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W physics at LEP

W physics at LEP. E.Barberio Southern Methodist University Dallas (USA) September 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 Dallas (USA) September 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. = 0.997 0.021 = 1.058 0.028 = 1.061 0.028 W branching fractions test of lepton universality at 3% (less precise than LEP1) SM: Wln and Wqq couplings are equal, but QCD correction enhance hadronic branching fraction: Br(Wqq’) = 67.8  0.28% SM: 10.83% SM: 67.51% E.Barberio

  5. 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| = 0.989 ± 0.014 dominated by the error on the Br E.Barberio

  6. =0.978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

  7. 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

  8. 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

  9. f1 qW WW production/decay angular distributions E.Barberio

  10. + 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

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

  12. TGC 3-D parameter fit results 2D contour: 3rd parameter at the minimum E.Barberio

  13. 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 polarisation in the SM  W boson longitudinally polarised unfold decay angle distribution spin density matrix sL/s =0.2100.0330.016 evidence for WL at 5s level ! E.Barberio

  14. e.g. OPAL • 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 s GeV 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 E.Barberio

  15. aem = 0.004 ppm • Gm = 9 ppm • mZ = 23 ppm very well measured! on-shell renormalization scheme O(em,s,mW, mZ, mHiggs, mtop ,Vij) Standard Model parameters e.w. process at tree level are computed from three parameters , GF , mZ and the CKM matrix elements Vij vacuum fluctuations modify the value of the observables -> when higher orders are included, observables are predicted as: contrary to ‘exact gauge symmetry’ theories (QED or QCD) the effect of heavy particles do not decouple: mtop was predicted by LEP1/SLD sensitivity to mHiggs or to any kind of “heavy new physics” at energies not accessible E.Barberio

  16. from data + theory from LEP from  decay tree level mW= 80.937 GeV wrong by 10s measurement of the W mass Dr radiative corrections r = -Da + Drew  3%  measure mW and mtop prediction of mH or new heavy objects which couple with the W as the Higgs does E.Barberio

  17. 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 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

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

  19. 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

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

  21. 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

  22. 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

  23. 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 also at the hadronization phase • 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 fm only phenomenological models E.Barberio

  24. expected effects of color reconnection the effect should be present in the data, but how strong it is ? It affects: - interaction between decay products at the parton level - final hadronic color singlets do not correspond to the initial W bosons effects: - change in particle particle multiplicity - depletion of soft momenta particles - anomalies in the particle flow /string effect modified - rapidity gaps - change in the reconstructed value of mW : the most sensitive observable unfortunately E.Barberio

  25. 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

  26. 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

  27. using mw for CR? mW is the most sensitive observable andwe can use it to measure/limit CR CR affects more particles in the interjet region exclude/change the weight of soft inter-W particles from jets! strategies to reduce CR bias: • - hybrid cone jet cone algorithm • remove low energy particle pcut • jet direction from Spk : • K>0 decreases sensitivity; • K<0 enhance it variable used mass difference: e.g. mW(k<0)-mW(K>0) this allows to use the qqqq channel to measure mW E.Barberio

  28. mW and CR Delphi (this summer): cone and 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

  29. 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

  30. 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

  31. 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

  32. conclusions and outlook • measurements at the Z peak demonstrate that the SM is a quantum field theory • measurements above the WW threshold demonstrate that the SM is a non-abelian gaunge theory • and as for the Z, measurements of the W properties at LEP has brought the quantitative test of the SM to a high level of accuracy: • no deviation are observed within that accuracy • LEP2 achievements were better than foreseen: • triple gauge coupling are now well determined: 5% measurement! • 5s evidence of the longitudinal polarisation of the W • measurement the W mass 42 MeV and 91 MeV for the width, with good prospects to improve mW to meet the 35 MeV error • …BUT LEP did not see the Higgs…. E.Barberio

  33. global fit of the SM to data  deduce mH which gives best 2 limit from direct searches mH> 114.4 GeV mH radiative corrections ~ log mH mHew < 219 GeV 95% C.L. largest discrepancy:  3 P(2) ~ 4.4% all P(2) ~ 27.3% without NuTeV E.Barberio

  34. LHC pp, s=14 TeV, start 2007? mW 2002 LEP2+Run1 5.1 GeV 33 MeV  2006 LEP2+Run2 2.5 GeV 25 MeV 2009 ? LHC 1.5 GeV 15 MeV ??? LC ? 0.2 GeV 7 MeV ~ 4 years ~1 year ~3 years 5 mtop full mass range accessible in 1 year ( 5)  final word LEP limit ~50% ~ 35% ~25% ~10% LHC and the electroweak interaction if Higgs discovered  comparison of measured mH with indirect measurement E.Barberio

  35. x2 p x1 p W p p n l mW at hadron colliders:Tevatron single W production through qq annihilation: p = Ebeam=s/2 mW measurement is performed in the leptonic channels using the transverse mass: pTv is inferred from the recoil system balancing the W the non-zero pT is due to gluon radiation from quarks E.Barberio

  36. Systematics: key issues calibration, energy scales and resolutions: challenge for detector alignment and calibration, use Z, , J/ mass peaks pTW distribution  Z bosons (fully reconstruct) plus models/theory for difference between Z and W (different initial state quarks) recoil pT distribution  Z bosons with study of underlying event ET distributions from proton remnants and multiple interactions HENCE major limitation on systematics from Z statistics… E.Barberio

  37. Tevatron results RunI (~100 pb-1,15-30k events per channel): CDF W and e, D0 We Tevatron (+UA2): mW= 80.454  0.059 GeV main systematics ‘almost’ uncorrelated E.Barberio

  38. mW at hadron machines: LHC mtop~2 GeV requires mW ~ 15 MeV statistical error for 10 fb-1 DmW<2 MeV W  l: 3 x 108 events Z  ll: 3 x 107 events one LHC experiment plus unknown effects ..… E.Barberio

  39. conclusions and outlook • LEP gave a very solid ground to the Standard Model of electroweak interactions • however: the Higgs is still missing…… • Tevatron is exploring a higher energy region and will reduce the uncertainties on mtop and mW (measurement uncorrelated with LEP) but has little chances to see the Higgs • LHC will explore a higher energy region: it will cover the full allowed range for the Higgs • if we find the Higgs at LHC we will need another e+e- machine for precision measurements E.Barberio

  40. LHC events previous machines in 1 year total statistics Z 108 LEP: 107 in ~10 yrs W 109 FNAL: 107 in ~7 yrs top 108 FNAL: 105 in ~7 yrs event rate and particle multiplicity • L = luminosity = 1034 cm-2 s-1 • bunch spacing = 25 ns • 22 events / bunch E.Barberio

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