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Electroweak PhysicsLecture 5
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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
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Top Event in the Detector
• 2 jets from W decay• 2 b-jets• ℓ±νℓ
Nicest decay mode: Ws decay to lepton+jets
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Top Event Reconstruction
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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 = 173.5 +2.7/-2.6 (stat) ± 2.5 (JES) ± 1.5 (syst) GeV/c2
JES = −0.10 +0.78/−0.80 sigma
~16% improvement on systematic error
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Top Mass: Matrix Element Method
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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:
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Top Mass: Template Method• Dependence of
reconstructed mass on true mass parameterized from fits to MC
• Include background templates constrained to background fraction
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Top Quark Mass Results
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Top Quark Cross Section• Test of QCD prediction:
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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
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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)
No discrepancies found, need more data for
precision
CDFII 200pb−1
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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!
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Two More Measurements for Our Plot
Extracted from σ(e+e−→ff)
Afb (e+e−→ℓℓ)
AL
R
τ polarisation asymmetry
b and c quark final states
From Tevatron
Tevatron + LEPII
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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 ViolationPlus: muon lifetime and muon magnetic moment
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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 measure sin²θeff
at different Q²
• For exchange diagram
~2.5%
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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:
meas R LLR e LR
R L
N NA P A
N N
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Tree Level Diagrams
• Photon exchange will be dominant• Asymmetry between L and R terms (parity violation) is from
Z-exchange → small asymmetry
24 4
(1 )1 sin
1 (1 )2R L F
LR WR L
G s y yA
y y
12 (1 cos )y
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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
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NuTeV
• NuTeV = neutrinos at the Tevatron• Inelastic neutrino-hadron scattering• Huge chunk of instrumented iron
– With a magnet!
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NuTeV Physics• Two interactions possible:• Neutral Current (NC) Charged Current
(CC)
• Pachos Wolfenstein Relationship• Requires both neutrino and anti-neutrino beams
No γ* interference
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NuTeV Beams
• Beam is nearly pure neutrino or anti-neutrino
• 98.2% νμ 1.8% νe
• Nu beam contamination < 10³
• Anti-nu beam contamination < 2 x 10³
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Events in the Detector
“Event Length” used to separate CC and NC interactions
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NuTeV Result
• Doesn’t agree with Z pole measurements
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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
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sin²θW(Q) Results
Some disquiet in the Standard Model, perhaps?
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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…
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• End of lecture
• Precision measurements on muons follow
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Muon Lifetime• The lifetime of the muon is one of the test predicted
parameters in the EWK
• μ+ → e+ νe νμ no 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
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1
2FG
v
τ(μ)=(2.19703 ± 0.00004)X10−6
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Prediction for the Lifetime
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TWIST Experiment
Highly polarized +
+ stop in Al target(several kHz)
Unbiased + (scintillator)
trigger
At TRIUMF in Vancouver
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Typical Decay Event
e+
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Muon Decay Spectrum
• SM predictions and measurements:
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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
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Very Precisely Predicted…
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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
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E821
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Decay Curve
Oscillations due to parityviolation in muon decay
Use ωa from fit
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aμ: Results and Comparison
10( ) 11659214 8 11 10a
Very precise measurement!
Another hint of a problem?
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