Electroweak Physics Lecture 5

1. Electroweak PhysicsLecture 5

2. Contents • Top quark mass measurements at Tevatron • Electroweak Measurements at low energy: • Neutral Currents at low momentum transfer • normally called low Q2 • Q is the four momentum of the boson • Precision measurements on muons • We didn’t get to this in the lecture • Slides are at the end

3. Top Event in the Detector Nicest decay mode: Ws decay to lepton+jets • 2 jets from W decay • 2 b-jets • ℓ±νℓ

4. Top Event Reconstruction

5. Top Mass: Largest Systematic Effect • Jet Energy Scale (JES) • How well do we know the response of the calorimeters to jets? • In Lepton+Jets channels: 2 b-jets, 2 jets from W→qq, ℓ+ν • Use jets from W decay (known mass) to calibrate JES • Example of CDF analysis: JES = −0.10 +0.78/−0.80 sigma Mtop = 173.5 +2.7/-2.6 (stat) ± 2.5 (JES) ± 1.5 (syst) GeV/c2 simulation ~16% improvement on systematic error

6. Top Mass: Matrix Element Method

7. Matrix Element Method in Run II • Probability for event to be top with given mtop: • Use negative log likelihood to find best value for mtop:

8. Top Mass: Template Method • Dependence of reconstructed mass on true mass parameterized from fits to MC • Include background templates constrained to background fraction

9. Top Quark Mass Results

10. Top Quark Cross Section • Test of QCD prediction:

11. Search for Single Top Production • Can also produce single top quarks through decay of heavy W* boson • Probe of Vtd • Search in both s and t channel • Currently limit set <10.1 pb @ 95%C.L. • Don’t expect a significant single until 2fb-1 of data are collected

12. W helicity in Top Decays • Top quarks decay before then can hadronise • Decay products retain information about the top spin • Measure helicity of the W to test V-A structure of t→Wb decay • F+α mb²/mW²≈0 • Use W→ℓν decays • Effects in many variables: • pT, cosθ* of lepton • mass of (lepton+jet) CDFII 200pb−1 No discrepancies found, need more data for precision

13. Tevatron Summary: mtop and MW • CDF and DØ have extensive physics programme • Most important EWK measurements are MW and mtop • Stated aim for RunII: • mtop ±2.5 GeV/c2 • MW to ±40 MeV/c2 • Probably can do better • Other EWK tests possible too!

14. Two More Measurements for Our Plot Extracted from σ(e+e−→ff) Afb (e+e−→ℓℓ) τ polarisation asymmetry b and c quark final states ALR Tevatron + LEPII From Tevatron

15. Electroweak Physics at Low Energy • Low momentum transfer, Q, of the boson • Test whether EWK physics works at all energy scales • Møller Scattering • Neutrino-Nucleon Scattering • Atomic Parity Violation Plus: muon lifetime and muon magnetic moment

16. Running of sin²θW • The effective value of sin²θeff is depend on loop effects • These change as a function of Q², largest when Q²≈MZ, MW • Want to measuresin²θeff at different Q² • For exchange diagram ~2.5%

18. E158: Møller Scattering • e−e−→e−e− scattering, • first measurement at SLAC E158 in 2002 and 2003 • Beam of polarised electrons <Pe> ≈ 90%, Ee=48.3GeV • Both L and R handed electron beams • Incident on liquid hydrogen target • Average Q² of 0.027 (GeV/c)² (Qboson~0.16 GeV/c) • Measure asymmetry between cross section for L and R beams:

19. Tree Level Diagrams • Photon exchange will be dominant • Asymmetry between L and R terms (parity violation) is from Z-exchange → small asymmetry

20. Measured Asymmetry • A = −131 ± 14 (stat) ± 10 (syst) ppb • sin2θWeff(Q2=0.026) = 0.2397 ± 0.0010 (stat) ± 0.0008 (syst) • cf 0.2381 ± 0.0006 (theory) +1.1σ difference

21. NuTeV • NuTeV = neutrinos at the Tevatron • Inelastic neutrino-hadron scattering • Huge chunk of instrumented iron • With a magnet!

22. NuTeV Physics • Two interactions possible: • Neutral Current (NC) Charged Current (CC) • Pachos Wolfenstein Relationship • Requires both neutrino and anti-neutrino beams No γ* interference

23. NuTeV Beams • Beam is nearly pure neutrino or anti-neutrino • 98.2% νμ1.8% νe • Nu beam contamination < 10³ • Anti-nu beam contamination < 2 x 10³

24. Events in the Detector “Event Length” used to separate CC and NC interactions

25. NuTeV Result • Doesn’t agree with Z pole measurements

26. Atomic Parity Violation • Test Z and γ interaction with nucleons at low Q² • Depends on weak charge of nucleon: • Large uncertainty due to nuclear effects • eg nucleon spin

27. sin²θW(Q) Results Some disquiet in the Standard Model, perhaps?

28. Low Energy Summary • Important to test EWK Lagrangian at different energy scale • Challenging to achieve the level of precision to compare with theory! • Experimental Challenges overcome, very precise results achieved • Some (small) discrepancies found between data and theory…

29. End of lecture • Precision measurements on muons follow

30. Muon Lifetime • The lifetime of the muon is one of the test predicted parameters in the EWK • μ+ → e+ νe νμno hadronic effects • One of the most precisely measured too, use it to set GF in the Lagrangian • No recent measurement of just lifetime, current investigations of decay spectrum τ(μ)=(2.19703 ± 0.00004)X10−6

31. Prediction for the Lifetime

32. Highly polarized + + stop in Al target(several kHz) Unbiased + (scintillator) trigger TWIST Experiment At TRIUMF in Vancouver

33. m e+ Typical Decay Event

34. Muon Decay Spectrum • SM predictions and measurements:

35. Muon Dipole Moment • The Dirac equation predicts a muon magnetic moment: • Loop effects make gμ different from 2 • Define anomalous magnetic moment: with gμ=2

36. Very Precisely Predicted…

37. The Experiment: E821 at Brookhaven • polarised muons from pion decay • procession proportional to aμ: ω=ω(spin)−ω(cyclontron) • Precise momentum tuning, γ=29.3

38. E821

39. Decay Curve Oscillations due to parity violation in muon decay Use ωa from fit

40. aμ: Results and Comparison Very precise measurement! Another hint of a problem?