1. (Final) achievements on Higgs boson searches at LEP2
Rosy Nikolaidou
CEA/DAPNIA/SPP
Introduction
The Standard Model Higgs boson
More exotic scenarios
Conclusion
Results will be finalised
this year

2. Introduction
Started operating in Stopped at November 2000
Energy spread 91<Ecm< Since 1995 Ecm > mZ
 1000 pb-1 per experiment,
 688 pb-1 /exp above WW
threshold
The rise of energy at LEP
open the possibility to explore a wide mass range
Combined results from the 4 experiments
ALEPH DELPHI L3 OPAL are denoted
LEP
ADLO
R Nikolaidou
Beyond 2002 conference

3. SM Higgs boson: production, decays
Higgsstrahlung WW,ZZ Fusion
mH s - mZ
We can go beyond
the kinematic limit
Hbb dominant
Channels:
Zq q
Hbb
Hbb
Z
Hbb
Ze+e-,+-
Hbb (+-)
Z+-,q q
R Nikolaidou
Beyond 2002 conference

4. Fundamental tools: b-tagging
Silicon vertex detectors (e.x DELPHI)
Hit precisions: 10 m in R
15 m in Rz
B-hadron characteristics :
B-hadron life-time 1.6 ps
Flight distance  3 mm
Impact Parameter 400 m
Decay products of B-hadrons:
large IP, PT , lead to tracks
originating from SV…
Combination of (SV mass, IP,
decay length,PT of leptons, e.t.c)
to a single b-tagging variable
DELPHI IP SV PV
DELPHI
Z events
u,d,s,c b
Data/MC agreement  5%
R Nikolaidou
Beyond 2002 conference

5. LEP2 cross sections Background cross sections SM Higgs cross section
qq, Z* dominant but can be reduced
ZZ irreducible for H, hA searches
WW irreducible for H+H- searches
Background cross sections
103
SM Higgs cross section
(… < 1pb )
102 10 1
10-1
 (pb)
s GeV
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Beyond 2002 conference

6. Tools :Multidimensional analyses
Multidimensional techniques used
to reach the best S/B discrimination
(Neural Network, Likelihood ratio)
H L3
Hqq
Kinematic fits with E,P conservation
and Z mass constraint performed to improve on signal mass reconstruction
R Nikolaidou
Beyond 2002 conference

7. SM Higgs boson results ADLO
3 selections with increasing purity of a 115 GeV/c2 signal
D/Bck 61/60.3
D/Bck 187/188.1
Signal
Signal
D/Bck 22/20.2
Signal
All channels contribute
Hqq,H,Hll,qq
 event select: A discriminant variable and a mass
R Nikolaidou
Beyond 2002 conference

8. Tools:Statistical analysis of the results
For a powerful statistical analysis of the combined results
(per channel and experiment) :
Selection of ‘signal-like’ events in each researched channel
 event 2d p.d.f’s of a discriminant variable and a mass
Test the compatibility of data with B-only or S+B hypotheses
from S and B rates and 2d pdf’s
using a likelihood ratio test statistics
which simplifies into
-2lnQ=2stot-2Niln(1+si/bi)
(stot total signal rate, si/bi =s/b prob per pdf bin)
e.g
Expectations on –2lnQ, one experiment in 2000
Kinematic 116 GeV
Powerful separation for MH=110 GeV/c2
More difficult for MH=115 GeV /c2
DELPHI
R Nikolaidou
Beyond 2002 conference

9. SM Higgs boson results ADLO Median, 1, 2 bands expected from
background only
Data consistent with
a Higgs signal of
MH=115.6 GeV/c2
Expected curve from
Bck+signal of 115 GeV/c2
LEP result corresponds to 2.1 over background
At 95%C.L mH>114.1 GeV/c2(obs) mH>115.4 GeV/c2(exp)
Compatibility with background fluctuation hypothesis : 3.4%
Compatibility with a signal of GeV/c2:44%
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10. A golden candidate in the SM search
Event compatible with SM Higgs boson decay
e+e-HZ bbqq
A 4-jet event with b-jets
Some properties:
MH=114.3 3 GeV/c2
Strong S/B probability ratio=4.6
R Nikolaidou
Beyond 2002 conference

11. SM Higgs boson searches : a summary
From indirect limits from
EW precision measurements
(LEP,SLC,Tevatron)
MH196 GeV/c2 95% C.L
From direct searches at LEP
MH114.1 GeV/c2 95% C.L
Likelihood test preferred mass
MH=115.6 GeV/c2
R Nikolaidou
Beyond 2002 conference

12. 2 Higgs Doublet Models
In these models: 2 complex Higgs doublets (1 2 )
Five Higgs bosons:(h0, H0, A0, H)
Parameters: tan (=2/1 VEV ratio) (mixing angle of h0 H0 )
(Higgs mass parameter) Ao ( trilinear Higgs-sfermion coupling)
e.g
Couples to
Type I
Type II
’d-type’ leptons 2 1
’u-type’ quarks
’d-type’ quarks
sin2(-) HZ(SM) cos2(-) HZ(SM)
Large at small tan Large at large tan
mA mh
SM topologies and New topologies
searches are used hA bbbb, bb
Also h0A0A0,invisible
e.x MSSM
ghff  cos
e.x fermiophobic ( =/2)
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13. MSSM Higgs searches ADLO Combine hZ , hA searches
Scenario mh-max: Allows max
theoretical bound on mh w.r.t tan
e+e- hA
e+e- hZ
mh91GeV/c2 , mA91.9 GeV/c2
0.5 < tan < 2.4 excluded
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14. MSSM Higgs searches ADLO Combine hZ , hA searches Scenario:No mixing
e+e- hA
e+e- hZ
mh91.5 GeV/c2 , mA92.2 GeV/c2
0.7 < tan < 10.5 excluded
R Nikolaidou
Beyond 2002 conference

15. Flavor independent Higgs search
ADLO
Flavor independent Higgs search
In some extensions to SM H-b coupling could be suppressed
e.g Search for e+e- hZ , h hadrons (qq, gg), Z anything
Assuming Br(h hadrons) =1
Min eff (h qq,gg ) used
No more btagging applied
Results
Mh112.9 GeV/c2 95% C.L
assuming hZ= hZSM
Strong cross section limits
112.9 GeV
R Nikolaidou
Beyond 2002 conference

16. Invisible Higgs searches
ADLO
Invisible Higgs searches
e+e- Z*  hZ, h(stable, undetectable particles)
h decays
In MSSM h  00
Other models h  JJ (Majorons)
Signatures
large Emiss + dileptons or dijets
with mass close to mZ
114.4 GeV
Assuming BR(h inv)=1
mh114.4 GeV/c2 or
Up to mh 110 GeV/c2
hZ/ SM  40%
R Nikolaidou
Beyond 2002 conference

17. Non fermionic Higgs searches
ADLO
Searches in which Higgs couples to bosons only
e.g Decay: e+e- hZ()(anything)
h0 small (0.1% at SM , mh=90 GeV)
can be enhanced in (2HDM type I models,
modified MSSM e.t.c)
Look for energetic isolated photons
Assuming SM production rates,BR(hff)=0
mh108.2 GeV/c2
R Nikolaidou
Beyond 2002 conference

18. Model independent search for scalars
e+e- S0Z, (Zee,)
(S0  had, inv.,, lept, stable)
Compare recoil mass spectrum
with an expected S0 signal
S0 masses down to 1KeV
Limits S0Z=  HSMZ <1 mS0 <81 GeV/c2 95%C.L
(absolute mass limit for
SM coupling strength)
R Nikolaidou
Beyond 2002 conference

19. Search for Charged Higgses
ADLO
Search for Charged Higgses
In 2HDM models
e+e- H+H-
3 channels
cscs, cs, 
Assume:(cs, ) saturate the decays
But BR’s are not constrained
Production cross section (mH)
Background from e+e-W+W-
Results
Assuming Br(Hcs,  )=1
m H >78.6 GeV/c2 95% C.L
78.6 GeV
R Nikolaidou
Beyond 2002 conference

20. Search for H  WA OPAL mA In a 2HDM type I model
H  WA, (A  bb,+-)
dominant if
tan>1, kinematically allowed
3 final states are studied:
H+H- WAWA qqbbqqbb
H+H- WAWA lbbqqbb
H+H- WA qqbb
Results on (mH, mA) plane
tan=1
tan=10
68 GeV 42 60 mA
tan=100
R Nikolaidou
Beyond 2002 conference 50 60
mH

21. Search for doubly charged H bosons
Appear in left right symmetric
models and can be relatively light
Decay to leptons, preferably 
Signatures: 4l, 4l+Emiss
h unknown Yukawa coupling
For h >10-7 H decays fast
Results
m H >98.5 GeV/c2
for BR(H   ±  ± )=1
R Nikolaidou
Beyond 2002 conference

22. Light Higgs Yukawa production
Processes I,II suppressed if
sin2(-) small, mA+mh>s
Look at Zbb h/A bb  I II
III
Then
III dominant
enhancement factor of the
bb h/A couplings
R Nikolaidou
Beyond 2002 conference

23. Still pending question
Conclusions
For the SM Higgs boson
MH<196 GeV/c2 , MH114 GeV/c2 , MH=115.6 GeV/c2
LEP has explored and constrained a lot of models beyond the SM
Is the Higgs boson responsible for the EW
breaking mechanism?
(Tevatron, LHC, Linear Colliders)
Still pending question
wait for