Z boson pair production

1
Z boson pair production
at DELPHI
Ivo van Vulpen
Outline

• LEP and its operation between 1997-2000
• ZZ production final states
• Discussion of the various analyses
• Combined results

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LEP2 data set
Above ZZ threshold
Lum. pb-1
?s GeV
year
1997
183 54.7
1998
189 158.0
192 25.9
196 76.9
1999
200 84.3
202 41.1
lt205.5 82.0
2000
gt205.5 142.2
LEP2 665
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NC02 diagrams on-shell ZZ production
NC02 diagrams
? Subset of all 4-fermion diagrams
M2(NC02)
ZZ ?
1) Define a ZZ signal at generator level
gt 0.5
M2(all 4f)
2) Measure ?zz
3)Translate ?zz to a full NC02 cross section
?zz --gt ?NC02
(few corrections)
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Motivation
? No ZZZ coupling in the SM
--gt constrain neutral TGCs
? Similar to Higgs production
experimental signature
cross section
--gt test Higgs analyses
--gt possible strong systematic effects
Most analyses are adapted Higgs analyses
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ZZ final states
LEP1 Z --gt qq (70) Z --gt ?? (20)
Z --gt ll- (10)

Except all final states covered (96)
ZZ branching fraction in

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four leptons
ZZ --gt 4 muons
Rare beauties
Branching ratio only 1
clean signal
(very small background)
Unfortunately also a
very small signal
side-view
--gt One or two events per year lt--
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qqll- (muons and electrons)
Front-view
? Clean events (2 isolated leptons)
Small backgrounds (Z? and WW)
? Efficiencies 70 (85) for e (?)s
Branching ratio 9.4
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qqll- (with taus)
Dedicated tau analysis
Branching ratio 4.7
? Main background WW
? probabilistic analysis
Isolation of ? candidates
?2 of kinematic fit
M??- Mqq Mz
? Efficiency ? 40
Purity ? 80
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qq??
2 jets missing energy
Branching ratio 28.0
? Large background from
WW qq ?? final state
qq(?) photon lost
We? electron lost
? Mass cuts in pre-selection
? 2 analyses in DELPHI
side-view
Likelihood ratio
Iterative Discriminant Analysis
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qq?? (IDA analysis)
Iterative Discriminant Analysis
LEP2 combined
Visible mass
Acoplanarity
Acollinearity
Visible energy
Recoil mass
etc etc.
signal
Compute correlations (NN like)
Second order discriminant function
Likelihood fit to IDA distribution
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4 quarks
4 (or more) jets
Branching ratio 48.6
? Large backgrounds
3d-view
WW 17 x ZZ signal
qq(?) with gluon radiation
5 x ZZ signal
? Pairing ambiguities
4 jets --gt 3 pairings
5 jets --gt 10 pairings
? DELPHI 5 jets treated as 5 jets and all
pairings used
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qqqq probability distribution
Likelihood ratio analysis
? b-tag information (per jet)
--gt reject WW
? Mass information (per pairing)
? Topological info. (per event)
--gt reject qq(?)
Event-by-event ZZ probability
Likelihood fit to ZZ prob. distr.
efficiency ? 42 , purity ? 35
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NC02 (qqbb) results (all LEP2 data)
Eff 60
NC02 (qqbb)
Pur 45
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Combined results (per energy)
Theoretical prediction YFSZZ
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Measured/Predicted NC02 cross section
4 leptons
qqll
4 quarks
? Expected contribution of
individual channels to
PRELIMINARY
combined uncertainty
qq??
? LEP2 combined DELPHI result on NC02 cross
section
PRELIMINARY
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Conclusions
? NC02 cross section in agreement with SM
expectation
96 of final states covered
? Irreducible background for Higgs searches
under control
? Constrained neutral TGCs
? Statistics dominated
(even the combined LEP result)