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Electroweak b physics at LEP

Electroweak b physics at LEP. V. Ciulli INFN Firenze. Introduction. Two electroweak observable in Z bb decays : Partial decay width: R b = s bb / s had measures Forward-bakward b asymmetry: A FB (b) measures Often results given as. Global fit probability ~ 4.5%

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Electroweak b physics at LEP

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  1. Electroweak b physics at LEP V. Ciulli INFN Firenze

  2. Introduction Two electroweak observable in Zbb decays: • Partial decay width: Rb = sbb /shad • measures • Forward-bakward b asymmetry: AFB(b)\ • measures • Often results given as

  3. Global fit probability ~ 4.5% (28% without NuTev) Measurements using Z  QQ decays Rb anomaly solved long ago… Forward-backward b asymmetry pull > 2 s The Electroweak Fit

  4. Rb vs Rc (LEP+SLD) Summer 1996 Summer 2003 Mt = 174.3  5.1 GeV

  5. AFB(b) • Very stable over the years • Errors significantly reduced • New analysis techniques • Full data reprocessing

  6. Asymmetries at the Z pole • From fermion direction and helicity: • With polarized beams f = e,m,t,b,c f e- q e+ f = t only f f any f e- q e+ f = e,m,t,b,c f

  7. AFB(b) measured by means of two different techniques • b-tagging from semi-leptonic b decays • b quark/anti-quark separation from lepton charge • Inclusive b-tagging (impact parameter, secondary decay vertex, etc…) • b quark/anti-quark separation from jet-charge, e.g. The two measurements are systematically uncorrelated Statistical correlation in the same experiment: 25% - 40%

  8. AFB (b) with leptons • Lepton momentum and transverse momentum separate different sources • b-quark direction measured from event thrust, signed by the lepton charge • Asymmetry measured from fit to the angular distribution • Separation is improved in recent measurements by the use of more variables and neural networks OPAL final (2003)

  9. AFB (b) with leptons - 2 • AFB diluted by mixing and bcl • Both mixing and semi-leptonic BR’s measured on same data • Modeling effects, important for those measurements, cancel out in AFB • AFB(c) also measured (correlation < 20%) • The measurement is largerly dominated by statistical error ALEPH final (2002)

  10. d2  s2 AFB(b) with jet-charge • AFB measured from forward-backward charge asymmetry d measured on data ! • Vertex and identified particles also used to tag the charge

  11. Main systematics b purity (efficiencies from double tag methods) hemisphere correlations (from MC, but much less important than for Rb) charge tagging calibration AFB(b) with jet-charge - 2 charge separation vs cosq Data vs MC

  12. QCD corrections • Measured asymmetry must be corrected for gluon radiation effects Thrust axis  quark direction correction ~3.5% > DAFB/AFB 1.7%

  13. QCD corrections • Bias due to event selection • Leptons: 50-70% of total correction • Jet-charge: ~25% only (corrections almost completely absorbed into charge correlations between the hemispheres) Lepton momentum

  14. LEP + SLD Combination The six electroweak observables measured at LEP and SLC are fitted together with Taking into account statistical and systematical correlations • BR’s and mixing mostly correlated to asymmetries • Charm hadron production correlated to Rc Rb, Rc, AFB(b), AFB(c), Ab and Ac BR(b  l), BR(b  c  l), BR(c  l), , f(D+), f(Ds), f(cbarioni), P(cD*+)xBR(D*+D0+)

  15. Results 14-parameter fit: c2/d.o.f.=51/91 (91/91 if only stat errors are used) A0FB(b)=0.09970.0016 Inclusive: A0FB(b)=0.10000.0019 Leptons: A0FB(b)=0.10000.0025

  16. AFB(b) vs centre-of-mass energy Peak data only (+0.6s): A0FB(b)=0.10060.0017

  17. Ab: are LEP and SLD compatible? • LEP only (indirect): • Ab=0.8980.021 • SLD direct: • Ab=0.9250.020 • Agree within 0.8s ! • LEP+SLD: • Ab=0.9030.013 • (0.935 SM)

  18. From all asymmetries (including quark ones): Final precision < 10-3 but average prob = 7% Most sensitive measurements ALR AFB(b) disagree by 2.9s ! (t polarisation in the middle of the two) sin2qelfefpt

  19. b-quark couplings anomaly? Large deviations in the right coupling gRb only! Difficult to explain…

  20. Conclusions • LEP b-asymmetry measurements almost final • 2nd iteration using reprocessed data and improved techniques • only DELPHI inclusive still preliminary • LEP results are consistent with SLD Ab measurement • O(3s) discrepancy between “hadronic” and “leptonic” mixing angle measurements confirmed • Interpretation of this electroweak result difficult

  21. Backup slides

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