Feynman Diagrams Topic 7.3

Feynman Vertices Each of the three basic interactions can be described using a symbol called a Feynman vertex. We can use the vertices in a non-mathematical way to illustrate how quarks and leptons interact with each other. There is an electromagnetic interaction vertex, a weak interaction vertex and a strong interaction vertex. IB Physics – Particle Physics

Drawing Feynman Diagrams Each vertex has an arrow going in and one going out. These represent a lepton – lepton or quark-quark transition. Quarks or leptons are solid straight lines Exchange particles are either wavy (Photons, W, Z) or curly (gluons). Time flows from left to right Arrows from left to right represent particles moving forward in time. Arrows from right to left represent antiparticles moving forward in time. (think of them as moving left to right). Vertices are linked by a line representing an exchange particle Charge and colour are conserved at each vertex. IB Physics – Particle Physics

Forces in particle physics Forces are explained by Emission/absorption of particles A particle is emitted “ spontaneously” Where does the energy to create this particle come from? (Uncertainty video) New law called the Heisenberg Uncertainty Principle The particle is known as a virtual particle. QED Video IB Physics – Particle Physics

What is happening here? a.) Electron absorbs a photon and accelerates (changes direction) b.) Electron emits a photon and changes direction

Electron and Positron Vertices

Examples of Feynman Diagrams

Examples of Feynman Diagrams

Rotate the vertex slightly to show a real interaction Space time IB Physics – Particle Physics

Use of Feynman diagrams Feynman diagrams may be used to calculate probabilities for fundamental processes. The picture represents a mathematical process called the amplitude. For the em interaction The amplitude of the diagram is the product of the interaction strength for each vertex i.e. Probability of taking place process = (amplitude)2 IB Physics – Particle Physics

EM vertex IB Physics – Particle Physics

Basic em interactions By rotating the arms of the vertices, the following interaction possibilities are generated. Note that the time still flows from left to right and a backwards facing arrow represents an antiparticle travelling forwards in time. IB Physics – Particle Physics

Weak Vertices Ws, Z and gluons video IB Physics – Particle Physics

IB Physics – Particle Physics

Strong vertices The left hand side represents BEFORE and the right hand side represents AFTER The gluon can be regarded as a pathway through which colour charge is exchanged between quarks and antiquarks. The quark gluon vertices could also show colour flow as quarks interact. IB Physics – Particle Physics

Strong interactions Annotate to show colour and flavour IB Physics – Particle Physics

Feynman Diagram Examples http://teachers.web.cern.ch/teachers/archiv/HST2002/feynman/examples.htm http://www.departments.bucknell.edu/physics/animations/Feynman_diagrams/ http://www2.slac.stanford.edu/vvc/theory/feynman.html You should be able to draw Feynman diagrams for the following interactions; Electron scattering Beta decay Pion decay Electron – positron annhilation Pair production Muon decay Quark interactions Photon – photon scattering IB Physics – Particle Physics

Electron Scattering

Beta Decay

Pion - Decay Pion 1 Decay Pion + Decary

Electron Positron Annihilation

Pair Production

Muon Decay

Quark Interactions

Photon Photon Scattering

Learn these ones Draw the Feynman diagram for beta (-) decay IB Physics – Particle Physics

Strong force and gluon exchange Color force and strong force are essentially the same thing Colour force binds quarks together in hadrons by exchange of gluons Strong force binds colour-neutral particles together e.g. protons and neutrons in the nucleus. IB Physics – Particle Physics

Gluons Bosons with spin = 1 and zero mass Gluons are themselves coloured Gluons bind quarks together Force between quarks increases as quarks are separated. Therefore isolated quarks and quarks cannot be observed. This is quark confinement IB Physics – Particle Physics

Gluon colour Quarks change colour through gluon exchange. There are 6 coloured quarks and 2 colour neutral gluons Note: time should be horizontal Click diagram for animation IB Physics – Particle Physics

Example A green s quark emits a gluon and becomes a blue quark. State the flavour of the new quark and the colours of the emitted gluon. A blue u quark absorbs this gluon. What is its final colour and flavour? Draw a labeled Feynman diagram for this process. IB Physics – Particle Physics

Feynman diagram practice Using the basic weak interaction vertex involving a W boson and two fermions (below) draw Feynman diagrams to represent the following processes Fermion out Fermion in W boson Using quarks, draw a Feynman diagram for: IB Physics – Particle Physics

Websites www.particleadventure.com http://teachers.web.cern.ch/teachers/archiv/HST2002/feynman/index.html IB Physics – Particle Physics