(Final) achievements on Higgs boson searches at LEP2

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 1989. Stopped at
  November 2000
• Energy spread 91ltEcmlt 209. Since 1995
  Ecm gt 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
3
SM Higgs boson production, decays

• Higgsstrahlung WW,ZZ Fusion

mH ??s - mZ
We can go beyond the kinematic limit
H?bb dominant
Channels
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
SV
PV
DELPHI
Z? events
u,d,s,c
b
Data/MC agreement ? 5
5
LEP2 cross sections
qq, Z? dominant but can be reduced ZZ
irreducible for H, hA searches WW irreducible
for HH- searches

• Background cross sections

103
SM Higgs cross section ( lt 1pb )
102
10
1
10-1
? (pb)
?s GeV
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
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 5.8
Signal 11.4
D/Bck 22/20.2
Signal 2.7

• All channels contribute
• Hqq,H??,Hll,qq??
• ? event select A discriminant variable and a mass

8
ToolsStatistical 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.fs of a discriminant variable
  and a mass
• Test the compatibility of data with B-only or
  SB hypotheses
• from S and B rates and 2d pdfs
• using a likelihood ratio test statistics
• which simplifies into
• -2lnQ2stot-2?Niln(1si/bi)
• (stot total signal rate, si/bi s/b prob per pdf
  bin)
• e.g
• Expectations on 2lnQ, one experiment in 2000
• Kinematic edge _at_ 116 GeV
• Powerful separation for MH110 GeV/c2
• More difficult for MH115 GeV /c2

DELPHI
9
SM Higgs boson results
ADLO
Median, ?1?, ?2? bands expected from background
only Data consistent with a Higgs signal of
MH115.6 GeV/c2 Expected curve from Bcksignal
of 115 GeV/c2

• LEP result corresponds to 2.1? over background
• At 95C.L mHgt114.1 GeV/c2(obs) mHgt115.4
  GeV/c2(exp)
• Compatibility with background fluctuation
  hypothesis 3.4
• Compatibility with a signal of 115.6 GeV/c244

10
A golden candidate in the SM search

• Event compatible with SM Higgs boson decay
• ee-?HZ ?bbqq
• A 4-jet event with 2 b-jets
• Some properties
• MH114.3? 3 GeV/c2
• Strong S/B probability ratio4.6

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
• MH115.6 GeV/c2

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 ?2 ?2
d-type quarks ?2 ?1
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)
13
MSSM Higgs searches
ADLO

• Combine hZ , hA searches
• Scenario mh-max Allows max
• theoretical bound on mh w.r.t tan?

ee- ?hA
ee- ?hZ
mh?91GeV/c2 , mA?91.9 GeV/c2 0.5 lt tan? lt
2.4 excluded
14
MSSM Higgs searches
ADLO

• Combine hZ , hA searches
• ScenarioNo mixing

ee- ?hA
ee- ?hZ
mh?91.5 GeV/c2 , mA?92.2 GeV/c2 0.7 lt tan?
lt 10.5 excluded
15
Flavor independent Higgs search
ADLO

• In some extensions to SM H-b coupling could be
  suppressed
• e.g Search for ee- ?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
16
Invisible Higgs searches
ADLO

• ee- ?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
17
Non fermionic Higgs searches
ADLO

• Searches in which Higgs couples to bosons only
• e.g Decay ee- ?hZ?(??)(anything)
• h0??? small (0.1 at SM , mh90 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
18
Model independent search for scalars

• ee- ?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
?lt1 mS0 lt81 GeV/c2 95C.L

(absolute mass limit for

SM coupling strength)
19
Search for Charged Higgses
ADLO

• In 2HDM models
• ee- ?HH-
• 3 channels
• cscs, cs??, ????
• Assume(cs, ??) saturate the decays
• But BRs are not constrained
• Production cross section ?(mH)
• Background from ee-?WW-
• Results
• Assuming Br(H?cs, ? ?)1
• m H? gt78.6 GeV/c2 95 C.L

78.6 GeV
20
Search for H? ? W?A
OPAL

• In a 2HDM type I model
• H? ? W?A, (A ? bb,??-)
• dominant if
• tan?gt1, kinematically allowed
• 3 final states are studied
• HH- ?W?AW?A ?qqbbqqbb
• HH- ?W?AW?A ?l?bbqqbb
• HH- ???W?A ???qqbb
• Results on (mH?, mA) plane

tan?10
tan?1
68 GeV
42
60
mA
tan?100
mH?
60
50
21
Search for doubly charged H?? bosons

• Appear in left right symmetric
• models and can be relatively light
• Decay to leptons, preferably ?
• Signatures 4l, 4lEmiss
• h?? unknown Yukawa coupling
• For h?? gt10-7 H?? decays fast
• Results
• m H?? gt98.5 GeV/c2
• for BR(H?? ? ? ? )1

22
Light Higgs Yukawa production

• Processes I,II suppressed if
• sin2(?-?) small, mAmhgt?s
• Look at Z?bb h/A ?bb ??

I
II
III
Then III dominant
enhancement factor of the bb h/A couplings
23
Conclusions

• For the SM Higgs boson
• MHlt196 GeV/c2 , MH?114 GeV/c2 , MH115.6 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