Quantum Field Theory Lecture Notes Joachim Kopp September 16, 2016 Contents 1 Introduction and Motivation 7 1.1 Do you recognize the following equations? . .7 1.2 A Note on Notation . .7 2 The Klein{Gordon Field 9 2.1 Necessity of the Field Viewpoint . .9 2.2 Elements of Classical Field Theory . 10 2.2.1 The Euler{Lagrange Equations . 10 2.2.2 The Hamiltonian . 11 2.2.3 Noether's Theorem . 12 2.3 Quantization of the Klein{Gordon Field . 13 2.3.1 Commutation Relations . 13 2.3.2 The Quantized Hamiltonian . 15 2.3.3 Time-Dependence of the Klein{Gordon Field Operator . 16 2.4 The Feynman Propagator for the Klein{Gordon Field . 17 2.4.1 Green's Functions of the Klein{Gordon Operator . 17 2.4.2 The Feynman Propagator . 19 2.4.3 Relation to Correlation Functions . 19 3 The Dirac Field 21 3.1 The Dirac Equation and its Solutions . 21 3.1.1 The Equation and the Corresponding Lagrangian . 21 3.1.2 Solutions of the Dirac Equation . 22 3.1.3 Spin Sums . 24 3.2 Quantization of the Dirac Field . 25 3.2.1 How Not to Quantize the Dirac Field . 25 3.2.2 Quantizing the Dirac Field with Anticommutators . 26 3.2.3 Physical Significance of the Quantized Dirac Field . 28 3.3 The Feynman Propagator for the Dirac Field . 28 3.4 Symmetries of the Dirac Theory . 30 3.4.1 Lorentz Invariance . 30 3.4.2 Parity (P )................................ 37 3.4.3 Time Reversal (T )........................... 38 3.4.4 Charge Conjugation (C)........................ 40 4 Interacting Fields and Feynman Diagrams 43 3 Contents 4.1 Time-Dependent Perturbation Theory for Correlation Functions . 43 4.1.1 φ4 Theory . 43 4.1.2 The Vacuum State of the Interacting Theory . 44 4.1.3 Correlation Functions . 44 4.1.4 Perturbation Theory . 45 4.2 Wick's Theorem . 50 4.3 Feynman Diagrams . 53 4.3.1 Basic Idea and Application to a Simple 4-Point Function . 54 4.3.2 An Example in φ4 Theory . 54 4.3.3 A More Advanced Example . 55 4.3.4 More Examples for Diagrams with Non-Trivial Symmetry Factors 56 4.3.5 Position Space Feynman Rules . 57 4.3.6 Momentum Space Feynman Rules . 58 4.3.7 Disconnected Feynman Diagrams . 59 4.3.8 The Denominator of the Correlation Function . 61 4.4 The LSZ Reduction Formula . 61 4.5 Computing S-Matrix Elements from Feynman Diagrams . 65 4.6 Feynman Rules for Fermions . 68 4.6.1 The Master Formula for Correlation Functions Involving Fermions 68 4.6.2 Wick's Theorem for Fermions . 69 4.6.3 The LSZ Formula for Fermions . 70 4.6.4 Yukawa Theory . 71 4.6.5 The Yukawa Potential . 78 5 Quantum Electrodynamics 81 5.1 The QED Lagrangian from Symmetry Arguments . 81 5.2 The Feynman Rules for QED . 83 5.3 e+e− ! µ+µ− .................................. 86 5.3.1 Feynman Diagram and Squared Matrix Element . 86 5.3.2 Trace Technology . 87 5.3.3 The Squared Matrix Element for e+e− ! µ+µ− (Part II) . 89 5.3.4 The Cross Section | General Results . 90 5.3.5 The Cross Section for e+e− ! µ+µ− ................. 91 5.3.6 e+e− ! µ+µ−: Summary . 92 5.4 More Technology for Evaluating QED Feynman Diagrams . 93 5.4.1 Scattering of Polarized Particles . 93 5.4.2 External Photons . 93 6 Path Integrals 95 6.1 Path Integrals in Quantum Mechanics . 95 6.2 The Path Integral for a Free Scalar Field . 97 6.3 The Feynman Propagator from the Path Integral . 100 6.4 Wick's Theorem from the Path Integral . 101 4 Contents 6.5 Interacting Field Theories in the Path Integral Formalism . 102 6.6 Quantization of the Photon Field . 103 6.7 Path Integrals for Fermions . 104 6.7.1 Grassmann Numbers . 105 6.7.2 Partition Function, Functional Derivative and Correlation Func- tions for Fermions . 107 6.8 The Quantum Equations of Motion: Schwinger{Dyson Equations . 109 6.9 The Ward{Takahashi Identity . 110 7 Weyl and Majorana Fermions 115 7.1 Spinor Indices . 116 7.1.1 Left-handed spinors . 116 7.1.2 Raising and lowering spinor indices . 117 7.1.3 Right-handed spinors . 118 7.1.4 Conjugate spinors . 119 7.1.5 Lorentz invariance of the Pauli matrices . 119 7.1.6 One more example: the vector current . 120 7.2 The QED Lagrangian in 2-Component Notation . 121 7.3 Majorana Fermions . 123 7.4 Application: Majorana Neutrinos and the Seesaw Mechanism . 124 7.4.1 Neutrino mass terms . 124 7.4.2 The seesaw mechanism . 125 7.4.3 Interlude: measuring neutrino masses . 127 7.5 Twistors . 131 7.5.1 Unifying spinors and momentum 4-vectors . 131 7.5.2 Twistor notation . 133 7.5.3 Examples . 135 8 Radiative Corrections 137 8.1 The Electron Vertex Function . 139 8.1.1 Preliminary Considerations . 139 8.1.2 Physical Interpretation . 141 8.1.3 Simplifying the Integrand . 143 8.1.4 The 4-Momentum Integral . 148 8.1.5 Lepton magnetic moments . 151 8.1.6 Renormalization: a First Glimpse . 154 8.1.7 Regularization of the Divergence . 156 8.1.8 Renormalization: The Counterterm . 159 8.1.9 Summary of our First 1-Loop Calculation . 160 8.2 Renormalization . 161 8.2.1 The renormalized QED Lagrangian . 161 8.2.2 1-Loop Corrections to the Fermion Propagator . 162 8.2.3 1-Loop Corrections to the Photon Propagator . 163 5 Contents 8.2.4 On-Shell Renormalization . 164 8.2.5 Renormalizability . 165 8.2.6 Renormalization of the Electric Charge . 169 8.2.7 Renormalization group evolution and the Callan{Symanzik equation173 8.2.8 The β function . 174 8.3 Infrared Divergences . 176 8.3.1 IR behavior of virtual (1-loop) corrections . 176 8.3.2 IR behavior of real corrections . 178 9 Non-Abelian Gauge Theories 183 9.1 Gauge Interactions from Symmetry . 183 9.2 Non-Abelian Gauge Transformations . 186 9.3 Lie Algebras and Lie Groups . 190 9.3.1 Special Representations . 192 9.3.2 The Casimir Operator . 193 9.3.3 Product Representations . 194 9.3.4 The SU(N) Groups . 195 9.4 Quantization of Non-Abelian Gauge Fields . 196 9.5 The Feynman Rules for Non-Abelian Gauge Theories . 200 9.5.1 The Gauge Boson Propagator . 200 9.5.2 Gauge Boson Self-Interactions . 200 9.5.3 Ghost Interactions . 201 9.5.4 Coupling to Fermions . 201 9.6 The Beta Function for Non-Abelian Gauge Theories . 202 9.7 Phenomenology of Quantum Chromodynamics (QCD) . 204 9.7.1 Deep-Inelastic Scattering . 205 9.7.2 Proton{Proton Interactions at the LHC . 207 9.7.3 Infrared divergences in QCD . 208 9.7.4 Multiple Splittings . 211 9.7.5 The DGLAP Equations . 212 10 Spontaneous Symmetry Breaking 217 10.1 The Abelian Higgs Mechanism . 217 10.2 Goldstone's Theorem . 219 10.3 The Glashow{Salam{Weinberg Theory of Electroweak Interactions . 221 10.3.1 Higgs Mechanism . ..