Search for SM and MSSM Higgs Bosons at LEP

Latest results from the LEP Collaboration on searches for neutral Higgs bosons predicted by the Standard Model and its minimal supersymmetric extension, the MSSM, are summarized.

4

002 raM 03 1v9403040/ex-pe:hviXraOPALConferenceReportCR520

11March,2004

SEARCHFORSMANDMSSMHIGGSBOSONSATLEP

GABRIELLAPASZTOR

´ PhysicsDepartment,UniversityofCalifornia,Riverside,CA92521,USA

Mailingaddress:CERN,Gen`eve23,CH-1211,Switzerland

E-mail:Gabriella.Pasztor@cern.ch

LatestresultsfromtheLEPCollaborationonsearchesforneutralHiggsbosonspredictedbytheStandardModelanditsminimalsupersymmetricextension,theMSSM,aresummarized.

1.Introduction

ThefourLEPexperiments(ALEPH,DELPHI,L3andOPAL)collected

around2.5fb 1dataintotalatenergies

√

s=206 209GeVin2000.Thedataareusedtosearch

forHiggsbosonsinavarietyofmodels.

IntheStandardModel(SM)theelectroweak(EW)symmetryisbrokenviatheHiggsmechanismgeneratingthemassesofelementaryparticles.Thisimpliestheexistenceofasingleneutralscalarparticle,theHiggsboson.WhiletheSMissuccessfulindescribingtheobservedphenomenathereareseveraltheoreticalargumentsrequiringitsextension.Theminimalsupersymmetric(SUSY)extensionoftheSM(MSSM)introducestwoHiggs elddoubletsleadingto veHiggsbosons:threeneutralandtwocharged.2.StandardModel

AtLEPenergies,theSMHiggsbosonisproducedviatheHiggs-strahlungprocesse+e →HZ.Vector-bosonfusionprocessese+e →He+e andHνeν¯eplayalsosomeroleclosetothekinematiclimit.Theexpectedtodecaypredominantlyintob¯Higgsbosonis

b

withsomecontributiontoτ+τ ,gg,W W andc¯c.ForaHiggsmassof115GeV,morethanhalfofthe

Higgseventsareexpectedinthefour-jetchannel(HZ→b¯bq¯

q).

Latest results from the LEP Collaboration on searches for neutral Higgs bosons predicted by the Standard Model and its minimal supersymmetric extension, the MSSM, are summarized.

DuringthelastyearofLEPoperationtheALEPHcollaborationob-servedanexcessoffour-jeteventscompatiblewithSMHiggsbosonpro-ductionwithaHiggsmassof115GeV.Forthismasshypothesis,theALEPHdatagiveabackgroundcon dence1 CLb=3.3×10 3,corre-spondingtoanapproximately3σexcess,andasignal-plus-backgroundcon denceCLs+b=0.87.WhencombinedwiththeresultsoftheotherLEPexperiments1thesigni canceoftheexcessdecreasesbelow2σ,andthecon dencesare1 CLb=0.09(seeFigure1(a))andCLs+b=0.15.Alowerboundof114.4GeVissetonthemassoftheSMHiggsbosonatthe95%CLwithanexpectedlimitof115.3GeVasshownonFigure1(b).ThedataarealsousedtoderivelimitsontheHZZcouplingforvariousassumptionsconcerningtheHiggsbosondecayproperties(seeFigure1(c)

¯decay).fortheassumptionof100%H→bb

1

95% CL limit on ξB(H→bb)

1-CLb

CLs

1

–

1010

1

10

σ

10101010

2

10

10

σ

10

10

bforSM)HZZ

¯b3.IntobeinvariantunderCP

tobreakCPsymmetryinascenarioprovidesa(CPV)scenariosarestud-bosonsareCPeigen-aremainlyproducedinandthepair-productionhowever,thethreeneutralHiggs eldsandthee+j(i,mayalloccur.Thedi erentinthe

Latest results from the LEP Collaboration on searches for neutral Higgs bosons predicted by the Standard Model and its minimal supersymmetric extension, the MSSM, are summarized.

twoscenarios,maintainacertainsimilarity:thelargestbranchingratios

¯andτ+τ andcascadedecays(h→AAandH2→H1H1)arethosetobb

occurandcanevenbedominantwhenkinematicallyallowed.

AlargenumberofsearchchannelsareusedintheMSSMHiggshunt:SMHiggssearchesarereinterpreted,searchesforhApair-production,bbhandbbAYukawaproductions, avourindependenthZandhA,decaymodeindependenthZsearchesaredeveloped.h→AAandA→hZdecaysareconsideredincascadedecaysleadingtosix-fermion nalstates.ThesearchforinvisibledecayofHiggsbosonsisalsousefultoexplorespeci careasoftheMSSMparameterspace.Ingeneral,searchesdesignedtodetectCPCHiggsproductioncanbereinterpretedintheCPVscenario.However,insomepartsoftheCPVparameterspacemodi edornewlydevelopedsearchesarealsonecessarytocovernew nalstatetopologies.

Theresultsofthedi erentHiggssearchesareinterpretedintheframe-workofaconstrainedMSSMwithsevenparameters.Attreeleveltwoparametersaresu cienttodescribetheHiggssector,theyarechosentobetheratioofthevacuumexpectationvalues(tanβ)andaHiggsmass(mAinCPCandmH±inCPVscenarios).Additionalparametersappearaftertheradiativecorrections:thesoftSUSYbreakingparameterinthesfermionsectorattheEWscale(mSUSY),theSU(2)gauginomassparam-eter(M2),thecommontrilinearHiggs-squarkcouplingparameter(A),thegluinomass(mg )andtheSUSYHiggsmassparameter(µ).

Insteadofvaryingalltheaboveparameters,onlyacertainnumberofrepresentativebenchmarksetsareconsideredwherethetreelevelparam-etersarescannedwhileallotherparametersare xed.Ontopofthetra-ditionalthreeLEPbenchmarkscenarios(large-µ,no-mixingandmh-max)severalnewscenarios,motivatedbylimitsfromb→sγand(g 2)µmea-surementsorbyfuturesearchestobeconductedattheLHC,arestudied.TherecentlyproposedCPVscenario,calledCPX,anditsseveralderivatesarealsoprobed.

Thelarge-µscenario,designedtobekinematicallyalwaysaccessiblebut

¯decaysuppressed,isentirelyexcludedbythepreliminarytohavetheh→bb

LEPcombination2thanksto avourindependentsearches.Theno-mixingscenariowheretheparametersarearrangedtohavenomixingbetweentheleft-andright-handedstop elds,isstronglyconstrainedevenbyasingleexperiment3,4asshowninFigure2.Inthemh-maxscenariowhichgivesthemaximalvalueofmhforgiventanβandmA,thelowerlimitontheHiggsbosonmassesaremh>91.0GeVandmA>91.9GeVandtherange0.5<tanβ<2.4isexcluded2.StudiesfromtheOPALexperimentshowthat

3

Latest results from the LEP Collaboration on searches for neutral Higgs bosons predicted by the Standard Model and its minimal supersymmetric extension, the MSSM, are summarized.

thenewlyproposedCPCbenchmark

atLEP:thederivedlowerlimitsonbenchmarkscen …… 此处隐藏：4358字，全部文档内容请下载后查看。喜欢就下载吧 ……