R-parity violation searches at LEP

HAL Id: in2p3-00012218

http://hal.in2p3.fr/in2p3-00012218

Submitted on 6 Feb 2003

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

R. Barbier

To cite this version:

R.BARBIER

Institutde PhysiqueNucleairede Lyon,Universite ClaudeBernard Lyon-1 4,rue Enrico Fermi,F-69622 VilleurbanneCedex,France

E-mail:[email protected]

The present results on R-parity violation searchesperformed by the four LEP experimentsarepresented. Thisconcernsmainlythepairandsingle production ofSUSYparticlesinthehypothesisofR-parityviolation(Rp/)viaexplicittrilinear couplings (LL  E, LQ  Dand  U  D 

D). The charginopair production searchinthe spontaneaous R-parityviolation byan eectivebilinearterm

i H u L i scenariois alsopresented. 1 Introduction

Themostgeneralwaytowriteasuperpotentialincludingthesymmetriesand particle content of the Minimal Supersymmetric extension of the Standard Model(MSSM) 1 is: W =W MSSM + ijk L i L j  E k + 0 ijk L i Q j  D k + 00 ijk  U i  D j  D k (1) whereW MSSM

representsinteractionsbetweenparticlesconsistentwithB L conservation(B=baryonnumber,L=leptonnumber)and theother terms describes interactions violating B or L conservation

2

. These latter terms of the superpotential break explicitly R-parity (R

p

= ( 1)

3B+2S+L ). In equation (1), i, j and k are the generation indices; L and



E denote the left-handed doublet lepton and the right-handed singlet charge-conjugated lepton supereldsrespectively, whereasQ,

 U and



D denote the left-handed doubletquarkandtheright-handedsingletcharge-conjugatedup-and down-type quark superelds; 

ijk ,  0 ijk and  00 ijk

are the Yukawa couplings. The rsttwotermsviolateLconservation, andthethird oneB conservation.

ThemajorphenomenologicalconsequencesofR p

/ istheproductionof sin-glesneutrinoandthedecayintostandardfermionsoftheLightest Supersym-metricParticle,LSP(see Fig.1). Thislast propertymodiesthesignatures of the supersymmetric particle decays compared to the expected signatures in caseofR -parityconservation.

However, an additional fourth term,  i

H u

L i

, can be added to the su-perpotentielofequation (1). Thistermcorrespondstoan eectivebilinear couplingbetweenthelefthandedleptonsupereldandtheup-typeHiggseld H

u

. This eective termcomes from the spontaneous R p

e l  (e)  r @ @ @ l 0 (l ) (l 0+ ) e q( e l)e  0 r @ @ @ q 0 l(q 00 )

q

λ

r

❅

❅

❅

_q

Table1. Sfermiondecaydiagramswithtrilinearcouplings

consideronlyspontaneousR p

/ inthethirdgeneration 3

. Inthismodel,called -modelthespontaneousR

p

/ leadstotheexistanceofaphysical,masslessand stableNambu-Golstoneboson,calledMajoron(J).

2 Decay of SUSY particles

TheSUSY particle decayscan beeither direct orindirect. In therstcase, the SUSY particle decays directly or via avirtual SUSY particle exchange to standard particles through an R

p

/ vertex. In the second case, the SUSY particledecaysthroughanR

p

/ conservingvertexto astandardparticleand anon-shell SUSY particle, which then decaysthroughan R

p

/ vertex. Inthe spontaneous R

p

/ scenario,the chargino is allowed to decayinto atau and a Majoron. Thebranchingratioofthis processdependsontheparameter

3 . Therefore,theexperimental signatureofthecharginopairproductionistwo acoplanartausandmissingmomentumfrom theundetected Majorons.

3 Searches framework R

p

/ LEPsearchesareperformedinaConstrainedMSSM.Themassspectrum andthecross-sectionoftheSUSYprocessesareobtainedwiththehypotheses of massuniversalityat GUTscale (,tan and m

0

). Thegauginorunning mass relation at electroweak scale is supposed to be M

1

 0:5M 2

. To be consistantwith the limit on the proton lifetime, the LEPanalyses consider onecouplingtobedominantatatime(LL

 E, LQ  D,  U  D  Dor i H u L i ). These analysesconsider thee

0 1

asthe LSPwhentheindirectdecaysof ~ f ore

 are searched whereasthe

~

f orthee 

analysisstrategies

LEP2correspondsto5yearsofdatatakingwithacentre-of-massenergyupto 209GeVandaluminosityperexperiment714pb

1

. Foralmostallsearches the four fermion nal states are the main backgrounds. The production of signalhasbeenperformedwiththeSUSYGEN generator

4

. Thenal states dependonthecouplings. Nevertheless,eachcoupling(LL

 E,LQ  D,or  U  D  D)is characterizebyadominanttopology. Thechannelsassumingthe

ijk

coupling have many leptons and missing energy. The channels with 

0 ijk

couplings havemany jets, leptonsand missing energy. Inthe 

00 ijk

analyses manyjets are expected. For all R

p

/ searches, we expect between 4(direct decay) and 10 fermions (indirect decay) in the nal state and the missing energy had to come from undetectableneutrino orMajoron(Spontaneous R

p

/ scenario). Thenumberof possiblenal statesis large. ThereforetheR

p

/ LEPsearches havetocovermanydierentchannels. Ingeneraloneanalysisisoptimizedfor dierentchannels. ForexampletheALEPHmulti-jetanalysiscorrespondsto the6,8,and10jettopologiesofthe

 U  D  Dcouplingsearches.

Thesignalselectionontherequiredtopologyisobtainedusingsequential cuts(ALEPH,DELPHI,L3,OPAL)andneuralnetworkmethods(DELPHI). Theselection arebased onleptonidentication and leptonisolation criteria forLL



E andLQ 

D searchesand onjet algorithmand b-taggingvariablesfor theLQ  Dand  U  D  Dsearches. 5 Results

Unfortunatly, thepresentpreliminary resultsofthe fourLEPexperiments 5 shownosignicantdeviationsfromtheSMexpectationsincaseofR

p

/ hypoth-esis. Thereforethese negativeresultsareinterpretedin aconstrainedMSSM framework by setting exclusionlimits at 95 % CL on cross-sections, SUSY particle masses(scalarsector) andexcluded MSSMregions(gaugini sector). Table2summarizethemostrecentresultsmainlybasedonALEPHand DEL-PHIpreliminary results. ThespontaneousR

p

/ search inDELPHI 7

obtained alimitonthecharginomassat 101GeVfordatataking atacentre-of-mass energyof202GeV.ThesinglesneutrinoproductionsearchesofDELPHIand ALEPHgiveslimtsonthecouplingsaround10

e  0 1 40 40 - - 38 38 e  + 1 103 103 103 103 102 102 e e R 96 96 - 93  94 e  R 87 96 - 90  85 e  R 87 95 - 76  -e  e 100 98 - 91  88 e   90 95 79 78  65 e   90 95 79 78  65 e t L  91 - 85 - 87 e b L  90 - 80 - 80

Table2. Summaryofexistingmasslimits inGeV=c 2

fromRp/ pairproduction searches ofsupersymmetricparticlesatLEP.Eachmasslimitisobtainedundersomehypotheses whicharenotpresented inthiscontribution. Thesignmeansnotallowed. Thesign -meansnotcovered.

References

1. For reviews, see e.g. H.P. Nilles, Phys. Rep.110 (1984)1; H.E.Haber andG.L.Kane,Phys. Rep. 117(1985)75.

2. P. Fayet, Phys. Lett. B69 (1977) 489; G. Farrar and P. Fayet, Phys. Lett. B76(1978)575.

3. A.MasieroandJ.W.F.Valle,Phys.Lett. B251 (1990) 273;F.de Cam-pos, O.J.P. Eboli, M.A. Garcia-Jareno, and J.W.F. Valle, Nucl. Phys. B546 (1999) 33.

4. S.Katsanevas,P.Morawitz,Computer Phys. Comm. 112(1998)227; N.GhodbaneSUSYGEN3.0hep-ph/9909499.

5. ALEPHCollab.,Eur.Phys.J13 (2000) 29;

DELPHICollab.,PreprintCERN, CERN-EP/2000-127; L3Collab.,PreprintCERN,CERN-EP/2000-130; OPALCollab.,Eur.Phys.J13 (2000) 553. 6. DELPHI2001-019CONF460;

ALEPH2001-011CONF 2001-008.