Associated Production of W + Charm in 13 TeV Proton-Proton Collisions Measured with CMS and Determination of the Strange Quark Content of the Proton PDF
Preview Associated Production of W + Charm in 13 TeV Proton-Proton Collisions Measured with CMS and Determination of the Strange Quark Content of the Proton
Springer Theses Recognizing Outstanding Ph.D. Research Svenja Karen Pflitsch Associated Production of W + Charm in 13 TeV Proton-Proton Collisions Measured with CMS and Determination of the Strange Quark Content of the Proton 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. 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More information about this series at http://www.springer.com/series/8790 fl Svenja Karen P itsch Associated Production of W + Charm in 13 TeV Proton-Proton Collisions Measured with CMS and Determination of the Strange Quark Content of the Proton Doctoral Thesis accepted by Deutsches Elektronen Synchrotron, Hamburg, Germany 123 Author Supervisor Dr. SvenjaKaren Pflitsch Dr. Katerina Lipka Particle Physics Particle Physics Deutsche Elektronen-Synchrotron DESY Deutsches ElektronenSynchrotron Hamburg,Germany Hamburg,Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-52761-7 ISBN978-3-030-52762-4 (eBook) https://doi.org/10.1007/978-3-030-52762-4 ©TheEditor(s)(ifapplicable)andTheAuthor(s),underexclusivelicensetoSpringerNature SwitzerlandAG2020 Thisworkissubjecttocopyright.AllrightsaresolelyandexclusivelylicensedbythePublisher,whether thewholeorpartofthematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseof illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmissionorinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilar ordissimilarmethodologynowknownorhereafterdeveloped. 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 structure of the nucleon is one of the most fundamental topics of particle physics. The mass of the nucleon, and therefore of all nuclear matter in the uni- verse, has its origin in the strong interaction of the elementary constituents of the nucleon—the quarks and the gluons. This interaction is described by the quantum field theory Quantum Chromodynamics (QCD). Studying the details of the proton structure gainshighestattentionin thecontextoftheinterpretation oftheStandard Model measurements and searches for New Physics at the CERN LHC. InQCD,thequantumpropertiesofthenucleonareascribedtothethreevalence quarks,whiletheenergyofthegluonsandtheseaquarksisresponsibleforitsmass. The proton structure is expressed in terms of universal Parton Distribution Functions(PDFs),representingprobabilitiestofindaparton(quarkorgluon)inthe proton,carryingafractionxofismomentumataparticularenergyscale.Whilethe scale dependence of the PDFs can be calculated in perturbation theory using QCD evolution equations, their x-dependence has to be extracted from the experimental measurements. The investigation of the flavour decomposition of the quark sea represents a major experimental challenge. In particular, the accurate knowledge of the strange quark content of the proton is crucial for determination of the electroweak parameters of the Standard Model in proton-proton collisions at the LHC. Before the LHC era, constraints on the strange-quark distribution were obtained from neutrino scattering experiments. These measurements probe the (anti)strange-quark density at x (cid:1)10(cid:3)1 and scales of approximately 10 GeV2, but their interpretation is complicated by nuclear corrections and uncertainties in the charm-quark fragmentation function. Production of electroweak bosons, W and Z, in proton-proton collisions at the LHC has complimentary sensitivity to the strange quark distribution at the scale ofthebosonmass,extendingprobedxto(cid:1)10(cid:3)3.Inthelastdecade,indirectprobes of strange quark at the LHC using inclusive W and Z boson production lead to controversial interpretations. While the experimental measurements are very pre- cise, their QCD interpretation is nontrivial, since quark combinations of different flavours are probed. In contrast, the measurements of associated W + charm v vi Supervisor’sForeword production in proton-proton collisions have great potential to access the strange-quark distribution directly through the leading order QCD process gþs!Wþc. Dr. Pflitsch has performed the measurement of associated production of W + charm in proton-proton collisions at the LHC using the data collected by the CMS detector at a center-of-mass energy of 13 TeV. Quarks can’t be observed as free particles, and the particular challenge of this work is the charm quark identi- fication. In the work by Svenja Pflitsch, the charm quarks are tagged via their hadronisation into D* mesons, which are fully detected in decay channel D(cid:4) !D0þ…,basedonthetrackreconstructiondowntothetransversemomentaof 0.5GeV.Thischannelprovidesthemostcleanexperimentalsignature.Forthefirst time, it was possible to measure W + charm cross section precisely using the full meson reconstruction. Using the results of her measurements, Svenja Pflitsch has performedtheglobalQCDanalysiswiththePDFdeterminationhavinginvestigated different variants of the parameterization. This way, the strange content of the proton is probed directly, providing unambiguous results and resolving the con- troversy in the field. The analysis strategy used by Svenja Pflitsch provides new insights into the flavour decomposition of the proton quark sea and is a major step towards better understanding of the matter structure. Hamburg, Germany Dr. Katerina Lipka April 2020 Abstract (cid:5) ThisthesispresentsthemeasurementofassociatedproductionofaW bosonanda charm quark (Wþc) in proton-proton collisions at a center-of-mass energy of 13TeV.ThedatausedinthisanalysishasbeenrecordedbytheCMSexperimentat theCERNLHCandcorrespondstoanintegratedluminosityof35.7fb(cid:3)1.TheW(cid:5) bosons are reconstructed by the presence of a muon and a neutrino, with the latter indicated by the missing transverse momentum in an event. Charm quarks are identified by the full reconstruction of D(cid:4)ð2010Þ(cid:5) mesons decaying via D(cid:4)ð2010Þ(cid:5) !D0þ…(cid:5) !K(cid:6)þ…(cid:5)þ…(cid:5). The fiducial phase space of the mea- surement is defined by the muon transverse momentum pl[26GeV, muon T pseudorapidity jgl j \2:4 and the charm quark transverse momentum pc [ 5GeV. The measurement is performed inclusively and differentially as a T (cid:5) functionoftheabsolutepseudorapidityofthemuonfromtheW bosondecay.The results are compared to theoretical predictions using different PDF sets. AsubsequentQCDanalysisisperformedtoextractthestrangequarkcontentofthe proton and assess possible improvements in the uncertainties associated with the distribution by including the new measurement. The extracted strange quark dis- tributioniscomparedtodistributionsobtainedinglobalPDFfits,whichutilizethe results of neutrino scattering experiments. vii Acknowledgements Iwanttothankmysupervisor,Dr.KaterinaLipka,forgivingmetheopportunityto work on this interesting topic which had “something to do with Ws” and beyond. Thankyoufortheadvice,discussionsandsupport,whichmadeitpossiblethatthe results of this thesis could already be presented at several conferences and were published in a scientific journal. I am grateful for the time you took to help me improve my research, as well as my presentation skills. ThankyoutoProf.Dr.Sven-OlafMochforreviewingthisthesisandbeingpart of my defence committee. I would also like to express my gratitude to the other membersofmydefencecommittee,Prof.Dr.GünterSigl,Prof.Dr.ElisabettaGallo and Dr. Hannes Jung. ThankyoutomyofficematesEnginEren,MykolaSavitskyiandTillArndt,who were always up for a good discussion, be it about physics, politics, or movies. Thank you to Benoit Roland for the many discussions about Wþc and Paolo Gunnellini for everything related to MC. I also want to thank my colleagues at DESY who always took thetimeto give advice when it was needed and generally creatingagreat working atmosphere.Inthiscontext,Iwouldliketogive a special thanks to the QCD group. And finally, another special thanks to my family for their continuous moral support and patience. ix Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 Theoretical Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 The Standard Model of Particle Physics. . . . . . . . . . . . . . . . . . . . 3 2.1.1 The Electromagnetic Interaction . . . . . . . . . . . . . . . . . . . . 5 2.1.2 The Weak Interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3 Electroweak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.4 The Strong Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 The Proton Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.1 Deep Inelastic Scattering . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.2 Quark Parton Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2.3 Parton Distribution Functions and Factorization. . . . . . . . . 15 2.2.4 Evolution of PDFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2.5 Strange Quark Content of the Proton . . . . . . . . . . . . . . . . 18 2.3 Theoretical Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3.1 Matrix Element Calculations: MADGRAPH, POWHEG, MCFM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3.2 Parton Shower Description. . . . . . . . . . . . . . . . . . . . . . . . 29 2.3.3 Fragmentation and Hadronization . . . . . . . . . . . . . . . . . . . 31 2.3.4 Underlying Event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.3.5 Detector Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.4 QCD Analysis Tool: xFitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.4.1 Heavy Quark Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.4.2 Methods of Error Estimation in xFitter . . . . . . . . . . . . . . . 36 2.4.3 Fast Grid Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 xi
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