1 Electroweak Physics Lecture 5

2 Contents Top quark mass measurements at Tevatron Electroweak Measurements at low energy: –Neutral Currents at low momentum transfer normally called low Q 2 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 2 jets from W decay 2 b-jets ℓ ± ν ℓ Nicest decay mode: Ws decay to lepton+jets

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 : simulation Mtop = /-2.6 (stat) ± 2.5 (JES) ± 1.5 (syst) GeV/c2 JES = − /−0.80 sigma ~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 m top : Use negative log likelihood to find best value for m top :

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 V td Search in both s and t channel Currently limit set < %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 m b ²/m W ²≈0 Use W → ℓν decays Effects in many variables: –p T, cos θ * of lepton –mass of (lepton+jet) No discrepancies found, need more data for precision CDFII 200pb −1

13 Tevatron Summary: m top and M W CDF and DØ have extensive physics programme Most important EWK measurements are M W and m top Stated aim for RunII: –m top ±2.5 GeV/c 2 –M W to ±40 MeV/c 2 –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) A LR τ polarisation asymmetry b and c quark final states From Tevatron Tevatron + LEPII

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 sin² θ W The effective value of sin² θ eff is depend on loop effects These change as a function of Q², largest when Q²≈M Z, M W Want to measure sin² θ eff at different Q² For exchange diagram ~2.5%

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18 E158: Møller Scattering e−e− → e−e− scattering, –first measurement at SLAC E158 in 2002 and 2003 Beam of polarised electrons ≈ 90%, E e =48.3GeV –Both L and R handed electron beams Incident on liquid hydrogen target Average Q² of (GeV/c)² (Q boson ~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 sin 2 θ W eff (Q 2 =0.026) = ± (stat) ± (syst) cf ± (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% ν μ 1.8% ν e 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 sin² θ W (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 + ν e ν μ no hadronic effects One of the most precisely measured too, use it to set G F in the Lagrangian No recent measurement of just lifetime, current investigations of decay spectrum τ(μ)=( ± )X10 −6

31 Prediction for the Lifetime

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

33 Typical Decay Event  e+e+

34 Muon Decay Spectrum SM predictions and measurements:

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

36 Very Precisely Predicted…

37 The Experiment: E821 at Brookhaven polarised muons from pion decay procession proportional to a μ : 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 ω a from fit

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