2. LESSON 3: INTERACTIONS What is an interaction? More about interactions. How do the particles actually interact? What is a Feynman diagram? Examples and Exercises.

3. What is an interaction? The notion of interaction is a generalization of the known notion of force. An interaction describes also decays and annihilations. Interactions –1. Gravitational, between masses. –2. Electromagnetic, between electrical charges.

4. –3. Strong, between color charges (the quarks bring color charge than electric one and mass). –4. Weak interaction, which is responsible for decay of massive particles belonging to a family to other particles belonging to the same family. (example: d quark is transformed into an u one). –BUT Standard Model describes only the three: strong, electromagnetic and weak interaction.

5. More about interactions. Every interaction is characterized by an interaction time t. Strong interactiont~10 -23 s Electromagnetic interactiont~10 -18 s Weak interactiont~10 -10 s

6. Strong Interaction Strong Interactions always affect color charges, so it is only between quarks. Strong Interactions CAN NOT transform a quark flavor into another one. The Strong Interaction only: –Rearranges quarks. –Creates a quark-antiquark pair from energy. –Creates a quark-antiquark pair from quark- antiquark pair.

7. Weak Interaction The Weak Interaction transforms a quark flavor into another one, following this scheme: The Weak Interaction transforms a lepton into another one. The pairs of leptons which can interact weakly are:

8. How Do Particles Actually INTERACT ? Which means, how do they pull or push each other or how do they feel each other ? Today we believe that two particles interact when they interchange another particle, specific for each type of interaction. These new particles, which are the force carriers, are called exchange bosons. INTERACTION Strong  Gluons (8) Electromagnetic  Photon Weak  W + W - Z 0 Gravitational  Graviton

9. Every gluon carries a color and simultaneously an anti-color charge. Example: red and anti-blue. Why do we believe that particles interact exchanging bosons? This theory (physical concepts + mathematical background) gives results in a very good accordance with the experiments.

10. What is a FEYNMAN DIAGRAM ? It is a simple way of visualizing, analyzing and interpreting what happens in an interaction. However a Feynman diagram obeys certain rules and it’s made of certain parts.

11. The parts of a Feynman diagram are: –1.Vertex (symbolized by. ) represents the event of the interaction. –2. Straight lines represent the initial (before interaction) or final (after interaction) particles.

12. Notes: The time passes from left to right. Forward facing arrows for particles and backwards facing arrows for antiparticles. Each interaction consists of at least two vertices. Lines do not represent trajectories. A Feynman diagram is not a space-time diagram.

13. –3. Wavy lines ( ) represent photon or W +, W -, Z 0. –Curly lines ( ) represent gluons Now let’s look at the complete interaction.

14. Examples and Exercises Rearranging only. Decay creating new pairs. Interaction creating new pairs. Annihilating and creating new pairs.

15. Rearranging only (example)

16. Rearranging only (exercise)

17. Decay creating new pairs (example) Energy

18. Decay creating new pairs (exercise) Energy

19. Decay creating new pairs (exercise)

20. Decay creating new pairs (example)

21. Interaction creating new pairs (example) Energy

22. Interaction creating new pairs (example)

23. Interaction creating new pairs (exercise)

25. Interaction creating new pairs (Cosmic rays one example of strong interaction)

26. Interaction creating new pairs (Cosmic rays)

27. Annihilating and creating new pairs (example)

28. Annihilating and creating new pairs (exercise)

29. 1. 2. 3. 4. 5. Strong interaction exercises For each of the following interactions, write out the quark continents of each particle and check that the number of quarks has remained the same, they have simply rearranged themselves.

30. 6. 7. 8. 9. Strong interaction exercises write out the quark content of the particles and work out which quark-antiquark pair must have been created. Sketch a simplified pair of gluon vertices for the process.

31. 10. 11. 12. 13. Strong interaction exercises For each of the following decays, write out the quark constituents of each particle and show where quark-antiquark pairs have been created. Sketch a simplified gluon vertex pair for the process.

32. Introduction to weak interaction ? This is not a strong interaction!

33. Introduction to weak interaction

34. W-W-