Quantum Electrodynamics Dirac Equation : spin 1/2

Feynmann Diagrams

Moller Scattering Interaction is mediated by exchange of a photon Bhaba Scattering electron positron annhilation

Symmetries, Groups and Conservation laws Closure: R 1,R 2 belong to G, R 1.R 2 also belongs to G Identity: There exists element I such that for all elements in G IR i =R i I=R i Inverse: For every element there exists an inverse element such that R i R i -1 =I Associativity:

Symmetries and Conservation Laws Noether's theorem: Every continuous symmetry implies a corresponding conservation law and conversely, every conservation law reveals a symmetry in the system. Rotation ---- Angular momentum Time translation Energy

SU(2) Symmetry Group: Isospin mass of proton ~ mass of neutron treating proton as a different charge substrate of neutron: Isospin (I) I=1/2. (conserved in strong interactions) I 3 =+/- 1/ > charge Q/e=1/2+ I 3 Analogous to a particle of ordinary spin 1/2 Generators of SU(2) are Pauli matrices:

SU (3) symmetry: Flavour symmetry Generators of SU(3):Gell Mann Matrices

Charge Conjugation: C Particle to antiparticle Parity Operation: P, reflects a system through the origin right handed system to left handed system axial system invariant under parity CPT invariance: All interactions are invariant under combined C,P,T Time Reversal: T. Changes the direction of motion C and P are conserved in EM and Strong Interactions

Weak Interactions All quarks, leptons carry "weak charge" Short range interaction Interact via exchange of W+,W- and Z bosons (massive) two kinds of weak interactions: charged (W bosons) and neutral (Z boson) Examples :decay processes

Weak Interactions: Leptons charged neutral

Conservation of electron, muon, tau numbers

Conservation laws in WI Conservation of charge Conservation of Colour Conservation of Baryon number Electron, muon, tau number

CP violation in Weak Interaction Weak Interactions are not always CP invariant. CP violation implies Time reversal must also be violated.