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Electroweak Physics (from an experimentalist!)

Electroweak Physics (from an experimentalist!). Victoria Martin SUPA/University of Edinburgh. SUPA Graduate Lectures Term 2 2005/06. The Electroweak Lagrangian. Q : How do we relate this to observables that we can measure in experiments?. A : Take one piece at a time!.

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Electroweak Physics (from an experimentalist!)

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  1. Electroweak Physics(from an experimentalist!) Victoria Martin SUPA/University of Edinburgh SUPA Graduate Lectures Term 2 2005/06

  2. The Electroweak Lagrangian Q: How do we relate this to observables that we can measure in experiments? A: Take one piece at a time! Often need to consider corrections from other terms

  3. Pull= [X(expt)-X(theory)] / X Experimental Measurements • Look how well EW theory explains our measurements! • But what are these measurements!? • How do we relate what the theory tells us and what experimentalists measure? Experiment Theory Observables & Pseudo- Observables

  4. The Blue Band Plot! • Electroweak theory is so good, it predicts the Higgs mass

  5. Course Contents • Measurements at the Z pole: LEP & SLD • LEP production of W+W- • Measurements at low energy: muon lifetime, g-2 • Electroweak and top physics at the Tevatron • The search for the Higgs & BSM • What the future holds • But first, back to the theory…

  6. Parameters of the Electroweak Sector • Three key parameters: • The two gauge coupling constants: gW and g’W • The vacuum expectation value of the Higgs field: v • These can be obtained through 3 measurements. • Choose the 3 most precise: • The electric charge, e- • measured by the electric dipole moment • The Fermi Constant, GF (precision: 0.9x10-5) • measured by the muon lifetime • The mass of the Z boson, MZ (precision: 2.3x10-5)

  7. Other Useful Combinations • Mass of the W boson: • Weak mixing angle • Relationship between W and Z mass:

  8. Other Parameters in the Model • The masses of the fermions: • Most influential is m(top) due to its huge size • Mass of the Higgs, mH • The EWK model tells us nothing about these values! mH=(2λ)½v λ is not specified

  9. First Topic: Physics at the Z Pole • What EWK theory tells us about Z • How to make and detect Zs • Physics Topics: • Z mass • Partial and Total Widths • Z couplings to fermion pairs • Asymmetries

  10. Z in the Lagrangian

  11. Z boson-fermion interactions • Piece of the Lagrangian that describes fermion – Z interactions: • Vector coupling to Z: Vf = T3-2Q sin2θW • Axial coupling to Z: Af = T3 T: Weak Isospin T3 :Third Component Q: Charge

  12. A Z-boson factory • LEP=Large Electron Positron Collider @ CERN • 1989 to 1995: LEPI • CM energy: 88 to 94 GeV • 7 energy points • 17,000,000 Zs produced • 1995: 1000 Z/h recorded by each experiment • 1996 to 2000: LEPII • CM energy 161 to 209 GeV

  13. LEP Experiments • 4 experiments: • ALEPH • DELPHI • OPAL • L3

  14. y y y θ θ θ φ φ φ z z z x x x The Aleph Experiment y θ φ z x

  15. Z bosons at LEP

  16. SLC & Mark II • SLAC Linear Collider • Only linear collider to date • First detector: Mark II • 1989: First to publish observation of e+e−→Z

  17. SLC & SLD • 1992: SLC polarised e+e− beams established! • Mark II replaced with SLD detector • 1992 to 1998: 600,000 Z decays • Complementary to LEP for some measurements

  18. Two Main Measurables • What happens to the Z once produced? • It decays • What into? • Any fermion: e, μ, τ, ν, quarks • What can we measure? • Two main quantities to measure: • Cross sections to fermion final states, σ(e+e−ff) • Decay Asymmetries eg, Afb and ALR

  19. Z Boson Decaye+e−

  20. Z Boson Decay+-

  21. Z Boson Decay +− (e− + jet)

  22. Z Boson Decay Hadrons (qq jets)

  23. Decay into Fermion anti-Fermion Pairs • At tree level not too hard to calculate! • Vertex strength: • Summing over the possible electron polarisations: • Integrate over available phase space (p, p’) to get:

  24. In reality… • Need to include: • Interference from the γ • QED corrections due to gamma radiation • For quarks: QCD corrections due to gluon radiation • Fermion masses • Correction factors: ~1

  25. Total Cross Section • The total cross section to a given fermion depends on: • The rate for e+e− to make Zs: Γ(e+e−) • The rate for the Z to decay to a given fermion type: Γff • The rate for the Z to decay to anything, ΓZ • We can parameterise this for different centre of mass energies, s: Parameterises final state QED corrections in Γ(e+e−)

  26. Widths • Total width of the Z is sum of widths of everything it can decay into: • Hadronic modes due to quarks: • (top is too heavy for Z to decay into) • Invisible modes due to neutrinos:

  27. Observables extracted from σ • Many parameters can be extracted from the measurement of σ(e+e−→hadrons) • Highly correlated! Choose to use just six: • If we assume the three lepton types have the same interactions (lepton universality), last three measurements are the same. (Small correction to required for lepton mass difference).

  28. More Ratios: R0q and R0inv • When the type of quark can be identified, we can define: • Define ratio of invisible width and charged leptonic width: • Related to number of neutrinos, Nν:

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