Feynman Diagrams

1. Feynman Diagrams

2. 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

3. 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

4. Uncertainty Principle Data booklet In quantum mechanics it is possible to “borrow” an amount of energy (from nowhere), DE for a limited amount of time Dt A virtual W boson (mass 80 GeVc-2) is emitted in an interaction. How long does it exist for? IB Physics – Particle Physics

5. Range The Range is the maximum distance travelled by a virtual particle. The formula is given in the data booklet Find the range of a virtual W boson Explain why a photon (em force) has infinite range (symbol = ) IB Physics – Particle Physics

6. 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

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

8. 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

9. EM vertex IB Physics – Particle Physics

10. 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

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

12. IB Physics – Particle Physics

13. 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

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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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

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