hep-ph9703288 LeptoquarkSquark Interpretation of H

BI-TP97/07DESY97-038WUE-ITP-97-02hep-ph/9703288Leptoquark/SquarkInterpretationofHERAEvents:VirtualEectsine+e−AnnihilationtoHadronsJ.Kalinowski1;2,R.R¨uckl3;,H.Spiesberger4;,andP.M.Zerwas11DeutschesElektronen-SynchrotronDESY,D-22607Hamburg2InstituteofTheoreticalPhysics,WarsawUniversity,PL-00681Warsaw3Institutf¨urTheoretischePhysik,Universit¨atW¨urzburg,D-97074W¨urzburg4Fakult¨atf¨urPhysik,Universit¨atBielefeld,D-33501BielefeldABSTRACTInreferencetotherecentlyobservedhighQ2,largexeventsindeep-inelasticpositron{protonscatteringatHERA,variousleptoquarkandsupersymmetricscenariosarediscussed.WestudytheimpactofvirtualleptoquarkorR-paritybreakingsquarkexchangeaswellasgenericcontactinteractionontheproduc-tionofquark{antiquarkpairsine+e−annihilation,inparticularatLEP2.SupportedbyBundesministeriumf¨urBildung,Wissenschaft,ForschungundTechnologie,Bonn,Ger-many,Contracts057BI92P(9)and057WZ91P(0).11IntroductionTherecentobservationofeventsindeep-inelasticpositron{protonscatteringwithveryhighQ2andlargexatHERA[1]hasrefuelledspeculationsonphysicsbeyondtheStan-dardModel,inparticularonlow-massleptoquark-typeparticles.Suchparticleshadbeensuggestedalongtimeagoinavarietyofphysicalscenarios:Pati{SalamSU(4)unicationofquarksandleptons[2],granduniedtheoriessuchasSU(5)orE6[3],andcompositemodels[4].Moreover,insupersymmetrictheoriessquarkscoupletolepton{quarkpairsiftheR-symmetryisbrokeninthetrilinearcouplingsofthesuperelds[5,6].Vectorlep-toquarksingranduniedtheorieswithbothlepton-quarkanddiquarkcouplingsmustbeveryheavytosuppressprotondecay;certainscalarleptoquarksinGUTmultipletscouldneverthelessberelativelylight[7](disregardingthenotorioushierarchyproblemforthetimebeing).SquarksinsupersymmetrictheoriesshouldnaturallybeexpectedinthemassrangeofafewhundredGeV.Ageneralclassicationofthesenovelstates1hasbeenpresentedinRef.[8].Inthisanalysisthecouplingsofleptoquarkstolepton{quarkpairsareassumedtobebaryon-andlepton-numberconservinginordertoavoidrapidprotondecay,familydiagonaltoexcludeFCNCprocessesbeyondtheCKMmixing,andchiraltopreservethehelicitysuppressioninleptonicpiondecay.Moreover,thecouplingsaretakendimensionlessandallinteractionsareassumedtorespecttheSU(3)CSU(2)LU(1)YsymmetryoftheStandardModel.Theallowedstatescanbeclassiedaccordingtospin,weakisospinandfermionnumber.WeadoptthenotationofRefs.[9]toconformwiththenotationgenerallyemployedinexperimentalpapers:vectorleptoquarksaredenotedbyVI,scalarleptoquarksbySI;isomultipletswithdierenthyperchargesaredistinguishedbyatilde.ForconveniencetheninepossiblestatesofscalarandvectorleptoquarksarelistedinTable1.Leptoquarksintheupper(lower)partofTable1carryfermionnumberF=2(F=0).ThecouplingsaredenotedgenericallybygRorgLwithR;Lreferingtothechiralityofthelepton.Eachstatecancouplewithdierentstrength;forsimplicity,theadditionalindicesaresuppressed.InprinciplethetwoscalarstatesS0andS1=2andthetwovectorstatesV0andV1=2couldhavebothchiralgRandgLcouplingsatthesametime;however,sincetheproductofthetwocouplingsisconstrainedverystronglybyraredecays[10,11,12,13],weassumeonlyoneofthetwocouplingstobenon-zero.Thespecialtypeofleptoquarkthatdoesnotinduceprotondecay(asaresultofitsquantumnumbers)andiscompatiblewiththerenormalizationoftheelectroweakmixinganglefromthesymmetryvalue3/8attheGUTscaledowntosin2W=0:23attheelectroweakscaleRef.[7],ismarkedbyanasterisk.OnlyasmallsubsetofallthesestatesisrealizedinsupersymmetrictheorieswithR-paritybreaking.Moreover,supersymmetryrequiresthesestatestohaveuniversalleft-handedcouplingstoleptons.IfleptoquarksorsquarksinR-paritybreakingsupersymmetrictheoriesexist,alarge1WeshallgenericallydenoteleptoquarksandsquarksinR-paritybreakingscenariosbyLQ.2LQQDecayBRCouplingLimitsHERAe−ie+!qkqModeejRef.[11,12]estimatesikS0~dR−1=3eLuLdeRu121gL−gLgRgL0:06gR0:10:400:28LL−RR~S0−4=3eRd1gRgR0:10.30RR+2=3Lu0p2gL−−S1−1=3LdeLu12−gL−gLgL0:090.40−LL−4=3eLd1−p2gL0.21LL−1=3LdeRu01gLgRgL0:09−0:30−RLV1=2−4=3eLdeRd1gLgRgR0:050:320:32LRRL+2=3Lu0gL−−~V1=2−1=3eLu1gLgL0:090.32LR−2=3LueRd01gL−gRgL0:1−0:052−RLS1=2−5=3eLueRu1gLgRgR0:090:0260:026LRRL~dL+1=3Ld0gL−−~S1=2~uL−2=3eLd1gLgL0:10.052LRV0−2=3eLdLueRd121gLgLgRgL0:05gR0:090:0800:056LL−RR~V0−5=3eRu1gRgR0:090.027RR+1=3Ld0p2gL−−V1−2=3eLdLu12−gLgLgL0:040.080LL−−5=3eLu1p2gL0.019LLTable1:Scalar(S)andvector(V)leptoquarks/squarkswithelectriccharges(Q),decaymodes,branchingratiosforchargedlepton+jetchannelswitheitherLorRcouplings,andtheYukawacouplings(gR;L)withthemoststringentlimitsfromraredecaysandestimatesfromtherecentHERAdata(seetext).Thehelicitycombinationsik(=L;R)contributingtotheprocesse−ie+!qkqaregiveninthelastcolumn.Alsoshownarepossiblesquarkassignmentsofleptoquark-typestates;thespecialleptoquarksingletoutinRef.[7]ismarkedbyanasterisk.3varietyofphenomenaareexpectedtobeobservedexperimentally.Inelectron/positron{protoncollisions,theseparticlesareproducedassingleresonances,withtheratedeterminedbythestrengthoftheLQ-l-qYukawacouplings[8,14].Pairproductionisanimportantproductionmechanismforleptoquarksinproton{(anti)proton[15],el