Highlights of the searches for Higgs bosons at LEP2

22/01/2001 Marumi Kado - CERN 1

Highlights of the Highlights of the searches for Higgs searches for Higgs

bosons at LEP2bosons at LEP2

Marumi KadoMarumi KadoCERNCERN


First hints of Higgs boson First hints of Higgs boson productionproduction

Part I


205 GeV205 GeV

206.5 GeV206.5 GeV


How it all beganHow it all began

July 20July 20thth LEPC: LEPC: First combinationFirst combination limit limit 113.3113.3 GeV/cGeV/c22 / /sensitivity sensitivity 113.4 GeV/c113.4 GeV/c22

June 14June 14thth: : First candidateFirst candidate at high energy and high at high energy and high reconstructed mass reconstructed mass (~115 GeV/c2 ~ (~115 GeV/c2 ~ 206.5-m206.5-mZZ))

September 5September 5thth LEPC: LEPC: Second combinationSecond combination A A 2.22.2 excess is observed excess is observed limit limit 112.3112.3 GeV/cGeV/c22 / /sensitivity sensitivity 113.8 GeV/c113.8 GeV/c22


Septembre 14th Research board: Septembre 14th Research board: LEP is awarded an LEP is awarded an additional month of running, to further check these additional month of running, to further check these results results

Octobre 10Octobre 10thth LEPC: LEPC: Update of the resultsUpdate of the results The September 5The September 5thth excess grows up to excess grows up to 2.62.6

November 2November 2dd: : end of LEP operationsend of LEP operations

November 3November 3dd LEPC: LEPC: The new data confirm the The new data confirm the excess again. The significance grows up to excess again. The significance grows up to 2.92.9

November 13November 13thth:: ALEPH publication ALEPH publication November 14November 14thth:: L3 publication L3 publication

December 22December 22dd:: OPAL and DELPHI publication OPAL and DELPHI publication

Repeated confirmationsRepeated confirmations

Additional 2001 running not grantedAdditional 2001 running not granted

LEP running in 2001 is requested LEP running in 2001 is requested


Radiated by a Z* (Higgs-strahlung)Radiated by a Z* (Higgs-strahlung)

Production process at LEPProduction process at LEP

«Threshold»«Threshold»

204202

208206

s-mZs-mZ

~ 0.04 pb

as a fonction of mH

en fonction de s

Dominant production process

ee++ee- - HZ HZ 3 Sensitivity:mH s-mZ

5

3 L~200pb-1


DecaysDecays

Higgs boson branching ratiosHiggs boson branching ratios

LEP has mostly LEP has mostly b quarkb quark or or leptonlepton

Gain sensitivity near 115 GeV/cGain sensitivity near 115 GeV/c22 OPAL and L3 have OPAL and L3 have also started the study of Walso started the study of W++WW- - decays of the Higgs decays of the Higgs bosonboson

For mmHH ~ 115 GeV/c ~ 115 GeV/c22

Br(Hbb) ~ 74%Br(H+-) ~ 7%

Br(HW+W-) ~ 7%


Background processesBackground processes

Doubly resonant production of W and Z bosonsDoubly resonant production of W and Z bosons

A priori irreducible processZbb~2 pb

Reducible process~15 pb

_


Search topologiesSearch topologies

Mainly determined by the Z boson decay modesMainly determined by the Z boson decay modes

The “four jets” topologyThe “four jets” topology

%70)qqZ(

qqZ bbH

Dominant channel:

Background processes near threshold:

b quark pair with2 hard gluons

Quark pair withgluon splitting into a quark pair

b

b_

q_

q


Missing energy channelMissing energy channel

%20)Z(

ZbbH

Second most prolific channel:

Main background near threshold:

Where the Higgs boson is produced alsoin the fusion process:

b quark pair2 photons ISR

2 ISR escape undetected

at low angle

non-negligible contribution near threshold

(Constructive interference) No kinematic threshold

b_

b

_


Leptonic channelsLeptonic channels

),e(

%7)Z(

Z

Good resolution in reconstructed mass:

)Z,bbH(

The H or Z decays to taus:

bbH

)qqZ,H(

Also produced by the fusion process:

Negligible contribution(destructive interference)

%7

b_

b,q_

b

b,q

e+,+

e-,-

+

-

ZbbH

ZqqZ

_


Combined resultsCombined results

Results as ofResults as of November 3 November 3dd

DELPHI TPC sector and OPAL VDET module problems were worked around

Analysed luminosity up to October 25Analysed luminosity up to October 25thth

The The ~30 pb~30 pb-1-1 collected between collected between 25/1025/10 and and 02/1102/11 are not are not included in this combinationincluded in this combination

Luminosité intégrée (pb-1)Expérience

>200 GeV >206 GeV

ALEPH 122

DELPHI206214 132

L3 201 124

OPAL 190 110

Total LEP 810 488

s s


Illustration of the searches Illustration of the searches resultsresults

Reconstructed mass histogramsReconstructed mass histograms

Rescalings or fits using the recoil to the Z boson

Each Each channelchannel in each in each experimentexperiment is optimized with different is optimized with different expected background ratesexpected background rates

A possible prescription to illustrate the results of the searches is:A possible prescription to illustrate the results of the searches is:

So that each channel and each experiment are represented in a fair way:

S/B for a Signal mass of mH=115 GeV/c2 and for a reconstructed mass greater than 109 GeV/c2

Fixed S/B ratio

These histograms are shown for illustration These histograms are shown for illustration purposes only purposes only they do not containthey do not contain all the all the

information used in the complete information used in the complete interpretation of the resultsinterpretation of the results


Reconstructed mass distribution (S/B>0.3)Reconstructed mass distribution (S/B>0.3)

MH=115 GeV/c2 Full spectrum MH>109GeV/c2

Signal 10.3 6.1

Background 202 20.4

Data 200 27

Good agreement data and Monte Carlo

Excess at high reconstructed mass

26.5


For S/B>1For S/B>1

High mass excessconfirmed


Signal 4.7 3.0

Background 55.3 3.6

Data 59 6

6.660


For S/B>2For S/B>2


Signal 2.7 1.8

Background 22.8 1.1

Data 24 4

2.925.5

ALEPH: 3 (4J) L3: 1 (H)

Most significantevents

(mH>109 GeV/c2)

High mass excessagain confirmed


Estimation of an experiment Estimation of an experiment s, b, Ns, b, N

Estimator Estimator Likelihood ratio: Likelihood ratio:

Complete interpretationComplete interpretation

N

1i i

isN

1i i

iib

)bs(

b

bs

b~s~

1eb~

b~

s~

e

eL

LQ

- si signal local value of the discriminating variable (reconstructed mass, b-tag, NN)

- bi local value for the expected background

~

~

N

1i

ii)bs(N

bs bsb~

s~e

!N)bs(

L

N

1i

ibN

b bb~

e!N

bL

Likelihood for the signal & background hypothesis

Likelihood for the background only hypothesis

For each event i:For each event i:

ALEPH


-2lnQ-2lnQ Estimator used Estimator used

Trivial combination of channels and experimentsTrivial combination of channels and experiments (Sum of the estimators)(Sum of the estimators)

For gedanken experiments:For gedanken experiments:

Signal & bckg. Bckg. only

Light Higgs and large cross section

Intermediate Higgs mass and cross section

Heavy Higgs andlow cross section

N

1ii )ws(2Qln2 where )

b~s~

1ln(wi

ii


Estimator for each Estimator for each experimentexperiment

For different Higgs boson mass hypotheses:For different Higgs boson mass hypotheses:

A, L & O: Significant excess at high massO: Also excess ~105 GeV/c2

D: Deficit


Back to the transverse vue Back to the transverse vue for mfor mHH=115 GeV/c=115 GeV/c22

The distribution of the four observations is The distribution of the four observations is compatiblecompatible with the signal hypothesis with the signal hypothesis

Illustration of the signal & background and Illustration of the signal & background and background only expectations for mbackground only expectations for mHH=115 GeV/c=115 GeV/c22

-2lnQ -2lnQ

-2lnQ-2lnQ


Combined estimatorCombined estimator

Minimum at 115 GeV/cMinimum at 115 GeV/c22 in in coincidence coincidence with the most likely value in the signal with the most likely value in the signal

and background hypothesisand background hypothesis

mmHH=115 GeV/c=115 GeV/c22+0.7+0.7-0.3-0.3

-2lnQ

-2lnQ

Q ~ 30Q ~ 30


How much do we trust that this observation is How much do we trust that this observation is background like?background like?

CLCLbb Official « discovery » estimator Official « discovery » estimator (deviation from (deviation from the Standard Model)the Standard Model)

CLb: Fraction of background only “gedanken” experiments with an estimator larger than observed

-2lnQ observed

Confidence in the background hypothesis

Confidence levelConfidence level

2.9 Standard 2.9 Standard deviationsdeviations

(m(mHH=115 GeV/c=115 GeV/c22))

1-Cl1-Clbb = 4.2 10 = 4.2 10-3-3

Minimum at 115GeV/c2:


Most significant candidatesMost significant candidates

(s/ b)115 Mrec(GeV/ c2) Chanel Exp.

4.7 114 (d) Hqq ALEPH

2.3 112 (c) Hqq ALEPH2.0 114 (b) H L30.90 110 (a) Hqq ALEPH0.60 118 Hee ALEPH0.52 113 Hqq OPAL0.50 111 Hqq OPAL*0.50 115 H ALEPH0.50 115 Hqq ALEPH0.49 114 H L30.47 115 Hqq L30.45 97 Hqq DELPHI0.40 114 Hqq DELPHI0.32 104 H OPAL

ALEPH: 6ALEPH: 6L3: 3L3: 3

OPAL: 3OPAL: 3DELPHI: 2DELPHI: 2

Hqq: 9Hqq: 9HH: 3: 3HlHl++ll--: 1: 1HH: 1: 1

(70%(70%))

(20%(20%))

(7%)(7%)(3%)(3%)

TheoreticalTheoreticalbranchingsbranchings

Candidates with (s/b)Candidates with (s/b)115115>0.3 >0.3 Observed: Observed: 1414Expected (s+b): Expected (s+b): 1414Signal: Signal: 77


Candidate -a- (21/08/2000)Candidate -a- (21/08/2000)

e+e-bbbb_ _

s = 206.7s = 206.7

Mrec=1103 GeV/c2

Three well b-tagged jetswith two reconstructed

displaced vertices


L3L3

ee++ee--bbbb206.6 GeV

Mrec

114.4 GeV/c114.4 GeV/c22

Missingmass

94 GeV/c94 GeV/c22

Candidate -b- (16/10/2000)Candidate -b- (16/10/2000)

Two b-tagged jets

with two reconstructe

ddisplaced vertices

(5 mm and 1 mm)


Candidate -c- (31/07/2000)Candidate -c- (31/07/2000)

e+e-bbbb_ _

s = 206.7s = 206.7

Mrec=1123 GeV/c2

Four well b-tagged jetswith tree reconstructed

displaced vertices


Candidate -d- (14/06/2000)Candidate -d- (14/06/2000)

Directionimpulsion manquante

e+e-bbqq_ _

s = 206.7s = 206.7

Mrec=1143 GeV/c2

Two well b-tagged jetswith two reconstructed

displaced vertices


How about the less significant How about the less significant events?events?

A: (s/b)115<0.7 B: (s/b)115>0.7

-2lnQA -2lnQB~

The effect is not The effect is not only due to the only due to the

four most four most significant eventssignificant events


Estimator for each channelEstimator for each channel

Excess at high Higgs mass hypotheses in all channels

most significant in the 4 jets channel

As a function of the Higgs boson mass hypothesis:As a function of the Higgs boson mass hypothesis:


All channels are All channels are compatiblecompatible with the signal with the signal hypothesishypothesis

-2lnQ-2lnQ

-2lnQ -2lnQ

Back to the transverse vue Back to the transverse vue for mfor mHH=115 GeV/c=115 GeV/c22


Evolution of the effect with Evolution of the effect with timetime

1.1

2.2

2.6

2.9

Results coherent with the presence of Results coherent with the presence of a Higgs boson with mass 115 GeV/ ca Higgs boson with mass 115 GeV/ c22

Coherence in terms of

amounts of datataken at high energy

Coherence in terms of

independent samples


Could there be a systematic bias Could there be a systematic bias near threshold?near threshold?

Question mainly answered by looking at data taken at Question mainly answered by looking at data taken at lower centre-of-mass energies:lower centre-of-mass energies:

Observed estimator as a functionof the distance to threshold

with 500pb-1 (per experiment) of data taken between 183

and 206 GeV

If a bias similar tothat observed above

206 GeVwas observed at all

centre-of-mass energies


Summary of the Standard Summary of the Standard Model Higgs search as of Model Higgs search as of

November 3November 3dd LEPC LEPC

Each experiment has a 3Each experiment has a 3sensitivity between sensitivity between 111.5111.5 and and 113.5 GeV/c113.5 GeV/c22

The combined sensitivity of the four experiments The combined sensitivity of the four experiments reaches reaches 33at at mmHH=115 GeV/c=115 GeV/c22

Coherent picture:Coherent picture:

Four experimentsAll channelsIn terms of production (threshold effect)Evolution in time and amount of accumulated data

Can we exclude? Can we exclude? LEP (rhetorical question) sensitivity 115.3, mH>113.5 GeV/c2 DELPHI sensitivity 113.4, mH>113.9 GeV/c2

A 3A 3 excess is observed by the combination excess is observed by the combination in agreement with the production of a Higgs in agreement with the production of a Higgs

boson weighing 115 GeV/cboson weighing 115 GeV/c22 +0.7+0.7-0.3-0.3


What have we learned since?What have we learned since? ALEPH ALEPH (PLB 495(2000)1):(PLB 495(2000)1):

- Combination: excess of 3.4/ published 3.0. Due to mainly two effects:

- Additional luminosity - Taking into account correlation between discriminating

variables

OPAL OPAL (CERN-EP-2000-156):(CERN-EP-2000-156): In the combination the significance was 1.3, but a quite significant event was detected after October 25th:

DELPHIDELPHI (to appear in PLB) : (to appear in PLB) : Observed limit 113.9114.3 GeV/c2

Expected limit 113.4113.5 GeV/c2

L3 L3 (PLB 495(2000)18):(PLB 495(2000)18): Reprocessing and analysis work is being completed.

OpalOpal

ee++ee--bbbbqqqq

29 Oct. 2000206.6 GeV

Mrec= 110.5 110.5 GeV/cGeV/c22

(1-Clb)03/11/2000=0.19(1-Clb)published=0.2


Beyond the Minimal Beyond the Minimal Standard ModelStandard Model

(Review of the topologies studied)(Review of the topologies studied)

Part II


Non minimal extensions of Non minimal extensions of the Standard Modelthe Standard Model

Any number of Higgs doublets is adequate to break Any number of Higgs doublets is adequate to break the EW symmetry the EW symmetry ((=1 at tree level)=1 at tree level)

Using two complex scalar Higgs doubletsUsing two complex scalar Higgs doublets Mass to gauge bosons and Five physical scalar fields are

left: h,H (CP even), A (CP odd) and H

The 6 free parameters of the model are: - Higgs masses - mixing between h and H - tan : ratio of the vev of the two doublets

To avoid FCNC fermions of a given charge must couple at most to one doublet only. There are therefore only two convenient types of models:

- Type I: One doublet does not couple to fermions -Type II: One doublet couples only to “up” type

fermions and the other to “down” type fermions.


In general In general 2HDM2HDM: Search for charged Higgs bosons: Search for charged Higgs bosons

The two main decay channels areThe two main decay channels are H H /cs/cs

ProductionProduction

Model dependence is embedded in Model dependence is embedded in BrBr(H (H )) assuming assuming

Topologies searched for:

cscs cs

Main background: W pair production

]1)csH()H([ BrBr


The L3 excessThe L3 excess

2000 L3 data confirm and reinforce this effect, but...2000 L3 data confirm and reinforce this effect, but...

PLB496(2000)34

Effect consistently seen at all centre-of-massenergies above 189 GeV


the effect is not confirmed by other experiments: the effect is not confirmed by other experiments: Each Each experiment individually excludes a possible signal atexperiment individually excludes a possible signal at

~70 GeV/c~70 GeV/c22

The combined exclusion in the The combined exclusion in the (m(mHH, Br(H , Br(H )))) as of as of

September 5September 5th th LEPC (with data since 1998):LEPC (with data since 1998):

(LEPC 05/09/2000)


In the In the 2HDM type I2HDM type I, if cos, if cos=0 then no mixing occurs =0 then no mixing occurs between the two CP-even Higgs bosons, thus one will between the two CP-even Higgs bosons, thus one will not couple to fermions whatsoever (fermiophoby)not couple to fermions whatsoever (fermiophoby)

- Produced by Higgs-strahlung process - Decays to a pair of photons if mh is small compared to 2mW

Topologies searched for:

qq~70% ~20% ~3%ll~7%

mqq~mZ m~mZ m,ll~mZ

Main background: two fermions production with two photons

Mass lower limit on a Higgs boson decaying to a pair of photons:107.7 GeV/c2 (obs)105.8 GeV/c2 (exp)

(LEPC 05/09/2000)


2HDM II, Flavour independent analyses (OPAL)2HDM II, Flavour independent analyses (OPAL)

- For certain parameter choices ( and tan0), the decay modes to a pair of b quarks and tau leptons can be suppressed

- Production process: Higgs-strahlung MSM

qqqq,qqgg~70% qq,gg~20% qq,gg~3%qq,ggll~7%

mqq~mZ m~mZ m,ll~mZ

Main backgrounds: Four fermion production

Mass lower limit on a Higgs boson decaying to any

quark flavour or a gluon pair:

75 GeV/c2 (obs)89.5 GeV/c2 (exp)

mh

CERN-EP-2000-92OPAL

170pb-1@183 and 189 GeV

Quasi model independent interpretation:


A complete 2HDM type II interpretation, the associated A complete 2HDM type II interpretation, the associated production has also been studied:production has also been studied:

Interpretation independent of the parameter Interpretation independent of the parameter ::

Associated production

Flavor independent

Standardsearches

Ingredients:

Flavourindependent

FlavourdependentNo mass constraints


The MSSMThe MSSM

A 115 GeV/A 115 GeV/ cc22 Higgs boson invites SUSY Higgs boson invites SUSY (J. Ellis and D. Ross (J. Ellis and D. Ross hep-ph/0012067)hep-ph/0012067)

The MSSM Higgs sector is a 2HDM type II, with only two The MSSM Higgs sector is a 2HDM type II, with only two free parameters at tree level (free parameters at tree level (mmAA and and tantan). ).

- mH< 130 GeV/c2 - At higher orders: Large number of parameters Most

conservative benchmark maximal mh

Topologies searched for to cover the High tanTopologies searched for to cover the High tan region region h/A h/A bb/ bb/ with with mmhh~m~mAA

bbbb~85% bb+- ~13%

ComplementaryProduction processes:

Higgs-strahlungsin2(-)

Associatedcos2(-)

Use standard searches


Exclusion regions Exclusion regions (combined as of October 10(combined as of October 10thth LEPC) LEPC)

The robustness of the limits on the masse of the h and A The robustness of the limits on the masse of the h and A Higgs bosons has been proven by large scans of all relevant Higgs bosons has been proven by large scans of all relevant parameters of the MSSM parameters of the MSSM (Eur. Phys. J. C 17(2000)223)(Eur. Phys. J. C 17(2000)223)

TheThe tan tan exclusion range is very sensitive to the top mass It exclusion range is very sensitive to the top mass It can be drastically reduced by assuming a large top masscan be drastically reduced by assuming a large top mass

tan exclusion [0.52,2.25]/expected [0.48,2.48]

mh> 89.9 (obs.) 93.8 (exp.) GeV/c2

mA> 90.5 (obs.) 94.1 (exp.) GeV/c2


Beyond… Beyond… Invisible decays of Higgs bosonsInvisible decays of Higgs bosons can occur in different can occur in different

models:models: - MSSM with non universal gaugino masses - Models with majorons

Produced by Higgs-strahlung processProduced by Higgs-strahlung process

Quasi model independent interpretation:Quasi model independent interpretation:

qq (invisible) ~70% ll (invisible) ~7%

Main backgrounds:Four fermions

MSMhZ

hZ2 )invisibleh( Br

For=MSM

Br(hinv.) = 100%

mh> 113.7 (obs.) mh> 112.8 (exp.)LEP

CombinationLEPC 05/09/2000


Anomalous couplings:Anomalous couplings: Assuming new physics appear at Assuming new physics appear at a scale a scale , low energy effects can be parametrised with , low energy effects can be parametrised with an effective Lagrangian:an effective Lagrangian:

nn

2n

eff Of

L

Where On are combinations of Higgs and Gauge fields

Anomalous couplings of the type h and hZ, are allowed

Hypotheses: One Higgs doublet and C and P conservation

Topologies:


DELPHIDELPHI search up to 183 GeV search up to 183 GeV (PLB 458(1999)431)(PLB 458(1999)431) interpretation (All finterpretation (All fnn’s are equal to F)’s are equal to F)


Interpretation of Interpretation of ALEPHALEPH’s MSM and Fermiophobic ’s MSM and Fermiophobic Higgs boson searches up 202 GeV Higgs boson searches up 202 GeV (Moriond QCD 2000)(Moriond QCD 2000)

Also Also L3L3 search @189 GeV search @189 GeV (PLB 489(2000)102)(PLB 489(2000)102) with with additional interpretationsadditional interpretations


ConclusionsConclusions

Numerous theoretical scenarios were investigated at Numerous theoretical scenarios were investigated at LEP and many nonstandard topologies, where the Higgs LEP and many nonstandard topologies, where the Higgs

boson could have escaped otherwise, were covered boson could have escaped otherwise, were covered

Hints of a 115 GeV/cHints of a 115 GeV/c22 standard-model-like Higgs boson standard-model-like Higgs boson were observed, as strong as could be expected at such were observed, as strong as could be expected at such

high mass, close to the operational limit of LEPhigh mass, close to the operational limit of LEP

This observation corresponds to roughly 3 standard This observation corresponds to roughly 3 standard deviations above the background expectationdeviations above the background expectation

These indications are not sufficient to claim a discovery These indications are not sufficient to claim a discovery

An additional year of LEP running in 2001 would have An additional year of LEP running in 2001 would have been essential to concludebeen essential to conclude

(Final results are scheduled for the forthcoming winter (Final results are scheduled for the forthcoming winter conferences)conferences)

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Highlights of the searches for Higgs bosons at LEP2

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