Search for charginos and neutralinos at LEP

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Search for charginos and neutralinos at LEP

B. Tellili

To cite this version:

BORHANTELLILI

IPNLyon,43bd11novembre 1918,69622VilleurbanneCedex,France PN2P, UniversityofTunisII,1006 Tunis,Tunisia

E-mail: tellili@in2p3.fr

Wereportontheresultofasearchforcharginosandneutralinosine +

e collisions atcentre-of-massenergiesbetween203GeVand208GeVatLEP.Noevidencefor suchparticlesisfoundina data sampleof220 pb

1

perexperiment. Improved upperlimitsfortheseparticlesaresetontheproductioncrosssections. New ex-clusioncontoursintheparameterspaceoftheMinimalSupersymmetricStandard Modelarederived,as wellasnewlowerlimitsonthe massesofthese supersym-metricparticles. Charginomassesbelow103GeVareexcludedoverlargeregions oftheparameterspaceoftheMinimalSupersymmetricStandardModel.

1 Introduction

One of the main goals of the LEP experiments is to search for new parti-cles predicted by theories beyond the Standard Model. We report on the searchesfor charginosand neutralinosat LEP.These particlesare predicted bytheMinimalSupersymmetricextensionoftheStandardModel(MSSM)

1 , in additionto the ordinaryparticles, there is asupersymmetric spectrumof particleswithspinsdieringbyonehalfwithrespecttotheirStandardModel partners. Forfermions,eachhelicityhasascalarsuperpartnerandforgauge bosons and Higgs particles their superpartners are called gauginos. There are two mass eigenstates formed by a mixing of the superparteners of the chargedgaugebosonsandchargedHiggsbosonscalledcharginos,~

 i

(i=1,2), andfourmasseigenstatesassociatedtotheneutralgaugebosonsandneutral Higgsbosonscalledneutralinos~

0 i

(i=1,...,4). wheretheindicesareorderedin increasingmass. Wehaveanadditionalmultiplicativequantumnumbercalled R-parity,whichinsurestheconservationoftheleptonandbaryonnumbers.

Inthis articleweonlytreatthecasewhere R-parityis conserved,which impliesthat supersymmetricparticles arepair-producedand thelightest su-persymmetricparticle,assumedtobethelightestneutralino,~

0 1

,isstable. Themassspectrumsandcouplingsarecompletelydenedbythe follow-ing parameters : gaugino masses M

1;2;3

at the electroweak scale, obtained byevolvingtheGUTscaleparameterm

1=2

(thegauginoscommonmass) us-ingrenormalizationgroupequations,thecommonsfermionmassattheGUT scalem

0

, the Higgsinomass term, theratio ofvaccumexpectation values ofthetwoHiggselds tan and A

0

parame-constrainedMSSMwithR-parityconservation.

2 Phenomenologyand searchstrategies

InCMSSMmodelwith R-parityconservation, thedecaychainof supersym-metricparticlesalwayscontains,besidestandardparticles,atleasttwo invis-ibleneutralinoscausingthemissingenergysignature.

Charginosare pairproducedvias-channel =Z ort-channel~exchange 2

, this two diagrams interfer destructivelyand the production cross section can be reduced by an order of magnitude when the t-channel exchange is important. Neutralino pairse

+ e !~ 0 i ~  0 j

(i;j =1;:::;4) areproduced via s-channelZ ort-channele~

L;R

exchange 3

,thediagramsinterferconstructively whenthet-channelexchangeisallowed.

When the masses of the scalar leptons and the charged Higgs bosons (H



)areverylarge,thecharginos~  1;2 decaythroughaW  : ~  1 !~ 0 1 W  ! ~  0 1 f  f 0

. Thedominanttopologiesarethenhadroniceventswithmissingenergy oreventswithhadrons,anisolatedleptonandmissingenergyortwoleptons andmissingenergy.

Ifthe ~ `



and~ massesarecomparabletoM W

, thecharginoalsodecays viavirtualscalarleptonorscalarneutrinoandtheleptonicbranchingfraction isenhanced. Finallyfor

~ `



and~ lighterthanthechargino,thedecaymodes ~   1 ! ~ `   or~  1 !~`  becomedominant.

Wehavealsotwoscenariosfortheneutralinodecays: whenthemassesof theneutralSUSYHiggs bosonsandofthescalarleptonsareverylarge(high m

0

),theheavierneutralinos(~ 0 j ; j2) decaysviaZ  : ~ 0 j !~ 0 k Z  !~ 0 k f  f withk <j. Thedominanttopologiesarethen hadroniceventswith missing energyortwoleptons(withsame avors)andmissingenergy. Forachargino lighterthanheavierneutralinos,thelatterdecaysviaW

 suchas~ 0 j !~  1 f  f 0 . IfthescalarleptonmassesarecomparablestotheZmass,theneutralino decaysalsoviaavirtualscalarlepton,enhancingtheleptonicbranching frac-tion. Finally, for ~ and

~ `



lighter than neutralinos the two-body decays ~  0 j ! ~ `  `  or ~ 0 j

! ~ (j  2) become dominant. The radiative decays ~  0 j !~ 0 k

arealsopossibleviahigher-orderdiagrams.

Themissing energyiscarriedoutbytheLSPsorbythepresenceof neu-trinosinthenalstateofSUSYsignals,whichimpliesthatthekinematic con-gurationofthesignaleventsandthedetectioneÆciencies dependsstrongly onthemassdierence betweentheheavierSUSYparticleproduced andthe LSP:
M=M

SUSY -M

LSP

vary signicantly withthe
Mrange considered. When
M 10GeVthe processesaredominant. When
Mislargethedominantbackgroundsare W + W , W  e   andZZ.

The selections are optimized for various
M ranges and for dierent topologies in order to reach the best signal to background ratio. Searches forcharginosandneutralinosinthevarioustopologieshavebeencarriedout byallfourLEPexperiments

4;5;6;7

3 Results

The results presented here were preliminarilyperformed with the data col-lectedbythefourLEPexperimentsduring 2000atacenter-of-massenergies rangingform 203GeVto208GeV.Thecollectedintegratedluminositiesare givenin table1

Experiment A D L O

Luminosity(pb 1

) 217.0 224.0 217.0 221.0 Table 1. Integrated luminosities (pb

1

) for the four LEP experiments (A: ALEPH, D:DEPHI,L:L3,O:OPAL)

Thenumbersof selectedcandidates and expectedeventsfrom Standard Modelprocessesinthedierentchannelsarereportedintable2. Nosignicant excess of events is observed. This results is then used to set upper limits on chargino and neutralino production cross sections in the framework of the CMSSM.Exclusion limits at 95 % C.L. are derived taking into account backgroundcontributions. Experiment ~ + 1 ~  1 ~  0 2 ~  0 1

Expected Observed Expected Observed

ALEPH 11.1 11 1.9 1

DEPHI 169.3 160 163.2 183

L3 109.7 112 26.3 24

OPAL 257.7 266 207.0 197 ADLO 547.8 549 398.4 405

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Preliminary

Figure 1. Upperlimitson the e + e ! ~ + 1 ~  1

production crosssection up to 208 GeV inthe (M ~   1 ,M ~  0 1

)mass plan(the leftplot)and the Upperlimits onthe e + e !~ 0 2 ~  0 1 productioncrosssectionupto208GeVinthe(M

~  0 2 ,M ~  0 1

)massplan(therightplot).

The L3upper limitscontours on thecharginos production crosssection assuming ~  1 ! W  ~  0 1

for the chargino decay with standardW branching fractions are shown in left plot of g 1. Similarly, the upper limits on the neutralinose + e !~ 0 2 ~  0 1

productioncrosssectionassuming~ 0 2 !~ 0 1 Z,with standardZdecaysareshowninrightplotofFig1. Thecomparisonofupper limitontheproductioncrosssectiontothetheoreticalcalculationsallowsto excludetheCMSSMparameterregionsandtoset limitsonthecharginoand neutralinomasses. Inorder to derive theabsolutelimits onthemasses,the following MSSM typical parameter space is investigated : 0  M

2

 2000 GeV, 2000  2000 GeV, 0 m

0

 500GeV, 0:7 tan  40. The standardgauginosectorofCMSSMisinsensitivetotheparameterA.Foreach point theupperlimitson productioncross sectionsare calculatedbytaking into account theexpected backgrounds, the numberof observed eventsand thebranchingratiosforeachtopologie.

InFig 2.a, theregionexcludedbyALEPH in (,M 2

)planeis shownfor tan=

p

2 and m 0

= 500 GeV. The dark grey region corresponds to LEP1 exclusion,thelightgreyregionistheexclusionfrom charginosearchandthe black region is excluded from the neutralino pair production search. This resultscanalsobeinterpreted to setalowerlimitonthe charginomass. In Fig 2.b, the lower mass limits as a function of M

2

are shown for tan = p

2 and  < 0. Charginos ~  1

are excluded up to the kinematic limit in large space of parameter by using the direct search (e

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-200

-100

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+

_χ

-χi

χj

excluded at 95% C.L.

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₎

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(GeV/c

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)

ALEPH preliminary

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(GeV/c

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(GeV/c

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)

χ

+

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-χ

i

χ

j

kinematic limit

ALEPH PRELIMINARY

b)

m

0

=500 GeV/c

2

µ<0

tan

β = √2

Figure2. Excluded regionof the CMSSMparameterspaceinthe (M 2

,)(the leftplot) andthelowerlimitsoncharginomassasfunctionofM2(therightplot)form0=500GeV andtan=

p 2

inaccessiblecharginomasses for<0. AllLEPexperimentsusuallypresent the excludedregion in the massplan (M

SUSY ,M

LSP

). In Fig 3, theregion excluded by L3 is shown in the (M

~   1 ,M ~  0 1 ) plane for m 0 =500 GeV and for tan =1 and for tan =40. Similarly, the exclusionin term of MSSM parameters can be interpreted to derive a lower limits on the neutralinos massesasshowninFig 4obtainedbyOPAL.

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not allowed

Figure3. Charginoexclusionsexpressedin(M ~   1 ,M ~  0 1

)planefortan =1(theleftplot) andtan=40(therightplot). Thedarkerregionsareexcludedforanyvalueofm0,and thelighterregionsareexcludedadditionallyinthecaseofm

0

500GeV

Figure4. Neutralinoexclusionsexpressedin(M ~  0 2 ,M ~  0 1

)planefortan =1.5andm 0

= 500GeV(theleftplot)andtan=45andm

0

=500GeV(therightplot). Theexclusions areobtainedbyOPALat208GeV

ForHiggsino-like~ 0 2 and~  1 (M 2

>>jj)theproductioncrosssectionsdo notdependonthesfermion masses,themassdierencebetweentheheavier SUSYparticleandtheLSPdecreaseswithincreasingM

2 asshowninFig2.b.

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~

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β

= 2

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= -200 GeV

L3

_Preliminary

Excluded at 95

%

C.L.

M

χ

~

1

+

_>103.2

M

χ

~

+

₁

_>98.6

Figure5. Excludedregionin(M ~   1 ,M ~ 

~  1 ~  1

than3GeV,thesignaland the interactionbackgroundare indistinguish-able. Inthiscaseonerequires intheeventanInitialStateRadiation photon withhightransversemomentuminordertoimprovethetriggereÆciencyand thesignaltobackgroundratio.

For Gaugino-like ~  1

(M 2

<< jj) the chargino cross section depends stronglyonthesneutrino(~)mass,ifthesneutrinoislight,thecrosssection decreases with M

~ 

due to the destructiveinterference ofthe t-channel with s-channel. Whenthe~ismassdegeneratewiththechargino~

 1

,oneusesthe sleptonsearchestoimprovethelimits. Fig5showstheevolutionoflowerlimit of chargino massasafunction of M

~ 

for tan =2and = 200GeV,the slepton search canexclude chargino pair production beyond its kinematical limitforlightestvaluesofM

~  (lowvaluesofm 0 ),atlargem 0

themasslimit is set from charginos search, and the lightest chargino are excluded up to kinematic limit. The limit independent of m

0

is obtained when the mass dierence between the chargino and the sneutrino is small, in this casethe lowerlimitoncharginomassis 98.6GeVforanym

0 . The limiton the lightest neutralino ~

0 1

obtained by L3asa function of tan and largevaluesof m

0

is shown in the left plotof Fig 6. The limitis constrained to beabove39.4 GeV,obtainedat tan =1. set from chargino (~ + 1 ~  1

) and neutralino searches (~ 0 2 ~  0 1 ,~ 0 4 ~  0 1 ,~ 0 2 ~  0 3

). One can improve

neu-20

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)

_DELPHI

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∼ 0

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limit (189-208) GeV

36.7 GeV/c

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38.7 GeV/c

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up to 1 TeV/c

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_t

= 174.3 GeV/c

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₀

= 1 TeV/c

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no mixing

ANY m

₀

PRELIMINARY

Figure6. Lowerlimit onthe lightest neutralinomass asfunction of tan for m0 =500 GeVobtenedbyL3(theleftplot)andtheresultsonlightestneutralinosearchobtainedby Delphi(therightplot)

tralinomasslimitincludingtheresultsfrom theHiggssearches. Intheright plotofFig6,thelowerlimitson~

0 1

obtainedbyDELPHIareshownforseveral scenarios. AtmaximalM

h 0

scenarioandM t

<tan<2.36isexcludedbytheHiggssearches . Thestandardlowerlimit onlightest neutralinois38.7GeVobtainedfor tan =1. Whenwecombine withthe Higgssearches,thelimitisobtainedat tan =2.36andM

~  0 1 >49 GeV. Atlowm 0

,thelimitisobtainedfromthesleptonandcharginosearches, theminimumisobtainedforA=0andhightan.

3.1 Conclusions

Searches for charginos and neutralinos in e +

e collisions at center-of-mass energiesup to 208GeVwere performedat LEP. Noevidence forsignalwas foundin anyofthechannelsand lowerlimitsat 95% C.Lon charginosand neutralinosmasseshavebeenderived. Asummary ofthelimitsobtainedfor thefourLEPexperimentsispresentedin table3.

Charginos DELPHI 95.0GeV
M 3GeV,anym

0 ,nomixingandM ~  M ~  0 1 >6 L3 85.9GeV anym 0

, any
M and tan<40 OPAL 93.9GeV
M >5GeVandm

0

<1TeVandtan<40 Neutralinos

ALEPH 39.6GeV form 0

>0.5TeVandtan<40 DELPHI 36.7GeV anym

0 andtan<40 L3 39.4GeV form 0 >0.5TeVandtan<40 OPAL 36.3GeV anym 0 andtan<40

Table3. Summaryoflowerlimitat95%C.Loncharginoandneutralinomassesandtheir validityconditionsforthefourLEPexperiments

References

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H.E. HaberandG.L.Kane,Phys. Rep. 11775(1985). 2. A.Bartl,H. FraasandW.Majerotto,Z.Phys. C30441(1986);

A.Bartl,H. FraasandW.Majerotto,Z.Phys. C41475(1988). 3. A.Bartl,H. FraasandW.Majerotto,Nucl. Phys. B2781(1986). 4. TheALEPHCollaboration,ALEPH2001-009CONF.

5. TheDELPHIcollaboration,DELPHI2001-085CONF513.