1. Lecture 14 – Neutral currents and electroweak unification
Number of neutrinos and fermion generation
The Standard Model
The Higgs boson
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2. 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
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3. Using the mixed states + = +
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4. Question e- Z0 s K0 K-
forbidden u d
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5. Neutral currents s
g,Z0 f
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6. Rare decays s u W u d
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7. First evidence for neutral current weak processes
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8. Electroweak theory
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9. Electroweak unificationne
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10. Electroweak theory predictions
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12. A small correction
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13. Strategy for directly observing Z0
Lepton
Charge (e)
Mass (GeV) e- -1
0.0005 ne 0 m-
0.105 nm t-
1.8 nt Y-
> 200 nY ?
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14. Measuring the mass and decays of a Z0
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15. Energy dependence
mostly
mostly
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16. LEP
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17. The Z0 resonance is what concerns us. What can we learn from it ?
Lower energy experiments
LEP
+ SLD (at Z0)
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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
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19. What an experiment observes
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20. The Z0 resonance
LEP
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22. There are three light neutrino species
Combined data from LEP experiments
This is where we end the story.
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23. The Standard Model
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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
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25. Summary Neutral currents
Flavour changing vertices
Electroweak unification and measurement of the Z0 resonance
3 light neutrinos
3 fermion generations
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