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

June 2003E.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:

June 2003E.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

June 2003E.Barberio W pair cross section 1% measurement clear evidence of WW  and WWZ vertices: probe of the non-Abelian structure of the Standard Model + + =0.998  (stat)  (syst) preliminary LEP

June 2003E.Barberio W branching fractions SM: 10.83% =  =  =  test of lepton universality at 3% (less precise than LEP1) SM: 67.51% hadronic branching fraction: Br(W  qq’) =  0.27%

June 2003E.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

June 2003E.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

June 2003E.Barberio WW production/decay angular distributions

June 2003E.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

June 2003E.Barberio TGC 1-parameter fit results dominant systematics O(  em ) g 1 Z,  :   : (partial statistics) - ALEPH - DELPHI - L3 - OPAL - LEP g 1 Z, k  2-5% measurement

June 2003E.Barberio TGC 3-D parameter fit results joint minimization of statistical error 2D contour: 3 rd parameter at the minimum

June 2003E.Barberio W polarization in the SM  W boson longitudinally polarized 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   SM: at  s=197 GeV cos  h *  L /  =0.210   unfold decay angle distribution

June 2003E.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  < a 0 /  2 < GeV < a c /  2 < GeV < a n /  2 < 0.15 GeV -2 new OPAL analysis of WW 

June 2003E.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 measure m W and m top  prediction of m H

June 2003E.Barberio reconstructed mass distributions DELPHI e qq ALEPH 4q L3  qq OPAL  qq

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

June 2003E.Barberio LEP: latest results direct measurements m H <210 95% C.L. SM fit m H > 114 GeV direct limit m W (GeV) m W world =  GeV  W constrained to SM relationship with m W :

June 2003E.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 other rad. corrections888 fragmentation1918 e c y detector c y LEP energy17 e c y systematics statistical total cross-LEP effort in progress to address these errors derive them from data whenever is possible

June 2003E.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

June 2003E.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 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

June 2003E.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

June 2003E.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

June 2003E.Barberio m W and CR strategies to reduce CR bias: - hybrid cone jet cone algorithm - remove low energy particle p cut 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

June 2003E.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

June 2003E.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

June 2003E.Barberio measuring the W width fit simultaneously for m W and  W  direct measurement of  W SM GeV  w world =2.139  GeV

June 2003E.Barberio conclusions and outlook LEP met the expectations and exceeded them many properties of the W boson are measured triple gauge coupling are well determined 5  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 m W to meet the 35 MeV error goal so far good agreement with the Standard Model predictions final analyses still going on …

June 2003E.Barberio CKM unitarity and V cs |V cs | = ± 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’ 