1. W physics at LEP E.Barberio Southern Methodist University Dallas (USA) September 2003

2. Nikhef, September 12 th, 2003 E.Barberio 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:

3. Nikhef, September 12 th, 2003 E.Barberio WW events semileptonic channel 43.8% missing energy low background hadronic channel 45.6% large background ambiguity in assigning jets to W leptonic channel 10.6% large missing energy WW  l l WW  qql WW  qqqq

4. Nikhef, September 12 th, 2003 E.Barberio W branching fractions = 0.997  0.021 = 1.058  0.028 = 1.061  0.028 test of lepton universality at 3% (less precise than LEP1) SM: 67.51% SM: Wl and Wqq couplings are equal, but QCD correction enhance hadronic branching fraction: Br(W  qq’) = 67.8  0.28% SM: 10.83%

5. Nikhef, September 12 th, 2003 E.Barberio CKM unitarity and V cs |V cs | = 0.989 ± 0.014 dominated by the error on the Br measurement of V cs the least know CKM element before LEP2 (11%): flavour changing transitions W on-shell CKM unitarity for elements not involving the top quark dominated by the error on the Br ∝ |V qq | 2 q W q’ 

6. Nikhef, September 12 th, 2003 E.Barberio W pair cross section 1% measurement clear evidence of WW  and WWZ vertices: probe of the non-Abelian structure of the Standard Model + + =0.978  0.006 (stat)  0.007 (syst) preliminary LEP

7. Nikhef, September 12 th, 2003 E.Barberio triple gauge couplings WW  WWZ W W  W W Z general WW  and WWZ interaction: 14 parameters electric quadrupole moment magnetic dipole moment applying C and P invariance & use low-energy constraints we are left with 3 parameters relation with the static W properties: SM values

8. Nikhef, September 12 th, 2003 E.Barberio measuring the coupling at LEP2 sensitive observables WW production: most constraining WW e-e- e+e+ W+W+ W-W-  WW f f W  decay angles (helicity) W + W - production angle cos  W W  rest frame  and  of W decay products

9. Nikhef, September 12 th, 2003 E.Barberio WW production/decay angular distributions WW 11

10. Nikhef, September 12 th, 2003 E.Barberio Single W single W production + 8% precision but it is very constraining for k   smaller cross section than WW: OPAL preliminary - single W - WW angles -  WW - combined kk

11. Nikhef, September 12 th, 2003 E.Barberio TGC 1-parameter fit results (almost final) - ALEPH - DELPHI - L3 - OPAL - LEP g 1 Z, k  2-5% measurement dominant systematics O(  em ) g 1 Z,  : 0.015   :0.039

12. Nikhef, September 12 th, 2003 E.Barberio TGC 3-D parameter fit results 2D contour: 3 rd parameter at the minimum

13. Nikhef, September 12 th, 2003 E.Barberio W polarisation in the SM  W boson longitudinally polarised spin density matrix evidence for W L at 5  level ! OPAL cos  W L  L =   00 d  /dcos  W dcos  W  T =  (  ++ +  -- )d  /dcos  W dcos  W  L /  =0.243  0.027  0.012 SM: 0.240 at  s=197 GeV cos  h *  L /  =0.210  0.033  0.016 unfold decay angle distribution

14. Nikhef, September 12 th, 2003 E.Barberio 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 couplings a 0, a c, a n ; physics scale  -0.020 < a 0 /  2 < 0.020 GeV -2 -0.053 < a c /  2 < 0.037 GeV -2 -0.16 < a n /  2 < 0.15 GeV -2 e.g. OPAL  s GeV

15. Nikhef, September 12 th, 2003 E.Barberio Standard Model parameters e.w. process at tree level are computed from three parameters , G F, m Z and the CKM matrix elements V ij contrary to ‘exact gauge symmetry’ theories (QED or QCD) the effect of heavy particles do not decouple: m top was predicted by LEP1/SLD sensitivity to m Higgs or to any kind of “heavy new physics” at energies not accessible vacuum fluctuations modify the value of the observables -> when higher orders are included, observables are predicted as: O(  em,  s,m W, m Z, m Higgs, m top,V ij ) on-shell renormalization scheme   em = 0.004 ppm  G  = 9 ppm  m Z = 23 ppm very well measured!

16. Nikhef, September 12 th, 2003 E.Barberio measurement of the W mass  measure m W and m top  prediction of m H or new heavy objects which couple with the W as the Higgs does  r radiative corrections  r = -  +  r ew  3% from data + theory from  decay from LEP tree level m W = 80.937 GeV wrong by 10 

17. Nikhef, September 12 th, 2003 E.Barberio excellent mass resolution comes from kinematic fit: constrain total (E,p) to (  s,0) need for precise knowledge of the beam energy from LEP mass of the W boson direct reconstruction : m W from the invariant mass calculated using the W decay products WW  qqqq and WW  qql (ALEPH and OPAL also WW  l l ) raw mass

18. Nikhef, September 12 th, 2003 E.Barberio reconstructed mass distributions DELPHI e qq ALEPH 4q L3  qq OPAL  qq

19. Nikhef, September 12 th, 2003 E.Barberio m W spectrum m W extraction calibrated with Monte Carlo simulation hadronisatio n W production and decay Pert.QCD decay W observation (DETECTOR) reconstructed mass distorted! - initial state radiation E 0 <E beam - m W (jet/recon. lepton)  m W (quark/lepton)

20. Nikhef, September 12 th, 2003 E.Barberio LEP: latest results m W (GeV) m W world =80.426  0.034 GeV  W constrained to SM relationship with m W : direct measurements m H <210 GeV @ 95% C.L. SM fit m H > 114 GeV direct limit

21. Nikhef, September 12 th, 2003 E.Barberio Systematic errors WW  qqqq weight channel in the combination: 9% experiments channels years qqlvqqqqcomb.corr. e c y CR-909 e- y BE-353e- y other454 --- rad. corrections888 fragmentation1918 e c y detector141014 - c y LEP energy17 e c y systematics3110131 statistical323529 total4410743 cross-LEP effort in progress to address these errors derive them from data whenever is possible

22. Nikhef, September 12 th, 2003 E.Barberio radiative corrections a new OPAL analysis tries to estimate on data the contribution of real  production using WW  events m W calibrated on Monte Carlo with O(  ) photon radiation but not all diagrams are completely included: estimated mass shift due to real photon production from data ~ 6-8 MeV

23. Nikhef, September 12 th, 2003 E.Barberio final state interactions (only 4q) possible interaction between the two W decays products not in the simulation  apparent shift in m w only phenomenological models fm 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

24. Nikhef, September 12 th, 2003 E.Barberio expected effects of color reconnection 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 m W : the most sensitive observable unfortunately It affects: - interaction between decay products at the parton level - final hadronic color singlets do not correspond to the initial W bosons the effect should be present in the data, but how strong it is ?

25. Nikhef, September 12 th, 2003 E.Barberio CR: particle flow in 4-jet events at LEP2 L3 30% R N =(A+C)/(B+D) is used to compare with models: various models and parameters! one experiment can exclude only extreme cases  LEP combination CR: modifies particle flow between Ws: W W

26. Nikhef, September 12 th, 2003 E.Barberio particle flow: LEP combination r=R N data /R N no-CR r=0 no CR, r  0 CR preferred value in data P rec min ~49% r between various models SK1 gives the largest m W bias: vary reconnection fraction mass bias calculated from P rec min +1  used in the m W combination: mass shift increases (90 MeV) but data driven

27. Nikhef, September 12 th, 2003 E.Barberio using mw for CR? m W is the most sensitive observable and we can use it to measure/limit CR CR affects more particles in the interjet region variable used mass difference: e.g. m W (k 0) this allows to use the qqqq channel to measure m W 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 p cut - jet direction from  p k : K>0 decreases sensitivity; K<0 enhance it

28. Nikhef, September 12 th, 2003 E.Barberio m W and CR SK1 parameter most probably LEP will use these strategies for the final m W  trade statistics for systematics: all CR model used behave as SK1! it also reduces BEC systematics! systematics are under study ~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics Delphi (this summer): cone and p cut

29. Nikhef, September 12 th, 2003 E.Barberio CR with m W combination with colour flow (almost uncorrelated) m W (no-CR) –m W CR  to study CR - higher sensitivity than colour flow - mass difference  still use the qqqq channel to measure m W ! 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 5  discovery for CR in WW events

30. Nikhef, September 12 th, 2003 E.Barberio Bose Einstein Correlations hadronic parts of qqln rotate/boost mix ‘WW’ event measure BEC between W comparing  (Q) (2-particle density) in 4q and ‘mixed’ WW events: R 2 (Q)=ρ(4q) /ρ(mix WW) noBE Δρ = ρ(4q)- ρ(mix WW) ALEPH, L3: no sign of BEC between Ws DELPHI: small BEC between Ws propagate results on BEC between Ws into m W systematics: work in progress however mass shift due to BEC is expected to be smaller than CR

31. Nikhef, September 12 th, 2003 E.Barberio measuring the W width fit simultaneously for m W and  W  direct measurement of  W SM 2.095 GeV  w world =2.139  0.069 GeV

32. Nikhef, September 12 th, 2003 E.Barberio 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! 5  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 m W to meet the 35 MeV error …BUT LEP did not see the Higgs….

33. Nikhef, September 12 th, 2003 E.Barberio global fit of the SM to data limit from direct searches m H > 114.4 GeV  deduce m H which gives best  2 radiative corrections ~ log m H m H ew < 219 GeV 95% C.L. mHmH largest discrepancy:  3  P(  2 ) ~ 4.4% all P(  2 ) ~ 27.3% without NuTeV

34. Nikhef, September 12 th, 2003 E.Barberio LHC and the electroweak interaction 55 full mass range accessible in 1 year (  5  )  final word ~1 year ~3 years ~ 4 years LHC pp,  s=14 TeV, start 2007? LEP limit ~50% ~ 35% ~25% ~10% 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  m top mWmW if Higgs discovered  comparison of measured m H with indirect measurement

35. Nikhef, September 12 th, 2003 E.Barberio m W at hadron colliders:Tevatron p T v is inferred from the recoil system balancing the W the non-zero p T is due to gluon radiation from quarks single W production through qq annihilation: m W measurement is performed in the leptonic channels using the transverse mass: p = E beam =  s/2 x 1 p x 2 p p p W l

36. Nikhef, September 12 th, 2003 E.Barberio Systematics: key issues p T W distribution  Z bosons (fully reconstruct) plus models/theory for difference between Z and W (different initial state quarks) recoil p T distribution  Z bosons with study of underlying event E T distributions from proton remnants and multiple interactions HENCE major limitation on systematics from Z statistics… calibration, energy scales and resolutions: challenge for detector alignment and calibration, use Z, , J/  mass peaks

37. Nikhef, September 12 th, 2003 E.Barberio Tevatron results error sourceCDF  CDFeD0 lepton E scale857556 lepton E resl202519 P T W distrib.2015 recoil model353735 selection bias18-12 backgrounds2559 PDFs / lumi15 8 radiative corr n 11 12 statisitcs1006560 total14411384 RunI (~100 pb -1, 15-30k events per channel): CDF W  and e, D0 W  e Tevatron (+UA2): m W = 80.454  0.059 GeV main systematics ‘almost’ uncorrelated

38. Nikhef, September 12 th, 2003 E.Barberio m W at hadron machines: LHC  m top ~2 GeV requires  m W ~ 15 MeV systematics  m W (MeV) statistics  2 2 E-p scale15? energy resolution5? recoil model5? lepton id5 pTWpTW 5 parton distr.func.  10? W width7 radiative decays10 background5 total  25 statistical error for 10 fb -1  m W <2 MeV W  l : 3 x 10 8 events Z  ll: 3 x 10 7 events plus unknown effects..… one LHC experiment

39. Nikhef, September 12 th, 2003 E.Barberio 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 m top and m W (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

40. Nikhef, September 12 th, 2003 E.Barberio event rate and particle multiplicity L = luminosity = 10 34 cm -2 s -1 bunch spacing = 25 ns 22 events / bunch LHC events previous machines in 1 year total statistics Z 10 8 LEP: 10 7 in ~10 yrs W 10 9 FNAL: 10 7 in ~7 yrs top 10 8 FNAL: 10 5 in ~7 yrs