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Precision Electroweak Measurements at LEP

WIN 05 Weak Interactions and Neutrinos 2005 Delphi - June 8, 2005 Working Group 1 : Electroweak Symmetry Breaking. Precision Electroweak Measurements at LEP. Paolo Azzurri – INFN Pisa. LEP1 & LEP2. LEP1 (1989-1995): 4  200pb -1 @ √ s=m Z → 4  5 Million Z decays

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Precision Electroweak Measurements at LEP

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  1. WIN 05 Weak Interactions and Neutrinos 2005 Delphi - June 8, 2005 Working Group 1: Electroweak Symmetry Breaking Precision Electroweak Measurements at LEP Paolo Azzurri – INFN Pisa

  2. LEP1 & LEP2 • LEP1 (1989-1995): 4200pb-1 @ √s=mZ • → 45 Million Z decays • LEP2 (1996-2000): 4800pb-1@ √s=161-209 GeV • →410,000 W pairs Precision on Z and W mass ΔmZ(1986)=1.7 GeV [SPS] → ΔmZ(1996)=2.1MeV [LEP] ΔmW(1986)=1.5 GeV [SPS] → ΔmW(2002)=39 MeV [LEP+TEV] P.Azzurri - Precision EW @ LEP

  3. EW Tree Level (R.Tenchini) 2 1986 2002 More and more evidence for EW radiative corrections ! 2 1986 2002 Tree Level P.Azzurri - Precision EW @ LEP

  4. One loop EW radiative corrections Need to introduce three additional parameters mtop mhiggs αS Observables Oiare Contribution of radiative corrections P.Azzurri - Precision EW @ LEP

  5. Z pole Forward-Backward Asymmetries P.Azzurri - Precision EW @ LEP

  6. B Asymmetry NEW FINAL RESULT Χ2/n=0.58 Error dominated by statistics P.Azzurri - Precision EW @ LEP

  7. Sin2ΘW from Asymmetries FINAL RESULT Historical difference between ALR(l) SLD and AFB(b) LEP Prob=3.7% now 3.2σ P.Azzurri - Precision EW @ LEP

  8. Hadronic Vacuum Polarization and α(MZ) P.Azzurri - Precision EW @ LEP

  9. Small angle Bhabha scattering Evidence for αQED running and for Δαhad in the t-channel P.Azzurri - Precision EW @ LEP

  10. EW at LEP2 P.Azzurri - Precision EW @ LEP

  11. Photons @ LEP2 Clean environment for new physics ! Constraints on: Coulomb Deviations, QED cutoffs, SUSY Neutralinos & Gravitinos, extra-dimensions Gravitons, excited electrons, …. (Λ>1TeV) P.Azzurri - Precision EW @ LEP

  12. Fermion Pairs @ LEP2 More constraints on: extra-dim Gravitons, Contact Interactions, Z’ bosons, squarks, leptoquarks, … (Λ>1TeV) P.Azzurri - Precision EW @ LEP

  13. Single W and Z In agreement with SM within 8% precision In agreement with SM within 7% precision P.Azzurri - Precision EW @ LEP

  14. Z-pairs No gauge self couplings involved in Standard Model Z-pair production In agreement with SM within 5% precision P.Azzurri - Precision EW @ LEP

  15. W-pair events Channel efficiency purity bkg ll 50-80% 80-90% ,,ll eqq 75-90% ~90% qq,Zee qq 75-90% ~95% qq qq 50-80% 80-85% qq,We qqqq 80-90% 75-80% qq P.Azzurri - Precision EW @ LEP

  16. Precision better than 1% Without O(α) R=0.974±0.009 Test of the SM radiative corrections to the CC03 diagrams W-pair cross sections Clear proof of SU(2)xU(1) gauge couplings ! WW (pb) s (GeV) P.Azzurri - Precision EW @ LEP

  17. Standard electroweak theory U(1) SU(2) triple and quartic SU(2) gauge boson self couplings are thesignature of the non-abelian SU(2) electroweak structure ! The most general Lorentz Invariant WWV (V=,Z) vertex has 7 complex couplings The WWZ and WW gauge couplings can be measured in W-pair events, fitting the W-pair event rates and the W production and decay angular distributions. Triple Gauge Couplings P.Azzurri - Precision EW @ LEP

  18. Standard measurements of three TGC couplings that conserve C and P, U(1)em and global SU(2)L⊗U(1)Y one dimensional fit results (LEP): Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings one-dimensional fit results for the SM non-zero TGC values (ALEPH data only) … all other 24 couplings are consistent with zero (within 5-20%) ! Triple Gauge Couplings P.Azzurri - Precision EW @ LEP

  19. W leptonic couplings tau BR is three sigmas larger than e/mu ! Direct test of W lepton universality at the 1% precision level P.Azzurri - Precision EW @ LEP

  20. Direct test of W quark-lepton universality at the 0.6% precision level W hadronic couplings P.Azzurri - Precision EW @ LEP

  21. W mass Direct kinematic reconstruction of the W mass in qqqq, qql  (and l l ) final states Improve the W invariant mass resolution: kinematic fit energy-momentum conservation (equal mass constraints) • mW value extracted with different methods • Breit-Wigner fit (with bias correction) • Monte Carlo reweighting • (with different observables M1,δM) • Probability Density function P(M1,M2,..) Statistical power of the LEP2 data: ΔmW(stat)=21 MeV P.Azzurri - Precision EW @ LEP

  22. mW(qqqq)= 80.420±107 MeV mW(qql )= 80.411± 44 MeV (ρ=0.18) ΔmW(qqqq-qql )= +22±43 MeV without CR and BE errors W mass systematics The fully hadronic channel is de-weighted to 0.10 for possible final state interconnection effects ! P.Azzurri - Precision EW @ LEP

  23. Colour Reconnection effects: measure particle flow in regions between W’s / inside W’s upper limits: ki<2.13  Prob(CR)<0.65  ΔmW=90 MeV Hadronic final state interactions Bose-Einstein correlations: measure two particle correlations between W’s / inside W’s full effect ΔmW=35 MeV measured fraction ΔmW=15 MeV P.Azzurri - Precision EW @ LEP

  24. CR insensitive jet reconstructions ? Prospects Can reduce the CR mass shifts by a factor 2-3 deteriorating the statistical precision by 20% CR systematics : ΔmW=90  40 MeV statistical error: ΔmW=35  40 MeV total mW(qqqq) error:ΔmW=110  60 MeV mW(qqqq) weight in combination : 0.10  0.30 combined mW error :42  38 MeV • Ways to reduce the CR bias: • Remove low energy particles (pcut) • Hybrid cone algorithm (R) P.Azzurri - Precision EW @ LEP

  25. W mass and width W measured with the same methods used for the mW extraction mW= 80.412±0.042 GeV/c2 W= 2.150±0.91 GeV/c2 P.Azzurri - Precision EW @ LEP

  26. Standard Model Fit New Tevatron run2 top mass … P.Azzurri - Precision EW @ LEP

  27. Conclusions 1989-2000: LEP data provided a large set of new electroweak measurements  LEP1 final results on the Z pole HF asymmetries  Many LEP2 final results on cross sections and couplings • Still waiting for final results on W mass  All results published by 2006? P.Azzurri - Precision EW @ LEP

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