1. Boson Gauge Couplings at LEP
Paolo SPAGNOLO
INFN Pisa
For the Four LEP Collaborations ADLO
40th Rencontres De Moriond On Electroweak Interactions And Unified Theories
8 Mar 2005, La Thuile, Aosta Valley, Italy
March
Paolo SPAGNOLO – INFN PI

2. LEP collider performance
GeV
GeV
189 GeV
183 GeV
GeV
LEP: e+e- collider at CERN
LEP1: ECM ~ MZ from 1989 to 1995
LEP2: ~ 700 pb-1 /exp above WW production threshold from 1996 to 2000
March
Paolo SPAGNOLO – INFN PI

3. Results from the LEP experiments
Main analyses (cross section, W mass, TGC) by all experiments
Results combined by joint LEP Working Groups
Common systematics and correlations
Almost all the statistics analyzed
A lot of results still preliminary, many final publications this year !!
ALEPH
DELPHI L3
OPAL 
2003
2004
2002 ff
2005 ZZ
W+W-
TGC Z mW
March
Paolo SPAGNOLO – INFN PI

4. Physics processes at LEP2
~100k evts
~10k evts
~1k evts
~100 evts
March
Paolo SPAGNOLO – INFN PI

5. Paolo SPAGNOLO – INFN PI
Boson Gauge Couplings
Standard Model : non-Abelian SU(2)xU(1) gauge symmetry
cTGC charged triple Gauge Couplings: WWg, WWZ
cQGC charged quartic Gauge Couplings: WWgg, WWZg, WWZZ, WWWW
(small or negligible at LEP)
Search for anomalous Gauge Couplings
charged: beyond the SM
neutral: not in the SM: nTGC: ZZZ, ZZg, Zgg and nQGC: ZZgg
Gauge bosons production required
LEP2 energy allowed these studies for the “first time”
March
Paolo SPAGNOLO – INFN PI

6. Charged Triple Gauge Couplings
Sensitivity to TGCs in 3 processes
WW pairs production
single W production
single photon production
March
Paolo SPAGNOLO – INFN PI

7. Paolo SPAGNOLO – INFN PI
W-pair events
WW enqq (~15%)
Channel
efficiency
purity
bkg
,, ll
ll 11%
50-80%
80-90%
eqq 15%
75-90%
~90%
qq, Zee
qq 15%
75-90%
~95% qq
qq 15%
50-80%
80-85%
qq,We
qqqq 45%
80-90%
75-80% qq
Gauge Couplings !
March
Paolo SPAGNOLO – INFN PI

8. Paolo SPAGNOLO – INFN PI
WW qqqq (~45%)
WW mnqq (~15%)
March
Paolo SPAGNOLO – INFN PI

9. Paolo SPAGNOLO – INFN PI
WW enmn (~4%)
WW tnqq (~15%)
March
Paolo SPAGNOLO – INFN PI

10. Backgrounds in WW cross section
All 4f diagrams (enqq)
March
Paolo SPAGNOLO – INFN PI

11. Paolo SPAGNOLO – INFN PI
LEP combination
sWW from each exp combined
Precision better than 1%
systematics correlation matrix
test the SM radiative correction to CC03 diagrams
March
Paolo SPAGNOLO – INFN PI

12. W-pair cross sections n exchange t channel ONLY March 8 2005
non divergent high energy xsection (unitarity ok)
n exchange t channel ONLY
March
Paolo SPAGNOLO – INFN PI

13. Paolo SPAGNOLO – INFN PI
W-pair cross sections
non divergent high energy xsection (unitarity ok)
n exchange and gWW vertex
March
Paolo SPAGNOLO – INFN PI

14. Clear proof of SU(2)xU(1) gauge couplings !
W-pair cross sections
non divergent high energy xsection (unitarity ok)
Clear proof of SU(2)xU(1) gauge couplings !
March
Paolo SPAGNOLO – INFN PI

15. Charged Triple Gauge Couplings
Sensitivity to TGCs in 3 processes
WW pairs production
single W production
single photon production
March
Paolo SPAGNOLO – INFN PI

16. Single W…. In agreement with SM within 8% precision
Sensitive to the WWZ
and WWg couplings
In agreement with SM within 8% precision
March
Paolo SPAGNOLO – INFN PI

17. … and single Z In agreement with SM within 7% precision March 8 2005
Paolo SPAGNOLO – INFN PI

18. Charged Triple Gauge Couplings
Sensitivity to TGCs in 3 processes
WW pairs production
single W production
single photon production
March
Paolo SPAGNOLO – INFN PI

19. Paolo SPAGNOLO – INFN PI
Single g n
Sensitive to the WWg couplings outside Z resonance !!
March
Paolo SPAGNOLO – INFN PI

20. Triple Gauge Couplings
Standard electroweak theory
U(1)
SU(2)
triple and quartic SU(2) gauge boson self couplings are the signature of the non-Abelian SU(2) electroweak structure !
The most general Lorentz Invariant WWV (V=  + Z) vertex
has 7+7 = 14 complex couplings
March
Paolo SPAGNOLO – INFN PI

21. Triple Gauge Couplings
C and P conservation  only 6 real out of 14 complex parameters
SU(2)L⊗U(1)Y  only 3 free parameters
March
Paolo SPAGNOLO – INFN PI

22. Paolo SPAGNOLO – INFN PI
Most important process for studying TGCs is
The WWZ and WW gauge couplings can be measured in W-pair events, fitting the W-pair event rates and the W production and decay angular distributions.
W Polar angle
fermion polar angle wrt the W direction
fermion azimuthal angle wrt the WW plane
March
Paolo SPAGNOLO – INFN PI

23. Paolo SPAGNOLO – INFN PI
Triple Gauge Couplings: Analysis
Use angular distributions from
all event topologies:
Do W charge tagging to obtain
direction in events
Use Monte Carlo to correct
for detector effects and to
implement radiative corrections
March
Paolo SPAGNOLO – INFN PI

24. Triple Gauge Couplings: 1d & 2d Fit LEP Results
3 parameters fit with C and P cons. and SU(2)L⊗U(1)Y
March
Paolo SPAGNOLO – INFN PI

25. Triple Gauge Couplings: 3d Fit Results
3 parameters fit with C and P cons. and SU(2)L⊗U(1)Y
March
Paolo SPAGNOLO – INFN PI

26. Triple Gauge Couplings
Relaxing all constraints and fitting for any of the 28
WWZ and WW couplings g
one-dimensional fit results
for the SM non-zero TGC values
(ALEPH data only)
… all other 24 couplings are
consistent with zero (within 5-20%) !
framework 1 for anomalous contributions. generic framework 2 to measure the SM contribution
Proof of the Standard Model SU(2)L⊗U(1)Y Structure
March
Paolo SPAGNOLO – INFN PI

27. Neutral TGC (excluded by SM)
Investigated in ZZ and Zg processes
ZZ on shell: 4 parameters f4Z, f4g, f5Z, f5g
Zg on shell: 8 parameters hiZ, hig (i=1,4)
Z angular distrib
in bosted frame ZZ
March
Paolo SPAGNOLO – INFN PI

28. Paolo SPAGNOLO – INFN PI
nTGC combined results
March
Paolo SPAGNOLO – INFN PI

29. Quartic Gauge Couplings
QGC at LEP: WWgg, WWZg, ZZgg
5 parameters: aW0, aWc, aWn, aZ0, aZc
Total cross section
Photon energy and angular distribution
OPAL
SM forbidden
March
Paolo SPAGNOLO – INFN PI

30. 95% C.L. Limits on quartic couplings in from
DELPHI L3
OPAL
95% C.L. Limits on quartic couplings in from ZZ gg
March
Paolo SPAGNOLO – INFN PI

31. Paolo SPAGNOLO – INFN PI
CONCLUSIONS
The Standard Model non-Abelian SU(2)xU(1) gauge symmetry PROVED!!!
MEASURED cTGCs charged triple Gauge Couplings: WWg, WWZ
NEGLIGIBLE cQGC quartic Gauge Couplings: WWgg, WWZg, WWZZ, WWWW
EXCLUDED anomalous Gauge Couplings
charged: beyond the SM
neutral: not in the SM: nTGC: ZZZ, ZZg, Zgg and nQGC: ZZgg
one of the most important results of LEP2
March
Paolo SPAGNOLO – INFN PI

32. Paolo SPAGNOLO – INFN PI
W Polarization in
Measure spin density matrices and extract polarized diff cross sec
test writing notes
Use avg value of projection op (OPAL)
Fit for (L3)
March
Paolo SPAGNOLO – INFN PI

33. Paolo SPAGNOLO – INFN PI
Form Factor for Vector Resonance Enhancement of
Map triangle onto plane
Triangle correponds to 95%
CL allowed region with
ALEPH Preliminary
95%CL contour
w/ center = (.96,-.15)
For the technirho mass at 95% C.L.
For the technirho mass at 95% C.L.
March
Paolo SPAGNOLO – INFN PI

34. W Boson Production at LEP2
e+e  Wene
e+e  W+W
March
Paolo SPAGNOLO – INFN PI

35. Paolo SPAGNOLO – INFN PI
Event selection: qqqq
Event properties
Large multiplicity
no missing momentum
4jet-like
Main background: qq(g)
Multidimensional techniques (NN, pdf, ...)
Eff. ~ 90%, purity ~ 85 %
Main systematics ~1.3 ÷ 2.%:
hadronization models (signal and bkg)
correlated among experiments !!
detector simulation
March
Paolo SPAGNOLO – INFN PI

36. Paolo SPAGNOLO – INFN PI
Event selection: lnqq
Two jets + high energy and isolated lepton (or t jet) + missing pt
Main backgrounds: qq(g) and 4f (non-WW)
Multivariable selections
Eff %, purity ~ 80-95%
Main systematics ~1.4 ÷ 2.5%:
lepton-id
background subtraction
March
Paolo SPAGNOLO – INFN PI

37. Paolo SPAGNOLO – INFN PI
Event selection: lnln
Low multiplicity
two leptons or thin jets (t jets)
Missing pt and acoplanarity
cuts depend on lepton flavour
Main background:
two photons ,di-leptons, 4f non-WW
Eff. ~50-80%, purity ~80-90%
Main systematics ~ %:
background subtraction
lepton identification
beam related detector noise
March
Paolo SPAGNOLO – INFN PI

38. O(a) radiative correction to WW production
TGC: ~1%
change in WW
production angle
Not available at the beginning of LEP2
W mass:
peak distortions
and radiation:
compensation:
< 5-10 MeV
No correction:
~2% higher
cross section.
No effect on
acceptance
March
Paolo SPAGNOLO – INFN PI