(Final) achievements on Higgs boson searches at LEP2

(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)