Springer Theses Recognizing Outstanding Ph.D. Research Francesco Pandolfi Search for the Standard Model Higgs Boson in the H  ZZ  (cid:2)+ (cid:2)− qq Decay Channel at CMS Springer Theses Recognizing Outstanding Ph.D. Research For furthervolumes: http://www.springer.com/series/8790 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 for its scientific excellence and the high impact of its contents for the pertinent fieldofresearch.Forgreateraccessibilitytonon-specialists,thepublishedversions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on specialquestions.Finally,itprovidesanaccrediteddocumentationofthevaluable 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 • They must be written in good English. • ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineering andrelatedinterdisciplinaryfieldssuchasMaterials, Nanoscience, Chemical Engineering, Complex Systems and Biophysics. • The work reported in the thesis must represent a significant scientific advance. • Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. • They must have been examined and passed during the 12 months prior to nomination. • Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. • The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. Francesco Pandolfi Search for the Standard Model Higgs Boson in the H ! ZZ ! ‘þ‘(cid:2)qq Decay Channel at CMS Doctoral Thesis accepted by Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy 123 Author Supervisor Dr. Francesco Pandolfi Dr. Danieledel Re CERN Dipartimentodi Fisica&INFN Sapienza Università diRoma Sapienza Università diRoma Rome Rome Italy Italy ISSN 2190-5053 ISSN 2190-5061 (electronic) ISBN 978-3-319-00902-5 ISBN 978-3-319-00903-2 (eBook) DOI 10.1007/978-3-319-00903-2 SpringerChamHeidelbergNewYorkDordrechtLondon LibraryofCongressControlNumber:2013942123 (cid:2)SpringerInternationalPublishingSwitzerland2013 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionor informationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purposeofbeingenteredandexecutedonacomputersystem,forexclusiveusebythepurchaserofthe work. Duplication of this publication or parts thereof is permitted only under the provisions of theCopyright Law of the Publisher’s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the CopyrightClearanceCenter.ViolationsareliabletoprosecutionundertherespectiveCopyrightLaw. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexempt fromtherelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. While the advice and information in this book are believed to be true and accurate at the date of publication,neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityfor anyerrorsoromissionsthatmaybemade.Thepublishermakesnowarranty,expressorimplied,with respecttothematerialcontainedherein. Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) We shall not cease from exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time. T. S. Eliot For Flavio, my nova Supervisor’s Foreword Particle physicists have been searching the Higgs boson since it was firstly suggested in 1964. This particle was the last missing ingredient of the so-called Standard Model of Particle Physics. Any particle which interacts with the Higgs field acquires a mass. The stronger the interaction with the field is, the larger the massoftheparticle.BeforetheoperationsoftheLargeHadronCollider(LHC),no experimental evidence of this hypothetical particle confirmed this theory. The CMS and ATLAS experiments at the LHC have been designed to give a final answeronthis,eitherdiscoveringorrejectingthepresenceofthiselusiveparticle. On the 4th of July of last year everything changed. CMS and ATLAS announced the discovery of a new boson with a mass of about 125 GeV. Its properties, as the production and decay rates, the spin and the parity look com- patible with the particle predicted by Peter Higgs. This crucial discovery was the result of years of analysis optimization, tuning of Monte Carlo simulations, and precise calibrations of the detectors. To reach this important goal, several prepa- ratory studies have been performed and the experiments worked in restricting the mass range where this state could fall. For this purpose, physicists used channels which have larger production rates but are less clean. Among these modes, the decayH!ZZ!2l2jetsresultedtobeoneofthemostchallengingandpowerful channelstoinvestigatethepresenceofaHiggsbosonwithmasseslargerthan200 GeV. Compared to the standard H!ZZ!4l channel, it is 20 times more fre- quentbutthe presenceofjetsinthefinalstatemakesitverycomplicated because of the large expected background contributions. Francesco Pandolfi started from the idea that this channel could play a major roleinCMS.He worked on thesimulation toverify itsfeasibility,identifying the most relevant issues for this analysis. They can be divided into two big areas: the determinationofthejetenergyscaleandresolutionandtheimplementationofjet- basedexperimentaltechniquestoimprovethesensitivityofthemeasurement.This thesis reflects the huge amount of work needed to deal with these two issues. It summarizes the work needed to reduce the jet energy calibration uncertainties down to 1 % level and gives details on jet calibration developed by Francesco, whichusedsamplesofthegamma?jeteventsandwascrucial atthestartofLHC collisions. In addition, the thesis describes novel techniques, as the analysis tool implemented to distinguish between jets originated by quarks and gluons. After ix x Supervisor’sForeword Francesco’s implementation, they have been later inherited by other analyses of the CMS experiment and they will become even more important in the future. In summary, Francesco’s thesis is an excellent piece of work, a paradigm for studiesoftheHiggsbosonwithfinalstateswithjets.Thenonexpertphysicistswill enjoyacompleteandmarvelouslyreadabledescriptionofaproton–protoncollider analysis. This is not common for a thesis at the LHC, given the average level of complexityoftheanalyses.Atthesametime,theexpertanalyzerwilllearnabout searchesdonewithjetsatCMSandwillbestimulatedtofurtherdevelopthenovel techniques described. 12 March 2013, Rome, Italy Daniele del Re Preface The standard model of elementary particles is one of the most successful scien- tificaltheorieselaboratedbymankind.Throughoutthepastcentury,itspredictions have been confirmed with astonishing precision by numerous experiments, in conceptually distant fields and with a variety of experimental techniques. Its theoretical foundations, though, depended on a particle which had not been discovered. ThemassoftheHiggsbosonisnotconstrainedbythetheory,andisallowedto vary in a wide range. Numerous experiments have unsuccessfully pursued its search,buthavebeenableonlytoexcludeitsexistenceincertainmassranges.The analysesconductedatthedetectorsoperatingattheLEPcollideratCERNhaveset a lower bound of 114.4 GeV, whereas the searches performed at the Tevatron at Fermilab have excluded the 156–177 GeV range. The Large Hadron Collider (LHC) is the particle accelerator which has been built with the aim of producing definitive proof regarding the Higgs boson’s existence.Itisasuperconductingprotoncollider,withacenterofmassenergyof7 TeV. It has the capability of spanning a wide energy range, up to the TeV scale. TheCompactMuonSolenoid(CMS)isoneofthefourmainexperimentswhich analyzesthecollisionsproducedattheLHC.Itisageneral-purposedetectorwhich has been designedinorder tomaximize itsperformanceinHiggsbosonsearches. The discovery of a Higgs boson depends on its decay products, as it is an unstable particle. If its mass is large enough, the decay to pairs of electroweak vector bosons dominates the particle’s decay channels. The production of a Z boson pair, in particular, constitutes the most promising final state in search- orientedanalyses.TherequirementthatatleastoneofthetwoZbosonsdecaystoa light charged lepton pair, in fact, significantly reduces the possible sources of background at a hadron collider. On July 4, 2012, the CMS and ATLAS experiments have both reported evi- dence for a narrow resonance with mass close to 125 GeV, with properties compatible with those of a Higgs boson. A crucial role in this discovery was played by the fully leptonic decay channel H!ZZ!4‘. In this thesis, we have conducted a search for a heavy Higgs boson in the H!ZZ!‘þ‘(cid:2)qqdecaychannel,with4.6fb-1ofdatacollectedbytheCompact Muon Experiment. The presence of jets in the final state poses a series of xi