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
