1. R b @ LEP2 Yoram Rozen – Technion, Israel for the OPAL Collab.

2. bb      qq ee ee R b   Rc Rb OPAL ALEPH DELPHI Avg. LEP1 Rb@LEP2 Experimental parameter SM check s dependence Probe the top, Higgs sector Z-  interference “Nice to have” Precision “Need to have” Precision Zfitter

3. Rb-Theoretical prediction  Exchange  & z Z Exchange Zfitter Including radiative events Without Z/  interference

4. LEP2 cross sections ww qq qq non-rad. zz 4 fermions (4f) Fermion pair (non-radiative) radiative

5.  Exchange  & z Z Exchange LEP Fermion Pair Production None Radiative Cross Section non-rad events Zfitter

6. 1. Estimate hadronic events, make corrections  N qq select hadronic events reject radiative events reject 4-fermions events subtract remaining 4f and radiative events 2. Estimate bb events, make corrections  N tag 3.  Estimate remaining background   c,  uds 4. Tagging efficiency   b

7. Full energy events Radiative return to the Z Radiation of an initial state photon decreases effective collision energy to  s’ If  s’ close to Z peak  more likely to interact  enhancement of CS s’ (GeV) N MC S=207 GeV missing energy  fake lower s’ LEP signal definition: s’ > 0.85*s,  s’ =mass of propagator, ISR-FSR interference subtracted

8. Reject events with ‘other’ topologies: ww  qqqq (46%) Four jets No missing momentum ww  qql (43%) Two jets. Isolated lepton Missing momentum ww  l l (11%) [ee, ,e , e  ] Two leptons Missing energy  qq =94%  4f =5 %  4f ’ =0.3 % 4f veto

9. B tagging b-ness from three independent sources: Lifetime information Lepton Pt Kinematic variables each source gives ANN The 3 ANNs are combined into a likelihood for b,c,uds

10. B tagging Each event is divided to hemispheres The lb algorithm is applied to each hemisphere Each event is given a b-being likelihood L:

11. Efficiency b c u 4fudscb% 16878684 s’+ Event selection 0.580 77 4f veto 0.031.2548 B-tagging @0.3  lb cut Including a cut on cos(  thr )

12. g cc 0 1 2 3 4 5 6 7 8 123456 ALEPH D* OPAL D* ALEPH l OPAL l L3 l L3 event-shape average gcc results 90GeV gcc % HERWIG JETSET ARIADNE 90 GeV : Analytical result ~ 1.4% 90 GeV : Measured ~ 3.0% 200GeV: Analytical result ~ 2.6% 200GeV: “Measured” ~ 5.6% gcc Predictions q q g c c -

13. g bb gbb Predictions HERWIG JETSET ARIADNE Average 90 GeV : Analytical result ~ 0.18% 90 GeV : Measured ~ 0.25% 200GeV: Analytical result ~ 0.52% 200GeV: “Measured” ~ 0.72% q q g b b _

14. Systematics (207 GeV)  R b /R b Source 3.2%Track reconstruction 0.6 %MC stat 0.1 %S’ 0.5%Lepton ID 3.3%Total detector effects  R b /R b Source 1.3%b,c physics modelling 0.2 %K 0,  rate 0.3 %Interference 0.1% gcc gbb 0.1%4f background 1.3%Total physics modelling Total systematic uncertainty = 3.5%

15. 0.207  0.018  0.007183 207 205 202 200 196 192 189 E (GeV) 0.169  0.014  0.006 0.158  0.018  0.006 0.154  0.024  0.005 0.164  0.016  0.006 0.181  0.017  0.006 0.174  0.025  0.006 0.165  0.010  0.006 Rb  sys  stat Results   =  5.0  76%

16. Conclusion  Rb was measured by OPAL at 8 new energy points (183-207 GeV) Using a highly efficient and background-free B-tagger.  These measurements are consistent with the SM prediction: Thanks  To the organizers for the great snow/food/program  To my daughter for sitting so quietly  To Hagar Landsman for all the help throughout