1. WW CROSS SECTIONS AND |Vcs| On behalf the LEP collaborations
Anne EALET
CPPM,France
OUTLINE
W Pair production at LEP
W selections
WW cross sections
W Branching ratios
Measurements of |Vcs|
Conclusions
On behalf the LEP collaborations

2. LEP PERFORMANCE

3. LEP DATA collected / experiment
 Luminosity/exp
700 pb-1 =>  WW

4. { WW PRODUCTION AT LEP e+ e+ W+ W+ ,Z e W- e- e- W- WW -> ffff
 WW are pair created mainly through 3 diagrams at Born level (CC03): e W+ W- e+ e- e+ e-
,Z W- W+
 Measurements of CC03 cross sections, are corrected for 4-fermion diagram interference.
 WW belongs to the more general CC 4-fermion final states:
WW -> ffff {
qqqq(46%)
qql(44%)
ll(11%)
With l= e,,

5. WW SELECTIONS WW -> ll l=e,, (11%) Characteristics
ALEPH WW-> 
208 GeV
Characteristics
2 acoplanar leptons
missing energy
OPAL WW-> ee
208.7 GeV

6. Typical efficiencies 60-80 % WW cross section level 2 pb
WW SELECTIONS
WW -> ll l=e,,
Selections
Cut based analyses:
Acoplanarity,lepton momentum,missing energy
Typical efficiencies %
background level 150 fb
WW cross section level 2 pb

7. WW SELECTIONS WW -> qql l=e,, (44%) Characteristics
2 hadronic jets
missing energy
isolated lepton or low multiplicity jet for 
L3 WW -> qqe
at 206 GeV
ALEPH WW -> qq
at 205 GeV

8. WW SELECTIONS WW -> qql l=e,, Selections  jet identification
Cut based analyses and/or probabilities/likelihodd
based on Elepton,isolation criteria
 jet identification
Typical efficiencies e/ 85 %
 60 %
background level e/ 100 fb
 200 fb
WW cross sections level 3 pb each channel

9. WW SELECTIONS WW -> qqqq (46%) Characteristics 4 hadronic jets
No missing energy
Huge QCD back.(100 pb)
ee -> qq(gg)
OPAL GeV
DELPHI GeV

10. Typical efficiencies 90 % WW cross section level 8 pb
WW SELECTIONS
OPAL 189 GeV
WW -> qqqq
Selections
Neural Networks, Likelihood analyses
DELPHI preliminary
Typical efficiencies 90 %
background level 2 pb
WW cross section level 8 pb

11. WW CROSS -SECTIONS

12. WW CROSS SECTIONS Extraction of the cc03 cross sections/channel:
Maximum likelihood fit between the number of observed events and the number of expected events
Luminosity
Efficiency
matrix
Background
+cc03-4f corr
with
Extraction of the total cross section and branching ratio:

13. WW CROSS SECTIONS

14. i.e. existence of tripel gauge boson couplings was proven
WW CROSS SECTIONS e+ e+ e-
,Z W- W+ W+ e W- e-
Theory at tree level
i.e. existence of tripel gauge boson couplings was proven

15. WW CROSS SECTIONS
Because of the high accuracy for LEP measurement, new level of precision was needed for the theory
(<1 %)
Major uncertainty is coming from EW radiative corrections
80 differents 4f channels ! was at 2 % level with gentle
use DPA (double pole approximation uncertainty => O(a) correction
Now uncertainty is at 0.5 % level (YFSWW and RacoonWW) g w g w g w w w
Real corrections
Virtual corrections

16. WW CROSS SECTIONS
Theoretical improvements:
New models with DPA calculation (RacoonWW and YFSWW)
Error =f(Ecm,Mw) 0.4%-0.7%
Now experimental precision is at same level than the theoritical one
Ref: RacoonWW:Nucl.Ph.B475(2000)
YFSWW 1.16: Com.Phys.Comm 140 (2001) 432 and ref in
Very good agreement with experimental values

17. WW CROSS SECTIONS Systematic uncertainties (preliminary values)
Statistic (LEP comb) >200 fb
Jets
-fragmentation model
-FSI effects fb
-Detector effect :calibration..
Leptons
-Identification fb
-reconstruction
others
Luminosity fb
Mc statistic
Theory
Correlated between exp or/and energy
Not Correlated
Correlated between energy
=> Systemetic is at least half the statistical error for all energies

18. WW CROSS SECTIONS

19. RWW = sCC3(exp)/scc3(Th)
Excellent agreement with DPA calculation
RWW(RacoonW) = / (stat) +/ (syst)

20. RWW = sCC3(exp)/scc3(Th)

21. WW Branching Fractions
Root:
WW Branching Fractions
67.92  0.27
SM = 67.5 %
10.69 0.09
SM = 10.8 %

22. |Vcs|
Measurement

23. Without assuming unitarity
|Vcs | MEASUREMENT
Indirect Measurement from Br(W -> qq)
Assuming world average of Vij :
without |Vcs |
Using the LEP Br(W -> qq)
Without assuming unitarity
|Vcs|= ± 0.013

24. |Vcs | MEASUREMENT
Direct measurement
from charm content in W decays

25. SUMMARY on |Vcs |

26. The least known CKM matrix element
SUMMARY on |Vcs |
The least known CKM matrix element

27. CONCLUSIONS LEP2 has worked very successfully up to 209 GeV
LEP combinations give :
WW Cross section measurements updated with 10 energies points
Good agreement with recent calculations of RacoonWW / YFSWW
RWW(DPA)= \
Experimental precision at the level of the theory !
SM is tested in the W at the loop level…
Branching ratios extraction combining all LEP data
Br(WW -> qq) =  0.27 % (SM: 67.5 %)
Br(WW -> l) =  0.09 % (SM: 10.8 %)
From WW production |Vcs|= ± 0.013
10 x more precise than with direct measurement

28. Correction factor f(Ecm,Mw)
WW CROSS SECTIONS
GENTLE 2.0
GENTLE 2.1
Correction factor f(Ecm,Mw)
No DPA calculation
New uncertainties
2% -> 0.7%
(only due to ISR)
Ref: see hep-ph/
and ref in

29. LEP PERFORMANCE YEAR 2000 ALEPH Ecm (GeV) <Ecm> = 206.7 GeV