Transcript Electroweak Physics Lecture 4

Electroweak Physics
Lecture 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
Nicest decay mode: Ws decay to
lepton+jets
• 2 jets from W decay
• 2 b-jets
• ℓ± ν ℓ
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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:
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
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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
CDFII 200pb−1
• Use W→ℓν decays
• Effects in many variables:
– pT, cosθ* of lepton
– mass of (lepton+jet)
No discrepancies found, need
more data for precision
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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)
τ polarisation asymmetry
b and c quark final states
ALR
Tevatron + LEPII
From Tevatron
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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 Violation
Plus: 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
~2.5%
• For exchange diagram
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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
LR
A
NR  NL

 Pe ALR
NR  NL
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Tree Level Diagrams
ALR 
 R  L
G s
y(1  y)
2
 F
1

sin
W 

4
4
R L
2 1  y  (1  y)
y  12 (1  cos )
• Photon exchange will be dominant
• Asymmetry between L and R terms (parity violation) is from Zexchange → small asymmetry
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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)
No γ* interference
• Pachos Wolfenstein Relationship
• Requires both neutrino and
anti-neutrino beams
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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
GF 
1
2v 2
τ(μ)=(2.19703 ± 0.00004)X10−6
• No recent measurement of just lifetime, current
investigations of decay spectrum
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Prediction for the Lifetime
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TWIST Experiment
At TRIUMF in Vancouver
Highly polarized +
+ stop in Al target
(several kHz)
Unbiased +
(scintillator)
trigger
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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:
with gμ=2
• Loop effects make gμ different from 2
• Define anomalous magnetic moment:
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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 parity
violation in muon decay
Use ωa from fit
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aμ: Results and Comparison
a(  )  11659214  8 111010
Very precise measurement!
Another hint of a problem?
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