Electroweak Physics Lecture 5

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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 didnt 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?l

Nicest decay mode Ws decay to leptonjets
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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 LeptonJets channels 2 b-jets, 2 jets from
  W?qq, l?
• 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 lt10.1 pb _at_ 95C.L.
• Dont 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 a mb²/mW²0
• Use W?l? decays
• Effects in many variables
• pT, cos? of lepton
• mass of (leptonjet)

CDFII 200pb-1
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 s(ee-?ff)
Afb (ee-?ll)
t 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 sin²?W

• The effective value of sin²?eff is depend on loop
  effects
• These change as a function of Q², largest when
  Q²MZ, MW
• Want to measure sin²?eff 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 ltPegt 90,
  Ee48.3GeV
• Both L and R handed electron beams
• Incident on liquid hydrogen target
• Average Q² of 0.027 (GeV/c)² (Qboson0.16
  GeV/c)
• Measure asymmetry between cross section for L and
  R beams

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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

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Measured Asymmetry

• A -131 14 (stat) 10 (syst) ppb
• sin2?Weff(Q20.026) 0.2397 0.0010 (stat)
  0.0008 (syst)
• cf 0.2381 0.0006 (theory) 1.1s 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 ? interference
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NuTeV Beams

• Beam is nearly pure neutrino or anti-neutrino
• 98.2 ?µ 1.8 ?e
• Nu beam contamination lt 10³
• Anti-nu beam contamination lt 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

• Doesnt agree with Z pole measurements

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Atomic Parity Violation

• Test Z and ? 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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sin²?W(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 ?e ?µ 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

t(µ)(2.19703 0.00004)X10-6
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Prediction for the Lifetime
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TWIST Experiment
At TRIUMF in Vancouver
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Typical Decay Event
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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 gµ2
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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 ??(spin)-?(cyclont
  ron)
• Precise momentum tuning, ?29.3

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E821
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Decay Curve
Oscillations due to parity violation in muon
decay Use ?a from fit
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aµ Results and Comparison
Very precise measurement!
Another hint of a problem?