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Measurement of the Energy Flow at Large Pseudorapidities for /S = 0.9 Measurement of the Energy Flow at Large Pseudorapidities for ps = 0:9 TeV and ps = 7 TeV at the Large Hadron Collider using the Compact Muon Solenoid Dissertation zur Erlangung des Doktorgrades des Department Physik der Universit¨atHamburg vorgelegt von Niladri Sen aus Birkenhead Hamburg 2011 Gutachter/Gutachterin der Dissertation: Prof. Dr. Peter Schleper Priv.-Doz. Dr. Hannes Jung Gutachter/Gutachterin der Disputation: Prof. Dr. Joachim Mnich Priv.-Doz. Dr. Hannes Jung Datum der Disputation: 24.10.2011 Vorsitzende/Vorsitzender des Pr¨ufungsausschusses: Prof. Dr. Caren Hagner Vorsitzender des Promotionsausschusses: Prof. Dr. Peter Hauschildt Dekanin/Dekan des Fachbereichs Physik: Prof. Dr. Heinrich Graener Abstract Energy flow, dE/dη, has been measured in proton-proton collisions at the lhc, for two centre-of-mass energies, ps = 0:9 TeV and 7 TeV, using an integrated luminosity of 239 µb−1 and 206 µb−1 respectively. The measure- ments were made in a previously unexplored phase space (3:15 < η < 4:9) j j using the CMS detector for two separate event topologies: minimum bias events and events with a hard scale set by the transverse momentum of the jets in a di-jet system. Data from each of the measurements have been com- pared to leading order Monte Carlo pp-collision event generators that use 2 2 kT -, Q - and angular-ordered parton showers. The forward energy flow mea- surements are shown to be sensitive to the models and tuning parameters in both their shape and magnitude. The necessity of underlying event models in order to describe data will be demonstrated. In addition, predictions from cosmic-ray event generators are shown to describe data consistently well for each of the measurements. Zuzamfassung In dieser Arbeit wurde der Energiefluss dE/dη in Proton-Proton-Kollisionen des Beschleunigers lhc f¨urzwei verschiedene Schwerpunktsenergien, ps = 0:9 TeV und 7 TeV, mit Hilfe einer integrierten Luminosit¨atvon 239 µb−1 und 206 µb−1 gemessen. Die Messungen wurden in einem zuvor uner- forschten Bereich des Phasenraums (3:15 < η < 4:9) mit Hilfe des CMS De- j j tektors f¨urzwei verschiedene Ereignistopologien durchgef¨uhrt:zum Einen wurden Minimum Bias Ereignisse betrachtet und zum Anderen wurden Zwei-Jet-Ereignisse analysiert, wobei die harte Skala durch den Transver- salimpuls der Jets definiert wurde. Die Daten dieser beiden Messungen wurden mit Monte Carlo pp-Ereignisgeneratoren in fuehrender Ordnung 2 2 verglichen, wobei kT -, Q - und nach Winkeln sortierte Parton Shower Ver- wendung fanden. Diese Analysen zeigten, dass die Verteilungen des En- ergieflusses im Vorw¨artsbereich sensitiv auf die unterschiedlichen physikalis- chen Modelle und die Tuning Parameter sowohl in ihrer Form als auch in ihrer Gr¨oßenordnung reagierten. Es wird gezeigt, dass die Verwendung passender Underlying Event Modelle zur Beschreibung der Daten notwendig sind. Ferner wird aufgezeigt, dass die Vorhersagen von Ereignisgenera- toren f¨urkosmische Strahlung die Daten der einzelnen Messungen konsistent beschreiben. Declaration I herewith declare that I have produced this thesis without the prohibited assistance of third parties and without making use of aids other than those specified; notions taken over directly or indirectly from other sources have been identified as such. This thesis has not previously been presented in identical or similar form to any other German or foreign examination board. The thesis work was conducted from 3rd October 2008 to 30th September 2011 under the supervision of Priv.-Doz. Dr. Hannes Jung at Deutsches- Elektronen Synchrotron. Signed: Date: To my mother and my father, Ranjini and Himadri Sen Contents List of Figures vii List of Tables xi 1 Introduction 1 2 Theoretical Overview 5 2.1 Quantum Chromodynamics . .5 2.1.1 Fundamentals of Quarks and Gluons . .5 2.1.2 Key Aspects of QCD ........................6 2.1.3 Perturbative QCD . .8 2.2 Proton-Proton Collisions . .9 2.2.1 Structure of the Proton . 10 2.2.2 Hard-Scattering Formalism: Factorisation Theorem . 10 2.2.3 Partonic Cross Section . 11 2.2.4 Parton Distribution Functions . 11 2.2.5 Parton Evolution . 12 2.2.6 Underlying Event in Proton-Proton collisions . 14 2.3 Physics in Event Generators . 16 2.3.1 Minimum pT ............................. 16 2.3.2 Parton Showers . 18 2.3.3 Hadronisation and Fragmentation . 18 2.3.4 Multi-Parton Interactions . 19 2.3.4.1 pythia ........................... 22 2.3.4.2 herwig ........................... 23 2.3.5 Brief Description of Monte Carlo Models and their Tunes . 24 iii CONTENTS 2.3.5.1 pythia6:4 ......................... 25 2.3.5.2 Alternative generators . 26 2.3.5.3 Cosmic-ray event generators . 27 2.4 Relevance of the Measurement . 28 2.4.1 Predictions for the lhc ....................... 28 2.4.2 Looking Forward . 30 2.4.3 Energy Flow . 31 2.4.3.1 Previous measurements . 32 2.4.3.2 At the LHC ........................ 33 3 The Experiment 37 3.1 Accelerator . 37 3.1.1 Large Hadron Collider . 37 3.2 Compact Muon Solenoid Detector . 38 3.2.1 Coordinate System . 38 3.2.2 Detector Overview . 39 3.2.3 Specific Components . 40 3.2.3.1 Tracking System . 40 3.2.3.2 Electromagnetic Calorimeter . 42 3.2.3.3 Hadronic Calorimeter . 45 3.2.3.4 Muon System . 46 3.2.4 Forward Calorimeters . 47 3.2.4.1 Hadronic Forward calorimeter . 47 3.2.4.2 CASTOR . 51 3.2.5 Trigger and Data Acquisition . 52 4 Event Simulation and Reconstruction 55 4.1 Event Simulation . 55 4.1.1 Monte Carlo Method . 55 4.1.2 Event Generation . 56 4.1.3 Detector Simulation with geant4 ................. 57 4.2 Event Reconstruction through Physics Object Reconstruction . 58 4.2.1 Physics Object Reconstruction . 58 4.2.2 RecHits and CaloTowers . 58 iv CONTENTS 4.2.3 Particle Flow Reconstruction . 59 4.2.4 Jet Reconstruction . 61 4.2.4.1 Jet Algorithms . 61 4.2.4.2 Jet Types . 63 4.2.4.3 Jet Energy Correction . 63 5 Event Selection & Measurement 65 5.1 Event Selection . 65 5.1.1 Data Collection . 65 5.1.2 Online Trigger Selection . 65 5.1.3 Offline Event Selection . 67 5.1.4 Signature of Events . 67 5.1.5 Di-jet Event Selection Criteria . 68 5.2 Measurement . 71 5.2.1 Technical Issues . 71 5.2.1.1 HF Fibre Response . 71 5.2.1.2 Non-uniformity Effects . 74 5.2.2 Treatment of Pile-up . 75 5.2.3 Energy Flow . 77 5.2.3.1 Energy Flow in Minimum Bias Events . 78 5.2.3.2 Energy Flow in Di-jet Events . 79 6 Correction to Hadron Level 81 6.1 Hadron-level Selection . 81 6.1.1 Impact of Hadron-level Selection . 83 6.1.1.1 Impact of hadron-level selection on correction factors . 84 6.1.1.2 Impact of hadron-level selection on model dependence . 86 6.2 Correction Factors . 88 6.3 Migrations . 89 6.3.1 Single Particle Response . 92 6.3.1.1 Pions . 93 6.3.1.2 Electrons . 94 6.3.1.3 Photons . 94 6.3.2 Overall Effect of Migrations on the Energy Flow . 97 v CONTENTS 7 Systematic Studies 101 7.1 Systematic checks . 101 7.1.1 Consistency between previous and current measurements . 101 7.1.2 Splash Triggers . 101 7.1.3 Vertex Selection . 103 7.1.4 HF Noise Cut-off . 103 7.1.5 Dead Material Effects . 104 7.1.6 Modelling of diffraction . 108 7.1.7 Independence via Least Squares Method . 108 7.1.8 Other Effects . 110 7.2 Systematic Uncertainties . 111 7.2.1 Energy Scale Uncertainty . 111 7.2.2 Primary Vertex z-position . 111 7.2.3 Channel-by-channel Miscalibration . 112 7.2.4 Noise Threshold . 112 7.2.5 Photomultiplier Hits . 112 7.2.6 Non-linearity Effects . 112 7.2.7 Non-uniformity Effects . 113 7.2.8 Fibre Response . 113 7.2.9 Model Dependence . 114 7.2.10 Jet Energy Scale Uncertainty . 115 7.2.11 Total uncertainty . 116 8 Discussion of Results 119 9 Coda: In Through the Out Door 129 9.1 Summary . 129 9.2 Outlook . 132 References 135 vi List of Figures 2.1 Elementary particles of the Standard Model . .6 2.2 Fundamental vertices in qcd ........................7 2.3 Components of a hadron-hadron collision . .9 2.4 hera parton distribution function at nlo ................. 12 2.5 Altarelli-Parisi splitting functions . 13 2.6 Increase in cross-sections due to mpi .................... 15 2.7 Pictographic representation of proton-proton collisions at high energies . 17 2.8 Integrated interaction cross section . 20 2.9 Schematic Representation of Interleaved Parton Showers and Multi-parton Interactions . 23 2.10 Standard Model cross sections at the tevatron and the lhc ...... 29 2.11 lhc parton kinematics . 30 2.12 Forward region . 31 2.13 Transverse Energy Flow in Minimum Bias Events . 34 2.14 Transverse Energy Flow in Dijet Events . 34 3.1 Graphical representation of the experiments at the lhc ......... 38 3.2 cms coordinate system . 39 3.3 Perspective of cms .............................. 40 3.4 Quadrant of cms in the r-z plane . ..
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