DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 1

Triple Gauge Couplings at LEP

Ulrich ParzefallUniversität Freiburg

DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 2

Charged Gauge Boson Couplings• Non-Abelian nature of EW

sector means couplings between gauge bosons

• One of the major LEP2 tasks• Accessible at LEP2:• Trilinear Gauge Couplings

(TGC)– Charged TGC from WWV

vertex (V=Z or γ) – Neutral TGC

• Quartic Gauge Boson Couplings (QGC)– ZZγγ (not in SM)– WWVγ (σ ~3fb in SM) – See Paul Bell’s talk

DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 3

Charged Trilinear Gauge Couplings

• Couplings associated with WWγ/WWZ vertex

• Most general description with 14 independent couplings

• Assume C, P conservation and gauge invariance ->

• 3 Parameters: g1z, λγ, κγ

• g1z gives weak charge of W

• W magnetic dipole moment μW ~(1+λγ +κγ)

• W electric quadrupole moment qW ~(λγ -κγ)

• Contributions from WW, single W, single γ

• single- and multi-parameter fits done by ADLO

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Charged TGC Measurements• TGC sensitivity comes from

– Total cross-section– Differential distributions

• W-production angle most important (cos θW)

• Fermion angles have also significant contribution

– WW-channel contributions differ in sensitivity

• qqlv most sensitive• qqqq also important

(limited W charge tagging and jet pairing efficiency reduce sensitivity)

• lvlv also used (ambiguities due to two neutrinos)

• Analysis methods:– Angular variables– Optimal observable

techniques jji

iij

ii

i cccd

d

)()()(),(210

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Charged TGC Results• Single LEP experiments combine

energies and WW decay channels

– Dominated by qqlv– Also sensitivity from qqqq and

lvlv channels, and as cross-check– Single W important for kγ

• ALEPH, DELPHI, L3 and OPAL have similar measurements

• LEP cTGC combination– full likelihood based (no straight

averaging)

CERN-EP-2003-42

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LEP-Combined TGC Results• Single-parameter fit

combination– Precision of 0.02 – 0.05– Consistent with SM– Sensitive to radiative

corrections• Long-standing issue: agree on

treatment of LEP correlated systematics– what to study– how to quantify – non-Gaussian problems– how to include in LEP-

combined fit– combination more complex

than cross-section measurements, eg RWW

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LEP-Combined TGC Results

• Systematics inclusion now based on joint minimization of stat & syst errors

• Dominant systematic: theory uncertainty in radiative corrections – Doube Pole Approximation

(DPA)– progress in 2003: replaced

conservative ‘full difference’ estimate with fullsim results

– Reduction of σDPA by ~50% • Now: g1

z, λγ, κγ statistics limited– stat & systematic errors similar

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Charged Trilinear Gauge Couplings

• LEP WG performs also 2D and 3D cTGC fits

• results agree with SM• ADLO Status

– Final papers out by L3, OPAL– ALEPH, DELPHI publications

expected this summer– LEP combination to follow

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Neutral Gauge Boson Couplings• Couplings: Zγγ, ZZγ, ZZZ• Neutral TGCs vanish in SM• Final States: ZZ, Zγ

– Populated by other SM processes– Place limits on anomalous couplings

• Coupling parameters

• It is possible to express the nTGCs as only two independent parameters– CP-violating & CP-conserving

e+e- Zγ e+e- ZZCP-viol. h1

γ,Z ,h2γ,Z f4

γ,Z

CP-cons. h3γ,Z ,h4

γ,Z f5γ,Z

SM:

Anomalous Couplings:

DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 10

Zγ-Couplings Measurement• Zγ-couplings are

sensitive to– Total cross-section– Diff. distributions:

• |cos θγ|, Eγ , angle(γ,jet)LEP combinationfor e+e- Zγ

DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 11

ZZ-Couplings Measurement• ZZ-Couplings are

sensitive to– Total cross-section– |cos θZ| LEP combination

for e+e- ZZ

DIS 2004, Štrbské Pleso 16 April 2004 Ulrich Parzefall 12

Conclusions• All four LEP experiments have measured charged and

neutral Triple Gauge Couplings– Results agree with SM predictions– Non-Abelian nature of electroweak sector proven by cTGCs– Neutral TGC vertices were not observed

• Final LEP combinations expected this year– Place limits on W magnetic & electric moments from cTGCs– Express nTGC results as two parameters

• LEP cTGC limits will be setting the standard for some time

LEP2 Tevatron

LHC TESLA

κγ 0.05 0.1 0.08 ~5•10-4

λγ 0.025 3•10-3 1•10-3 ~5•10-4

g1z

0.025 ~5•10-4

(FERMILAB-PUB-96-082, CMS NOTE 2001/056, ATLAS IN 060, LC-PHSM-2001-022 )

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How Did We Do?

• The LEP EWWG has combined ADLO results and deals with systematics to achieve maximal precision

• LEP2 has done its homework:– Couplings (cTGCs to 2-5%)– Cross-sections (RWW to 1%)– mW = 80.412 ± 0.042 GeV

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THE END

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DIS2004 SparesDELPHI

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Radiative Corrections• Success of LEP σWW (RWW) and cTGC

combinations showed precision would reach theory error in MC (2% at start of LEP2)

• Challenge for theorists to do better and include radiative corrections– Double Pole Approximation (DPA)

calculations were performed– YFSWW & RacoonWW emerged,

reducing theory error from 2% to 0.5%

• Cross-section prediction reduced by ~2%

• This shifts the TGCs by 0.02 to 0.05– Comparable to stat. precision

G.Passarino, summary of LEP MC workshop

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TGC Sensitivity