(Final) achievements on Higgs (Final) achievements on Higgs boson searches at LEP2boson searches at LEP2
Rosy Nikolaidou CEA/DAPNIA/SPP
IntroductionThe Standard Model Higgs bosonMore exotic scenariosConclusion
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Introduction Introduction Started operating in 1989. Stopped at November 2000
Energy spread 91<Ecm< 209. 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
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SM Higgs boson: production, decaysSM Higgs boson: production, decays Higgsstrahlung WW,ZZ Fusion
Channels:
mH s - mZWe can go beyond the kinematic limit
Hbb dominantZq q
Hbb Hbb
Z
Hbb
Ze+e-,+-
Hbb (+-)
Z+-,q q
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Fundamental tools: b-taggingFundamental tools: b-taggingSilicon 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 tracksoriginating from SV… Combination of (SV mass, IP,
decay length,PT of leptons, e.t.c) to a single b-tagging variable
DELPHI
Z events u,d,s,c
b
IP
SV
PV
Data/MC agreement 5%
DELPHI
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LEP2 cross sectionsLEP2 cross sections
Background cross sections
SM Higgs cross section (… < 1pb )
qq, Z* dominant but can be reduced
ZZ irreducible for H, hA searches
WW irreducible for H+H- searches
10-1
1
10
102
103
(p
b)
s GeV
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Tools :Multidimensional analysesTools :Multidimensional analysesMultidimensional techniques used
to reach the best S/B discrimination
(Neural Network, Likelihood ratio)
Kinematic fits with E,P conservation
and Z mass constraint performed to improve on signal mass reconstruction
L3
Hqq
H
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SM Higgs boson resultsSM Higgs boson results 3 selections with increasing purity of a 115 GeV/c2 signal
•All channels contribute Hqq,H,Hll,qq event select: A discriminant variable and a mass
D/Bck 61/60.3Signal 5.8
D/Bck 22/20.2Signal 2.7
D/Bck 187/188.1Signal 11.4
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Tools:Statistical analysis of the resultsTools: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’susing a likelihood ratio test statistics which simplifies into
-2lnQ=2stot-2Niln(1+si/bi)(stot total signal rate, si/bi =s/b prob per pdf bin)
e.gExpectations on –2lnQ, one experiment in 2000Kinematic edge @ 116 GeV
Powerful separation for MH=110 GeV/c2
More difficult for MH=115 GeV /c2
DELPHI
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SM Higgs boson resultsSM Higgs boson results
Median, 1, 2 bandsexpected from background only
Data consistent witha Higgs signal of MH=115.6 GeV/c2
Expected curve fromBck+signal of 115 GeV/c2
LEP result corresponds to 2.1 over backgroundAt 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 115.6 GeV/c2:44%
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A golden candidate in the SM searchA golden candidate in the SM search
Event compatible with SM Higgs boson decay
e+e-HZ bbqq
A 4-jet event with 2 b-jets
Some properties:
MH=114.3 3 GeV/c2
Strong S/B probability ratio=4.6
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SM Higgs boson searches : a summarySM Higgs boson searches : a summary
From indirect limits from
EW precision measurements
(LEP,SLC,Tevatron)
MH196 GeV/c2 95% C.L
From direct searches at LEP
MH114.1 GeV/c2 95% C.L
Likelihood test preferred mass
MH=115.6 GeV/c2
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2 Higgs Doublet Models2 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)
Couples to Type I Type II
’d-type’ leptons 2 1
’u-type’ quarks 2 2
’d-type’ quarks 2 1
e.x MSSMghff cos
e.x fermiophobic ( =/2)
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, bbAlso h0A0A0,invisible
e.g
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MSSM Higgs searches MSSM Higgs searches Combine hZ , hA searches Scenario mh-max: Allows max
theoretical bound on mh w.r.t tan
mh91GeV/c2 , mA91.9 GeV/c2
0.5 < tan < 2.4 excluded
e+e- hZ
e+e- hA
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MSSM Higgs searches MSSM Higgs searches Combine hZ , hA searches Scenario:No mixing
~~, RL tt
mh91.5 GeV/c2 , mA92.2 GeV/c2
0.7 < tan < 10.5 excluded
e+e- hZ
e+e- hA
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Flavor independent Higgs searchFlavor independent Higgs searchIn 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 Mh112.9 GeV/c2 95% C.L assuming hZ= hZ
SM
Strong cross section limits
112.9 GeV
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Invisible Higgs searches Invisible Higgs searches e+e- Z* hZ, h(stable, undetectable particles) h decays
In MSSM h 00
Other models h JJ (Majorons) Signatures
large Emiss + dileptons or dijets
with mass close to mZ
114.4 GeV
Assuming BR(h inv)=1 mh114.4 GeV/c2
or
Up to mh 110 GeV/c2
hZ/ SM 40%
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Non fermionic Higgs searchesNon fermionic Higgs searchesSearches 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(hff)=0 mh108.2 GeV/c2
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Model independent search for scalarsModel independent search for scalars e+e- S0Z, (Zee,)
(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)
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Search for Charged Higgses 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(Hcs, )=1
m H >78.6 GeV/c2 95% C.L
78.6 GeV
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Search for HSearch for H W WAA In a 2HDM type I model
H WA, (A bb,+-)
dominant if
tan>1, kinematically allowed 3 final states are studied:
H+H- WAWA qqbbqqbb
H+H- WAWA lbbqqbb
H+H- WA qqbb Results on (mH, mA) plane
mH
mA
OPALtan=10tan=1
tan=100
68 G
eV
42 60
6050
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Search for doubly charged HSearch for doubly charged H bosonsbosons Appear in left right symmetric models and can be relatively light
Decay to leptons, preferably Signatures: 4l, 4l+Emiss h unknown Yukawa couplingFor h >10-7 H decays fast Results
m H >98.5 GeV/c2
for BR(H ± ± )=1
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Light Higgs Yukawa productionLight Higgs Yukawa production
Processes I,II suppressed if
sin2(-) small, mA+mh>s
Look at Zbb h/A bb
I IIIII
ThenIII dominant
enhancement factor of the bb h/A couplings
ConclusionsConclusions
For the SM Higgs bosonMH<196 GeV/c2 , MH114 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)