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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?