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Exploring Neutral Currents and Electroweak Unification in Particle Physics

This lecture delves into the concept of neutral currents and the electroweak force unifying electromagnetic and weak interactions. It covers the significance of neutrino species, fermion generations, and predictions from the Standard Model. Key topics include the mechanisms of flavor-changing processes, the significance of the Z₀ boson resonance, and the experimental evidence from LEP. Understanding these principles is crucial for grasping the interactions that govern particle physics and the unification of forces at a fundamental level.

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Exploring Neutral Currents and Electroweak Unification in Particle Physics

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  1. FK7003 Lecture 14 – Neutral currents and electroweak unification • Neutral currents • Electroweak unification • Number of neutrinos and fermion generation • The Standard Model • The Higgs boson

  2. FK7003 Basic diagrams for weak neutral currents Same quark flavour: u,d,s,c,b,t Flavour changing: forbidden u c u c Same quark flavour: u,d,s,c,b,t Flavour changing: forbidden

  3. FK7003 Using the mixed states = + +

  4. FK7003 Question e- Z0 s K0 K- forbidden u d

  5. FK7003 g,Z0 s s Neutral currents f

  6. FK7003 Rare decays s u W u d

  7. FK7003 First evidence for neutral current weak processes e-

  8. FK7003 Electroweak theory

  9. FK7003 Electroweak unification ne

  10. FK7003 Electroweak theory predictions

  11. FK7003

  12. FK7003 A small correction

  13. FK7003 Strategy for directly observing Z0 ?

  14. FK7003 Measuring the mass and decays of a Z0

  15. FK7003 Energy dependence mostly mostly

  16. FK7003 LEP

  17. FK7003 The Z0 resonance is what concerns us. What can we learn from it ? Lower energy experiments LEP + SLD (at Z0)

  18. Resonances reminder From lecture 4 Short-lived particles simply don’t have well-defined masses. Their masses follow a Breit-Wigner distribution 770 MeV – ”nominal” mass FK7003

  19. FK7003 What an experiment observes

  20. FK7003 The Z0 resonance LEP

  21. FK7003

  22. FK7003 There are three light neutrino species Combined data from LEP experiments This is where we end the story.

  23. FK7003 The Standard Model

  24. The most rigorous test of the Standard Model to date: g-2 Basic interaction with B field photon Next to leading order correction Higher order corrections FK7003

  25. FK7003 Summary • Neutral currents • Flavour changing vertices • Electroweak unification and measurement of the Z0resonance • 3 light neutrinos • 3 fermion generations

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