review of the
Higgs mechanismandthephenomenologyoftheHiggsbosonattheLHC,aswell
as presenting an up-to-date summary of recent measurements of Higgs boson
production and properties. A description of the LHC accelerator and CMS exper-
iment is given in Chap. 2. In Chap. 3 the reconstruction of the proton-proton
collisions recorded by the CMS experiment is described, with emphasis on the
final-state physics objects used inthe analysis. To enrich the t(cid:1)tH signal,the author
developed dedicated trigger algorithms essential in the extraction of the
v
vi Supervisor’sForeword
(cid:1)
fully-hadronicttHeventsfromtheproton-protoncollisiondata.Detailsofthiswork
are also described in Chap. 3. Chapter 4 is dedicated to describing the imple-
(cid:1)
mentation of the matrix-element approach to aid discrimination between ttH and
SM background events. This is a crucial component of the thesis as the large
numberofjetsinthesignatureleadstoahugenumberofpossiblecombinationsfor
(cid:1)
specifying ttH and thus an impractical computational time. Chapter 5 concentrates
on the analysis strategy from data collection and the Monte Carlo (MC) samples
used to the calculation of the systematic uncertainties. In particular, Chap. 5
describes the implementation of a new technique to estimate and reduce the
strongly produced QCD multijet background based on the quark-gluon discrimi-
nationofjets,whichenablesdifferentiationbetweengluon-richbackgroundevents,
and quark-rich t(cid:1)tH signal events. The final results are presented in Chap. 6 after a
detailed explanation of the statistical treatment used to extract the results. This
chapteralsoincludesacomparisonwithpreviousresultsbyCMSincomplementary
channelsandbytheATLASexperiment.Chapter7isreservedforconclusions.The
finalChap.8isanoutlooksummarizingthepossibleimprovementsandsensitivity
reach of the analysis for future LHC datasets.
ThetopicsstudiedinDanielSalerno’sthesiscoveravastarrayofinvestigations
and creative developments. They are described in a detailed, complete, and
accessible manner and I believe this work will be useful for beginners and experts
alike in the area offundamental science.
Zürich, Switzerland Prof. Dr. Florencia Canelli
July 2019
Abstract
I present my work at the CMS experiment on a search for the standard model
(SM) Higgs boson produced in association with top quarks. The search is targeted
towardsfinalstatescompatiblewiththeH!b(cid:1)bdecayandthefullyhadronicdecay
(cid:1)
channel of the tt pair, and uses data from proton-proton collisions at a
centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of
35.9 fb(cid:2)1.Thisisachallengingsearchwithmanyfinalstateparticlesthatcannotbe
uniquelyidentifiedandwithlargecontaminationfromSMbackgroundprocesses.It
pffiffi
isperformedforthefirsttimeatCMSandthefirsttimeanywhereat s¼13TeV,
(cid:1)
and contributes to the overall sensitivity of the ttH cross-sectional measurement,
which constitutes a crucial test of the SM.
TheCMSapparatusisamultipurposedetectoroperatingattheLHC,whichisa
hadron collider at CERN. The CMS detector operates a 3.8 T superconducting
solenoid, and includes dedicated subsystems for charged particle tracking near the
interaction point, measurements of electromagnetic and hadronic energy deposits,
and muon tracking outside the solenoid, all of which provide nearly 4… coverage.
The trigger and data acquisition system of CMS, to which I made original contri-
butions,efficientlyreducestheeventratefromthe40 MHzcollisionratetoaround
1 kHz for permanent storage and offline analysis. I also developed dedicated jet
(cid:1)
based triggers for the fully hadronic ttH search.
I performed all aspects of the search, making original contributions to all
techniquesandmeasurementsspecifictoit.Ajetbasedquark-gluondiscriminatoris
used in an event-based likelihood ratio for the first time in a CMS search to
differentiate between events containing jets originating from light-flavour quarks
andeventscontainingjetsfromgluons.Auniquemethodtoestimatethedominant
QCD multijet background from data is developed. Selected events with 7 or more
jetsand3ormoreb-taggedjetsareallocatedtooneof6categoriesbasedonjetand
b-tag multiplicity, with different levels of signal purity.
Amatrixelementmethod(MEM)isusedforoptimaldiscriminationbetweenthe
(cid:1)
ttH signal and SM background processes and for the ultimate signal extraction. It
assigns a signal and background probability density to each event using the full
vii
viii Abstract
event information and leading order matrix amplitudes of the t(cid:1)tH and t(cid:1)tþb(cid:1)b pro-
cesses. It sums over all combinations of jet-quark associations to reduce the
uncertaintyofmatchingthecorrectpairs,anditintegratesoverpoorlymeasuredor
missing variables. A likelihood ratio of these two probability densities is used to
formthefinalMEMdiscriminant.ThisisthefirsttimethattheMEMhasbeenused
in a fully hadronic final state at CMS.
(cid:1)
TheresultsareinterpretedviaanobservedttHsignalstrengthrelativetotheSM
cross section under the assumption of m ¼125GeV, i.e. l¼r=r . A binned
H SM
maximum likelihood fit is performed to the MEM discriminant in all categories to
extractabest-fitvalueofl^ ¼0:9(cid:3)1:5.ThisiscompatiblewiththeSMprediction
of l¼1, and corresponds to observed and expected significances of 0.6 and 0.7
standard deviations, respectively. Under the background-only hypothesis, upper
exclusion limits on the signal strength of l\3:8 and l\3:1 are observed and
expected, respectively, at the 95% confidence level.
Acknowledgements
First and foremost, I would like to express my deepest gratitude to my supervisor,
Prof. Dr. Florencia Canelli, for giving me the opportunity to work on such an
impressivesearchwithinahighlyesteemedcollaborationattheforefrontofparticle
physicsresearch.Herguidancehasbeeninstrumentalingettingmetothispoint,in
developing my skills as a physicist, and in bringing this complex search to
completion.
I would also like to express my sincere gratitude and appreciation to Dr. Lea
Caminadawhohassupportedmethroughoutthisendeavourwithallaspectsofthe
search and physics research in general.
I would like to thank the members and former members of ETH, with whom I
sharedaclosecollaborationforthedurationofmystudies.Inparticular,mydeepest
thanks to Dr. Lorenzo Bianchini for not only introducing me to data analysis at
CMS, but also for his development of the matrix element algorithm used in the
search. I would also like to express my gratitude to Joosep Pata, for his develop-
ment of the framework used in the search and his support in all technical aspects
of the search.
Iwouldliketothankmyuniversitycolleagues,especiallyDr.SilvioDonatofor
his guidance and support and Korbinian Schweiger for his contribution to the
search. In addition, a special thank you to my office mates Deborah Pinna and
Camilla Galloni, who created the most enjoyable working environment during my
studies.
Last but by no means least, I would to like express my deepest gratitude to
Valentina Zingg for her unconditional love and support throughout my studies.
ix
