Entwicklung von Objekt orientierten C++-Algorithmen zur b-Quark-Identifikation [Elektronische Ressource] = Development of methods for b-quark identification in high energy e_1hn+e_1hn- and pp collisions / vorgelegt von Arno Heister

Diplom Physiker Arno Heister

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2007
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 5 Mo

Extrait

Entwicklung von Objekt orientierten C++
Algorithmen zur b-Quark Identi kation
+(Development of methods for b-quark identi cation in High Energy e e and
pp collisions)
Von der Fakult at fur Mathematik, Informatik und Naturwissenschaften der
Rheinisch-Westf alischen Hochschule Aachen zur Erlangung des akademischen
Grades eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom Physiker Arno Heister
aus Duren-Birk esdorf
Berichter:Universit atsprofessor Dr. St.Schael
J.W.Rohlf, Professor of Physics
Tag der mundlic hen Prufung 16.November 2007
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek
verfugbar.Contents
List of Figures v
List of Tables vii
Introduction 1
1 The Standard Model: Basic concepts and their application 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2.1 The Jade jet algorithm . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2.2 The inclusive k jet algorithm . . . . . . . . . . . . . . . . . . . 6?
1.1.2.3 Jet recombination schemes . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 Identi cation of b-hadrons . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Testing the Standard Model: R . . . . . . . . . . . . . . . . . . . . . . . . . . . 11b
01.2.1 The measurement of R at the Z resonance . . . . . . . . . . . . . . . . 13b
1.2.2 Past and recent R measurements . . . . . . . . . . . . . . . . . . . . . . 14b
1.3 Searching the missing link of the Standard Model: the Higgs . . . . . . . . . . . 16
1.3.1 Status of the Standard Model Higgs search . . . . . . . . . . . . . . . . . 16
1.3.2 Standard Model Higgs searches in pp collisions . . . . . . . . . . . . . . . 18
1.3.2.1 Associated Higgs production in the W H channel . . . . . . . . 19
+2 e e physics 21
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 The LEP electron-positron collider . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 The ALEPH experiment at LEP . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4 The investigation of a new b-tag for ALEPH . . . . . . . . . . . . . . . . . . . . 27
2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4.2 The benchmark b-tag: the ALEPH lifetime mass tag . . . . . . . . . . . 28
2.4.3 Event selection and reconstruction . . . . . . . . . . . . . . . . . . . . . 28
02.4.3.1 The hadronic Z -decay and track selection . . . . . . . . . . . . 28
2.4.3.2 Jet reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4.3.3 Primary vertex reconstruction . . . . . . . . . . . . . . . . . . . 29
2.4.3.4 Secondary vertex . . . . . . . . . . . . . . . . . . 31
2.4.3.5 Association of tracks with the secondary vertex . . . . . . . . . 32
2.4.3.6 Signi cance of the signed 3D impact parameter . . . . . . . . . 33
2.4.4 Description and performance of the new ALEPH b-tag . . . . . . . . . . 34
2.4.4.1 Observables used in the new b-tag . . . . . . . . . . . 34
2.4.4.2 Combination of the observables used in the new ALEPH b-tag . 35
2.4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
iiiContents
3 Hadron collider physics 41
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2 The LHC pp collider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.3 The CMS experiment at LHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3.1 The magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.3.2 The inner tracking system . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.3.3 The muon system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.4 The calorimeter system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3.5 The trigger system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3.6 The luminosity measurement . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.4 b-hadron identi cation in CMS . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.4.1 Full simulation of CMS events . . . . . . . . . . . . . . . . . . . . . . . . 53
3.4.2 Charged track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 54
3.4.2.1 The track reconstruction in the inner tracker . . . . . . . . . . . 55
3.4.2.2 The muon rec . . . . . . . . . . . . . . . . . . . . . 57
3.4.2.3 The electron reconstruction . . . . . . . . . . . . . . . . . . . . 58
3.4.3 Vertex reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.4.3.1 Primary vertex reconstruction . . . . . . . . . . . . . . . . . . . 60
3.4.3.2 Secondary Vertex . . . . . . . . . . . . . . . . . 62
3.4.3.3 Association of tracks with the secondary vertex . . . . . . . . . 62
3.4.3.4 Vertex reconstruction from pixel tracks and signal vertex tag . . 62
3.4.4 Missing energy . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.5 Jet reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.4.5.1 Jet input: split UE TrackTowers . . . . . . . . . . . . . . . . . 65
3.4.5.2 Jet algorithm: inclusive k jet algorithm, E scheme . . . . . . 67? T
3.4.5.3 Jet calibration: 2-dimensional detector map . . . . . . . . . . . 69
3.4.6 b-hadron identi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.4.6.1 Signi cance of the signed 3D impact parameter . . . . . . . . . 72
3.4.6.2 Observables used in the new combined b-tag and its performance 76
3.4.7 Physics Analysis of WH, H! bb . . . . . . . . . . . . . . . . . . . . . . 79
3.4.7.1 L1 and HLT trigger selection . . . . . . . . . . . . . . . . . . . 79
3.4.7.2 W mass constraint . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.4.7.3 Further constraints to improve the event selection . . . . . . . . 80
3.4.7.4 Higgs mass reconstruction performance . . . . . . . . . . . . . . 83
3.4.7.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.4.7.6 Comparison with the ATLAS Physics TDR result . . . . . . . . 84
3.4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4 Conclusions 87
A Analytical solution of the double tag method equations 89
B The object-oriented b-tag framework 91
C The UE curve for the CMS low-luminosity phase 93
D The 2-dimensional detector map jet correction function 95
Bibliography 97
ivList of Figures
1.1 Topology of a b-hadron decay and the layout of the 3D impact parameter. . . . 8
+ 1.2 Feynman graph at Born level of the weak interaction e e ! ff. . . . . . . . . . 11
0 1.3 F graphs of the two dominant Z bb vertex corrections, which introduce
the top mass dependence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
01.4 Feynman graph of the dominant top quark contribution to the Z vacuum polar-
ization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.5 R analysis results from 1993 till 1997 [CERN97]. . . . . . . . . . . . . . . . . . 15b
1.6 Recent R measurements and the world average value [EWWG05]. . . . . . . . . 15b
21.7 Dependency of versus Higgs boson mass m [EWWG06]. . . . . . . . . . . 17H
1.8 The 18 input measurements included in the Standard Model t [EWWG06]. . . 17
1.9 Cross sections and branching ratios of the Standard Model Higgs [SZ97, DKS98]. 18
1.10 Main gluon fusion process incorporating a top quark at Born level. . . . . . . . . 18
1.11 Feynman graph at Born level of the W H production. . . . . . . . . . . . . . . . 19
2.1 LEP experimental setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2 ALEPH detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
0 2.3 event display showing a typical Z ! bb decay. . . . . . . . . . . . . . . 27
2.4 ALEPH lifetime mass tag P . . . . . . . . . . . . . . . . . . . . . . . . . . . 28H; mass
2.5 hemisphere primary vertex and b-hadron jet reconstruction performance. 30
2.6 Signi cance S of reconstructed secondary vertices for ALEPH. . . . . . . . 312ndvtx
2.7 The S observable for tracks which pass the ALEPH QIPBTAG track selection. 33vtd
D2.8 ALEPH signed 3D impact parameter signi cance ( ). . . . . . . . . . . . . . . . 33

2.9 Distributions of P , M , and X for ALEPH. . . . . . . 34jet 2ndvtx 2ndvtx;track;jet energy
2.10 P , PM and PMR b-tag distributions for ALEPH. . . . . . . . . . . . . . 36tag tag tag
2.11 PMRX b-tag distribution for ALEPH. . . . . . . . . . . . . . . . . . . . . . . 37tag
2.12 Comparison of the b-purity versus b-e ciency for the di erent b-tags for ALEPH. 37
2.13 of working points of recent R measurements. . . . . . . . . . . . . 38b
2.14 R versus b-e ciency taken from data only for the PMRX . . . . . . . . . . 39b b tag
3.1 LHC experimental setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2 CMS detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.3 Layout of the CMS inner tracker. . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.4 Layout of the CMS pixel vertex detector. . . . . . . . . . . . . . . . . . . . . . . 46
3.5 r-z view of the CMS muon sy