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QCD Studies at L3 QCD Studies at L3 Swagato Banerjee Tata Institute of Fundamental Research Mumbai 400 005 2002 QCD Studies at L3 Swagato Banerjee Tata Institute of Fundamental Research Mumbai 400 005 A Thesis submitted to the University of Mumbai for the degree of Doctor of Philosophy in Physics March, 2002 To my parents Acknowledgement It has been an extremely enlightening and enriching experience working with Sunanda Banerjee, supervisor of this thesis. I would like to thank him for his forbearance and encouragement and for teaching me the different aspects of the subject with lots of patience. I thank all the members of the Department of High Energy Physics of Tata Institute of Fundamental Research (TIFR) for providing various facilities and support, and ideal ambiance for research. In particular, I would like to thank B.S. Acharya, T. Aziz, Sudeshna Banerjee, B.M. Bellara, P.V. Deshpande, S.T. Divekar, S.N. Ganguli, A. Gurtu, M.R. Krishnaswamy, K. Mazumdar, M. Maity, N.K. Mondal, V.S. Narasimham, P.M. Pathare, K. Sudhakar and S.C. Tonwar. Asesh Dutta, Dilip Ghosh, S. Gupta, Krishnendu Mukherjee, Sreerup Ray Chaudhuri, D.P. Roy, Sourov Roy, Gavin Salam and K. Sridhar provided valuable theoretical insight on various occasions. My heartfelt thanks to them. I thank the European Organisation for Nuclear Research (CERN) for its kind hospitality and all the members of the Large Electron Positron (LEP) accelerator group for successful operation over the last decade. The members of the L3 collaboration and the LEP QCD Working Group provided me a stim- ulating working environment. In particular, I thank Pedro Abreu, Satyaki Bhattacharya, Gerard Bobbink, Ingrid Clare, Robert Clare, Glen Cowan, Aaron Dominguez, Dominique Duchesneau, John Field, Ian Fisk, Roger Jones, Mehnaz Hafeez, Stephan Kluth, Wolfgang Lohmann, Wes- ley Metzger, Peter Molnar, Oliver Passon, Martin Pohl, Subir Sarkar, Chris Tully and Micheal Unger. I thank all my friends who helped me on many occasions with various issues, and made my graduate days worthwhile and interesting. Finally, I thank my parents for their love, understanding, patience and encouragement, without which this thesis would have never been completed. Contents List of Figures iv List of Tables viii Synopsis i Overview x 1 A Brief History of QCD: from Pions to Jets 1 1.1 The Illustrious 1947 . 1 1.2 1947 onwards . 2 1.2.1 The Theory of Strong Interaction . 3 1.2.2 QCD with e−e+ Annihilations . 4 2 Jets, Shapes and Asymmetries 10 2.1 Jets in e−e+ Annihilation . 10 2.2 Global Event Shape Variables . 12 2.2.1 Relative Contributions from n-jet Configurations . 14 2.2.2 αs from Event Shapes . 14 2.3 Inclusive Momenta Variables . 16 2.3.1 Theoretical Predictions for ξ Distribution . 16 2.4 Gaps in e−e+ Annihilations . 18 2.4.1 Particles and Angles . 18 2.4.2 Octet vs Singlet . 20 3 e−e+ Annihilation at LEP 23 3.1 Introduction . 23 3.2 Large Electron Positron Collider (LEP) . 24 3.2.1 Acceleration Stages . 25 3.2.2 LEP Specifications . 26 3.2.3 Energy Record at LEP . 28 4 L3: Apparatus and Data Taking 31 4.1 L3 Detector . 31 4.1.1 Silicon Microvertex Detector . 33 4.1.2 Time Expansion Chamber . 33 4.1.3 Z Chambers . 34 4.1.4 Forward Tracking Chambers . 34 4.1.5 Electromagnetic Calorimeter . 35 4.1.6 Gap Calorimeter . 37 4.1.7 Scintillation Counters . 38 4.1.8 Hadron Calorimeter . 38 4.1.9 Muon Chambers . 40 4.2 Energy Resolution of L3 . 42 4.3 Trigger & DAQ . 42 4.4 Data Flow . 44 5 The Process: e−e+ γ=Z? hadrons 48 ! ! 5.1 L3 Data . 49 5.2 Modelling the Process . 50 5.3 Monte Carlo Tuning . 52 5.3.1 Tuning of Herwig 5.9 and Jetset 7.4 . 54 5.4 Energy Flow in Hadronic Events . 60 5.4.1 Event Reconstruction . 60 5.4.2 Clusters in the L3 Detector . 61 5.4.3 Energy Calibration . 63 5.4.4 Performance of ASRC and ECLU . 63 6 Hadronic Event Selection at ps > MZ 70 6.1 Hadronic Event Selection . 71 6.2 ISR Background . 73 6.2.1 Photons Collinear with Beam Direction . 74 6.2.2 Energetic γ Seen in the Detector . 76 6.3 W−W+ Background . 78 6.3.1 Semileptonic Decays: W−W+ qqlν . 78 ! 6.3.2 Hadronic Decays: W−W+ qqqq . 79 ! 6.4 Event Statistics . 80 7 Systematics and Corrections 85 7.1 Backgrounds at Detector Level . 85 7.2 Corrections to Event Shape Distributions . 86 7.2.1 Corrected Distributions . 87 7.3 Systematic Studies . 87 7.3.1 Sources and Estimates . 87 ii 7.3.2 Statistical Component of Systematic Error . 88 7.3.3 Combining the Errors . 89 8 Aspects of Hard Gluon Radiation 91 8.1 Jet Rates . 91 8.2 Event Shape Variables . 92 8.2.1 Energy Evolution . 93 2 8.3 Resummed LL & NLL + (αs ) . 96 O 8.4 αs from Event Shape Distributions . 97 8.5 Energy Evolution of αs . 100 8.5.1 One Parameter Fit . 101 8.5.2 Simultaneous Fit for nf . 101 8.6 Power Law Behaviour . 102 9 Aspects of Soft Gluon Radiation 111 9.1 Good Tracks . 111 9.2 Charged Particle Multiplicity. 112 9.3 Inclusive Momentum Spectra . 113 10 Search for Colour Singlet Exchange 119 10.1 Introduction . 119 10.2 L3 Data . 119 10.3 Flower Plots and Angle Rescaling . ..
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