Springer 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. 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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. 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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