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W physics at LEP

W physics at LEP. E.Barberio Southern Methodist University PIC2003 Zeuthen 28 th June 2003. the LEP program. LEP1: 18 Million Z boson decays (89-95 ) LEP2: 36 Thousand W pairs (96-00). W pair production triple and quartic gauge couplings W mass and width measurements

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W physics at LEP

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  1. W physics at LEP E.Barberio Southern Methodist University PIC2003 Zeuthen 28th June 2003

  2. the LEP program LEP1: 18 Million Z boson decays (89-95) LEP2: 36 Thousand W pairs (96-00) • W pair production • triple and quartic gauge couplings • W mass and width measurements • final state interactions this talk: E.Barberio

  3. WW events WWlnln leptonic channel 10.6% large missing energy semileptonic channel 43.8% missing energy low background WWqqln hadronic channel 45.6% large background ambiguity in assigning jets to W WWqqqq E.Barberio

  4. =0.9980.006(stat)0.007(syst) W pair cross section + + preliminary LEP clear evidence of WWg and WWZ vertices: probe of the non-Abelian structure of the Standard Model 1%measurement E.Barberio

  5. = 1.000 0.021 = 1.052 0.029 = 1.052 0.028 SM: 10.83% hadronic branching fraction: Br(Wqq’) = 67.92  0.27% SM: 67.51% W branching fractions test of lepton universality at 3% (less precise than LEP1) E.Barberio

  6. W W g Z W W SM values triple gauge couplings WWg WWZ general WWg and WWZ interaction: 14 parameters applying C and P invariance & use low-energy constraints we are left with 3 parameters relation with the static W properties: magnetic dipole moment electric quadrupole moment E.Barberio

  7. W- qW e- e+ f W+ q W f measuring the coupling at LEP2 WW production: most constraining sensitive observables W+W- production angle cosW W decay angles (helicity) W rest frame q and  of W decay products E.Barberio

  8. WW production/decay angular distributions E.Barberio

  9. + OPAL preliminary • - single W • WW angles • sWW • combined 8% precision kg Single W single W production smaller cross section than WW: but it is very constraining for kg: E.Barberio

  10. TGC 1-parameter fit results - ALEPH - DELPHI - L3 - OPAL - LEP (partial statistics) g1Z, kg 2-5% measurement dominant systematics O(em) g1Z,lg: 0.015 kg: 0.039 E.Barberio

  11. TGC 3-D parameter fit results 2D contour: 3rd parameter at the minimum joint minimization of statistical error E.Barberio

  12. sL/s =0.2430.0270.012 SM: 0.240 at s=197 GeV cosqh* OPAL L sL=r00ds/dcosqWdcosqW sT=(r+++r--)ds/dcosqWdcosqW cosqW W polarization in the SM  W boson longitudinally polarized unfold decay angle distribution spin density matrix sL/s =0.2100.0330.016 evidence for WL at 5s level ! E.Barberio

  13. Quartic Gauge Coupling in SM these couplings exist but too small to be seen at LEP look for anomalous contributions parameterised by additional terms in the Lagrangian new OPAL analysis of WWg • couplings a0, ac, an; • physics scale  • -0.020 < a0/2 < 0.020 GeV-2 • -0.053 < ac/2 < 0.037 GeV-2 • -0.16 < an/2 < 0.15 GeV-2 E.Barberio

  14. excellent mass resolution comes from kinematic fit: constrain total (E,p) to (s,0) need for precise knowledge of the beam energy from LEP raw mass mass of the W boson measure mW and mtop prediction of mH direct reconstruction: mW from the invariant mass calculated using the W decay products WW  qqqq and WW  qqln (ALEPH and OPAL also WW  lnln) E.Barberio

  15. L3 tnqq ALEPH 4q OPAL mnqq DELPHI enqq reconstructed mass distributions E.Barberio

  16. mW spectrum W observation (DETECTOR) W production and decay Pert.QCD hadronisation decay reconstructed mass distorted! - initial state radiation E0<Ebeam - mW(jet/recon. lepton)  mW(quark/lepton) mW extraction calibrated with Monte Carlo simulation E.Barberio

  17. direct measurements LEP: latest results mWworld=80.4260.034 GeV GW constrained to SM relationship with mW: mH<210 GeV @ 95% C.L. SM fit mH > 114 GeV direct limit mW(GeV) E.Barberio

  18. qqlv qqqq comb. corr. e c y rad. corrections 8 8 8 e c y fragmentation 19 18 18 - c y detector 1 4 10 1 4 e c y LEP energy 17 17 17 e - y CR - 90 9 BE - 35 3 e - y - - - other 4 5 4 systematics 31 101 31 statistical 3 2 3 5 29 total 44 107 4 3 Systematic errors experiments channels years WWqqqq weight channel in the combination: 9% cross-LEP effort in progress to address these errors derive them from data whenever is possible E.Barberio

  19. radiative corrections mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included: a new OPAL analysis tries to estimate on data the contribution of real  production using WWg events estimated mass shift due to real photon production from data ~ 6-8 MeV E.Barberio

  20. final state interactions (only 4q) possible interaction between the two W decays products not in the simulation  apparent shift in mw Colour Reconnection (CR): • W decay~0.1fm<< hadronization scale~1fm  colour flow between Ws • seen at ep,pp colliders (rapidity gaps) and in heavy meson decays Bose Einstein Correlation (BEC): • favours production of pairs/multiplets of identical particles close together • well established in single Z and W only phenomenological models fm E.Barberio

  21. W W L3 30% CR: particle flow in 4-jet events at LEP2 CR: modifies particle flow between Ws: RN=(A+C)/(B+D) is used to compare with models: various models and parameters! one experiment can exclude only extreme cases  LEP combination E.Barberio

  22. r particle flow: LEP combination between various models SK1 gives the largest mW bias: vary reconnection fraction preferred value in data Precmin~49% mass bias calculated from Precmin+1s used in the mW combination: mass shift increases (90 MeV) but data driven r=RNdata/RNno-CR r=0 no CR, r0 CR E.Barberio

  23. mW and CR strategies to reduce CR bias: - hybrid cone jet cone algorithm - remove low energy particle pcut all CR model used behave as SK1! it also reduces BEC systematics! systematics are under study SK1 parameter most probably LEP will use these strategies for the final mW  trade statistics for systematics: ~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics E.Barberio

  24. CR with mW - higher sensitivity than colour flow - mass difference  still use the qqqq channel to measure mW! mW(no-CR)–mWCR  to study CR combination with colour flow (almost uncorrelated) use this combination to get the CR systematics for the W mass: the exact procedure is under discussion all experiments are working on similar analyses it will be difficult to achieve a 5s discovery for CR in WW events E.Barberio

  25. Δρ = ρ(4q)- ρ(mix WW) hadronic parts of qqln rotate/boost Bose Einstein Correlations measure BEC between W comparing r(Q) (2-particle density) in 4q and ‘mixed’ WW events: R2(Q)=ρ(4q) /ρ(mix WW)noBE mix ‘WW’ event ALEPH, L3: no sign of BEC between Ws DELPHI: small BEC between Ws propagate results on BEC between Ws into mW systematics: work in progress however mass shift due to BEC is expected to be smaller than CR E.Barberio

  26. measuring the W width fit simultaneously for mW and GW  direct measurement of GW Gwworld=2.1390.069 GeV SM 2.095 GeV E.Barberio

  27. conclusions and outlook • LEP met the expectations and exceeded them • many properties of the W boson are measured • triple gauge coupling are well determined • 5s evidence of the longitudinal polarisation of the W • for the measurements of the W mass and width • there are good prospects to improve the results and for mW to meet the 35 MeV error goal • so far good agreement with the Standard Model predictions • final analyses still going on … E.Barberio

  28. q ∝|Vqq|2 W  q’ CKM unitarity and Vcs CKM unitarity for elements not involving the top quark flavour changing transitions W on-shell dominated by the error on the Br measurement of Vcs the least know CKM element before LEP2 (11%): |Vcs| = 0.966 ± 0.013 dominated by the error on the Br E.Barberio

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