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SEARCH FOR PAIR PRODUCTION OF SCALAR TOP QUARKS IN JETS AND MISSING TRANSVERSE ENERGY CHANNEL WITH THE D0 DETECTOR by MANSOORA SHAMIM Diploma, The Abdus Salam ICTP –

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  • SEARCH FOR PAIR PRODUCTION OF SCALAR TOP QUARKS IN JETS AND MISSING TRANSVERSE ENERGY

    CHANNEL WITH THE D0 DETECTOR

    by

    MANSOORA SHAMIM

    Diploma, The Abdus Salam ICTP – Italy, 2000 M.S., University of the Punjab – Pakistan, 1998

    AN ABSTRACT OF A DISSERTATION

    submitted in partial fulfillment of the requirements for the degree

    DOCTOR OF PHILOSOPHY

    Department of Physics College of Arts and Sciences

    KANSAS STATE UNIVERSITY Manhattan, Kansas

    2008

  • Abstract

    This dissertation describes a search for the pair production of scalar top quarks, t̃1, using a lumi-

    nosity of 995 pb−1 of data collected in pp̄ collisions with the D0 detector at the Fermilab Tevatron

    Collider at a center-of-mass energy √ s = 1.96 TeV. Both scalar top quarks are assumed to decay

    into a charm quark and a neutralino, ( χ̃01), where χ̃ 0 1 is the lightest supersymmetric particle. This

    leads to a final state with two acoplanar charm jets and missing transverse energy. The yield of

    such events in data is found to be consistent with the expectations from known standard model pro-

    cesses. Sets of t̃1 and χ̃01 masses are excluded at the 95% confidence level that substantially extend

    the domain excluded by previous searches. With the theoretical uncertainty on the t̃1 pair produc-

    tion cross section taken into account, the largest limit for mt̃1 is mt̃1 > 150 GeV, for mχ̃01 = 65

    GeV.

  • SEARCH FOR PAIR PRODUCTION OF SCALAR TOP QUARKS IN JETS AND MISSING TRANSVERSE ENERGY

    CHANNEL WITH THE D0 DETECTOR

    by

    Mansoora Shamim

    Diploma, The Abdus Salam ICTP – Italy, 2000 M.S., University of the Punjab – Pakistan, 1998

    A DISSERTATION

    submitted in partial fulfillment of the requirements for the degree

    DOCTOR OF PHILOSOPHY

    Department of Physics College of Arts and Sciences

    KANSAS STATE UNIVERSITY Manhattan, Kansas

    2008

    Approved by:

    Major Professor T. A. Bolton

  • Abstract

    This dissertation describes a search for the pair production of scalar top quarks, t̃1, using a lumi-

    nosity of 995 pb−1 of data collected in pp̄ collisions with the D0 detector at the Fermilab Tevatron

    Collider at a center-of-mass energy √ s = 1.96 TeV. Both scalar top quarks are assumed to decay

    into a charm quark and a neutralino, ( χ̃01), where χ̃ 0 1 is the lightest supersymmetric particle. This

    leads to a final state with two acoplanar charm jets and missing transverse energy. The yield of

    such events in data is found to be consistent with the expectations from known standard model pro-

    cesses. Sets of t̃1 and χ̃01 masses are excluded at the 95% confidence level that substantially extend

    the domain excluded by previous searches. With the theoretical uncertainty on the t̃1 pair produc-

    tion cross section taken into account, the largest limit for mt̃1 is mt̃1 > 150 GeV, for mχ̃01 = 65

    GeV.

  • Table of Contents

    Table of Contents v

    List of Figures viii

    List of Tables xii

    Acknowledgements xv

    Dedication xvi

    1 Overview 1

    2 Theoretical Background 6 2.1 An Overview of Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 The SM Lagrangian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    2.2.1 Fundamentality of the SM . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2.2 Physics Beyond the SM . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    2.3 Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 What is Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.2 Why do we need Supersymmetry . . . . . . . . . . . . . . . . . . . . . 14

    2.4 Formulation of SUSY Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.4.1 Representation of SUSY Algebra . . . . . . . . . . . . . . . . . . . . . 18 2.4.2 Chiral, Anti-Chiral, and Vector Superfields . . . . . . . . . . . . . . . . 19

    2.5 Supersymmetric Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.6 Minimal Supersymmetric Standard Model . . . . . . . . . . . . . . . . . . . . . 22

    2.6.1 The Superpotential and Supersymmetric Interactions in the MSSM . . . . 22 2.6.2 R-Parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.6.3 Soft SUSY Breaking in MSSM . . . . . . . . . . . . . . . . . . . . . . 26

    2.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

    3 The Scalar Top Quark 28 3.1 A Light t̃ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    3.1.1 t̃ Mass Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.2 t̃1 Production at the Tevatron . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.3 Decay Modes of t̃1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.4 t̃1 → cχ̃01 Decay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.5 Current Experimental Limits on t̃1 Mass . . . . . . . . . . . . . . . . . . . . . . 35

    v

  • 4 Experimental Apparatus 38 4.1 Fermilab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.2 Fermilab Accelerator Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

    4.2.1 Pre-accelerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.2.2 Linac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.2.3 Booster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.2.4 Main Injector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.2.5 Tevatron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

    4.3 D0 Coordinate System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.4 The D0 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    4.4.1 Central Tracking System . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.4.2 Preshower Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.4.3 Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.4.4 Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.4.5 Luminosity Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    4.5 Trigger and Data Acquisition System . . . . . . . . . . . . . . . . . . . . . . . . 51

    5 Object Identification and Event Reconstruction 54 5.1 Tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

    5.1.1 Isolated Tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5.2 Primary Vertex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.3 Electron and Photon Identification . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.4 Muon Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.5 Jet Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

    5.5.1 Jet Energy Scale Correction . . . . . . . . . . . . . . . . . . . . . . . . 61 5.6 Missing Transverse Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

    6 Data and Simulated Samples 63 6.1 Simulated Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

    6.1.1 Generation of Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.1.2 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.1.3 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

    6.2 SM Background Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.3 Data Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 6.4 Simulated Samples for t̃1 Signal . . . . . . . . . . . . . . . . . . . . . . . . . . 74

    7 Z/γ∗(→ e+e−) + jets Study for Background Normalization in t̃1 Search 77 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 7.2 Z/γ∗(→ e+e−) + jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

    7.2.1 Data Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.2.2 MC Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.2.3 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 7.2.4 Z pT Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

    vi

  • 7.2.5 Z Boson pT Reweighting . . . . . . . . . . . . . . . . . . . . . . . . . . 83 7.3 QCD Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 7.4 W Boson pT Reweighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

    8 t̃1 → cχ̃01 Analysis 91 8.1 Data Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 8.2 Background Normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

    8.2.1 V+Jets Background Normalization . . . . . . . . . . . . . . . . . . . . . 92 8.2.2 Normalization of Z(→ νν̄)+ Jets Events . . . . . . . . . . . . . . . . . 93 8.2.3 Normalization of W (→ `ν)+ Jets Events . . . . . . . . . . . . . . . . . 95 8.2.4 Normalization of tt̄, Diboson and Single Top Backgrounds . . . . . . . . 95

    8.3 Kinematical Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 8.4 Background Estimat

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