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An experimental and phenomenological dissection of beauty-quark decays into light leptons


Lancierini, Davide. An experimental and phenomenological dissection of beauty-quark decays into light leptons. 2023, University of Zurich, Faculty of Science.

Abstract

Table of contents
List of figures ix
List of tables xvii
1 Introduction 1
1.1 History of the Standard Model..................................................................... 2
1.1.1 A new player inthe game .............................................................. 2
1.1.2 Objects in the mirror don’t look as you’d expect......................... 3
1.1.3 A heavier sibling of the electron..................................................... 5
1.1.4 The "strange” path to new fundamental particles......................... 7
1.1.5 Symmetry as a guiding principle..................................................... 10
1.1.6 A “massive” problem........................................................................ 12
1.1.7 The colour and flavour of particle physics..................................... 13
1.1.8 Yet another symmetry is questioned............................................... 15
1.1.9 A third heavier family.................................................................... 17
1.2 The Standard Model.................................................................................... 20
1.2.1 The electroweak sector..................................................................... 20
1.2.1.1 Assigning particles to representations of SU(2)l x U(l)y 20
1.2.1.2 Electroweak unification .................................................. 22
1.2.2 Spontaneous symmetry breaking..................................................... 26
1.2.3 Fermion mass generation and the flavour sector......................... 30
1.2.3.1 The CKM matrix............................................................... 31
1.2.3.2 Lepton masses................................................................. 32
1.3 Beyond the Standard Model........................................................................ 35
1.3.1 Unanswered questions in the Standard Model............................ 35
1.3.2 The Standard Model as a low energy effective theory................ 38
1.3.2.1 A systematic approach to identifying the New Physics structure ........................................................................... 38
1.3.2.2 b —> sí+ír transitions......................................................... 39
1.3.2.3 Sandwiching the currents ................................................ 41
1.4 The Large Hadron Collider.......................................................................... 43
1.4.1 The LHCb experiment at the LHC............................................... 44
1.4.1.1 Tracking system ................................................................. 46
1.4.1.2 Particle identification ........................................................ 49
1.4.1.3 Calorimeters ........................................................................ 51
1.4.1.4 The muon system................................................................. 52
1.4.1.5 The trigger system.............................................................. 53
1.4.2 Overview of latest b —» sO.+t~ results............................................... 54
1.4.2.1 Differential semi-leptonic branching fractions ................ 54
1.4.2.2 Angular observables........................................................... 56
1.4.2.3 Purely leptonic branching fractions.................................. 58
1.4.2.4 Ratios of branching fractions ............................................ 60
1.5 Personal contributions................................................................................. 64
References 67
Appendix l.A Appendix to Chapter 1............................................................... 79
l.A.l The optical theorem ....................................................................... 79
l.A.2 Applications of the optical theorem............................................... 81
l.A.2.1 Decay widths and unstable particles ............................... 81
l.A.2.2 Unitarity bound ............................................................... 82
l.A.3 Generalities about SU(N)............................................................... 85
l.A.4 Symmetries and interactions............................................................ 87
l.A.4.1 global vs. local gauge invariance...................................... 87
l.A.4.2 Non-abelian (Yang-Mills) gauge theories...................... 89
2 QED in />’ - LFU ratios:Theory versus Experiment, a Monte Carlo Study 93
2.1 Introduction....................................................................................................... 93
2.2 Monte Carlo Framework................................................................................. 96
2.2.1 Generalities.......................................................................................... 96
2.2.2 Basic strategy of the Monte Carlo approach................................ 97
2.2.3 Numerical procedure........................................................................... 98
2.2.3.0.1
2.3 Direct Comparison with PHOTOS at the Short Distance Level................... 100
2.3.1 Parameterisation of the short distance amplitude ...................... 100
2.3.2 Comparison of our Monte Carlo with PHOTOS..................................100
2.4 Adding Long Distance (Charmonium Resonances)..................................... 104
2.4.1 Parameterisation of the charm amplitude........................................ 105
2.4.2 Study of the J/T-resonance interference term in our Monte Carlo 106
2.4.3 J/T and T(2S), including the resonant mode via a semi-analytic
approach................................................................................................ 109
2.5 Outlook and Conclusions ..............................................................................113
References 115
Appendix 2.A Appendix to Chapter 2................................................................. 118
2.A.1 Kinematics .......................................................................................... 118
2.A.2 More Detail on the CharmParameterisation . . .............................. 119
2.A.3 Supplementary plots.......................................................................... 121
2. A.4 Values of /th used in theMonte Carlo Simulations......................... 121
3 Test of lepton flavour universality in beautyquark-decays 125
3.1 Methods............................................................................................................ 135
3.1.1 Experimental setup............................................................................. 135
3.1.2 Analysis description............................................................................. 136
3.1.2.1 Event selection.................................................................... 136
3.1.2.2 Multivariate selection ........................................................138
3.1.2.3 Calibration of simulation......................................................139
3.1.2.4 Likelihood fit........................................................................ 140
3.1.2.5 Additional cross-checks........................................................ 145
3.1.2.6 Systematic uncertainties..................................................... 145
References 149
Appendix 3.A Appendix to Chapter 3................................................................ 163
Appendix 3.B Branching fraction measurements.............................................. 163
Appendix 3.C Fits to the B+ —> ^(2S)K+ resonant mode ........................ 164
Appendix 3.D Effect of q2 migration................................................................... 166
Appendix 3.E Overview of RK measurements.....................................................168
4 On the significance of new physics in b —> si+l~ decays 171
4.1 Introduction...................................................................................................... 171
4.2 Effective Lagrangian and selection of the observables...............................173
4.3 Statistical Method ..........................................................................................176
4.4 Results............................................................................................................... 178
viii Table of contents
4.5 Conclusion and discussion................................................................................179
References 183
5 A general effective field theory description of b —» si+i~ lepton universality ratios 189
5.1 Introduction..........................................................................................................189
5.2 General expression of Rx in terms of Wilson coefficients ......................... 190
5.2.1 Numerical estimate of the .............................................................194
5.2.2 Impact of four-fermion operators.......................................................... 195
5.3 Global combination of current measurements................................................197
5.4 Impact of future measurements...................................................................... 199
5.5 Conclusions......................................................................................................... 201
References 205
Conclusions 209

Abstract

Table of contents
List of figures ix
List of tables xvii
1 Introduction 1
1.1 History of the Standard Model..................................................................... 2
1.1.1 A new player inthe game .............................................................. 2
1.1.2 Objects in the mirror don’t look as you’d expect......................... 3
1.1.3 A heavier sibling of the electron..................................................... 5
1.1.4 The "strange” path to new fundamental particles......................... 7
1.1.5 Symmetry as a guiding principle..................................................... 10
1.1.6 A “massive” problem........................................................................ 12
1.1.7 The colour and flavour of particle physics..................................... 13
1.1.8 Yet another symmetry is questioned............................................... 15
1.1.9 A third heavier family.................................................................... 17
1.2 The Standard Model.................................................................................... 20
1.2.1 The electroweak sector..................................................................... 20
1.2.1.1 Assigning particles to representations of SU(2)l x U(l)y 20
1.2.1.2 Electroweak unification .................................................. 22
1.2.2 Spontaneous symmetry breaking..................................................... 26
1.2.3 Fermion mass generation and the flavour sector......................... 30
1.2.3.1 The CKM matrix............................................................... 31
1.2.3.2 Lepton masses................................................................. 32
1.3 Beyond the Standard Model........................................................................ 35
1.3.1 Unanswered questions in the Standard Model............................ 35
1.3.2 The Standard Model as a low energy effective theory................ 38
1.3.2.1 A systematic approach to identifying the New Physics structure ........................................................................... 38
1.3.2.2 b —> sí+ír transitions......................................................... 39
1.3.2.3 Sandwiching the currents ................................................ 41
1.4 The Large Hadron Collider.......................................................................... 43
1.4.1 The LHCb experiment at the LHC............................................... 44
1.4.1.1 Tracking system ................................................................. 46
1.4.1.2 Particle identification ........................................................ 49
1.4.1.3 Calorimeters ........................................................................ 51
1.4.1.4 The muon system................................................................. 52
1.4.1.5 The trigger system.............................................................. 53
1.4.2 Overview of latest b —» sO.+t~ results............................................... 54
1.4.2.1 Differential semi-leptonic branching fractions ................ 54
1.4.2.2 Angular observables........................................................... 56
1.4.2.3 Purely leptonic branching fractions.................................. 58
1.4.2.4 Ratios of branching fractions ............................................ 60
1.5 Personal contributions................................................................................. 64
References 67
Appendix l.A Appendix to Chapter 1............................................................... 79
l.A.l The optical theorem ....................................................................... 79
l.A.2 Applications of the optical theorem............................................... 81
l.A.2.1 Decay widths and unstable particles ............................... 81
l.A.2.2 Unitarity bound ............................................................... 82
l.A.3 Generalities about SU(N)............................................................... 85
l.A.4 Symmetries and interactions............................................................ 87
l.A.4.1 global vs. local gauge invariance...................................... 87
l.A.4.2 Non-abelian (Yang-Mills) gauge theories...................... 89
2 QED in />’ - LFU ratios:Theory versus Experiment, a Monte Carlo Study 93
2.1 Introduction....................................................................................................... 93
2.2 Monte Carlo Framework................................................................................. 96
2.2.1 Generalities.......................................................................................... 96
2.2.2 Basic strategy of the Monte Carlo approach................................ 97
2.2.3 Numerical procedure........................................................................... 98
2.2.3.0.1
2.3 Direct Comparison with PHOTOS at the Short Distance Level................... 100
2.3.1 Parameterisation of the short distance amplitude ...................... 100
2.3.2 Comparison of our Monte Carlo with PHOTOS..................................100
2.4 Adding Long Distance (Charmonium Resonances)..................................... 104
2.4.1 Parameterisation of the charm amplitude........................................ 105
2.4.2 Study of the J/T-resonance interference term in our Monte Carlo 106
2.4.3 J/T and T(2S), including the resonant mode via a semi-analytic
approach................................................................................................ 109
2.5 Outlook and Conclusions ..............................................................................113
References 115
Appendix 2.A Appendix to Chapter 2................................................................. 118
2.A.1 Kinematics .......................................................................................... 118
2.A.2 More Detail on the CharmParameterisation . . .............................. 119
2.A.3 Supplementary plots.......................................................................... 121
2. A.4 Values of /th used in theMonte Carlo Simulations......................... 121
3 Test of lepton flavour universality in beautyquark-decays 125
3.1 Methods............................................................................................................ 135
3.1.1 Experimental setup............................................................................. 135
3.1.2 Analysis description............................................................................. 136
3.1.2.1 Event selection.................................................................... 136
3.1.2.2 Multivariate selection ........................................................138
3.1.2.3 Calibration of simulation......................................................139
3.1.2.4 Likelihood fit........................................................................ 140
3.1.2.5 Additional cross-checks........................................................ 145
3.1.2.6 Systematic uncertainties..................................................... 145
References 149
Appendix 3.A Appendix to Chapter 3................................................................ 163
Appendix 3.B Branching fraction measurements.............................................. 163
Appendix 3.C Fits to the B+ —> ^(2S)K+ resonant mode ........................ 164
Appendix 3.D Effect of q2 migration................................................................... 166
Appendix 3.E Overview of RK measurements.....................................................168
4 On the significance of new physics in b —> si+l~ decays 171
4.1 Introduction...................................................................................................... 171
4.2 Effective Lagrangian and selection of the observables...............................173
4.3 Statistical Method ..........................................................................................176
4.4 Results............................................................................................................... 178
viii Table of contents
4.5 Conclusion and discussion................................................................................179
References 183
5 A general effective field theory description of b —» si+i~ lepton universality ratios 189
5.1 Introduction..........................................................................................................189
5.2 General expression of Rx in terms of Wilson coefficients ......................... 190
5.2.1 Numerical estimate of the .............................................................194
5.2.2 Impact of four-fermion operators.......................................................... 195
5.3 Global combination of current measurements................................................197
5.4 Impact of future measurements...................................................................... 199
5.5 Conclusions......................................................................................................... 201
References 205
Conclusions 209

Statistics

Additional indexing

Item Type:Dissertation (monographical)
Referees:Serra N, Isidori Gino, Baudis L
Communities & Collections:07 Faculty of Science > Physics Institute
UZH Dissertations
Dewey Decimal Classification:530 Physics
Language:English
Date:2023
Deposited On:18 Jan 2024 15:35
Last Modified:18 Jan 2024 15:36
Number of Pages:209
OA Status:Closed
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