LeptoquarkSquark Interpretation of HERA Events Vir

arXiv:hep-ph/9703288v212Mar1997BI-TP97/07DESY97-038WUE-ITP-97-02hep-ph/9703288Leptoquark/SquarkInterpretationofHERAEvents:VirtualEffectsine+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)unificationofquarksandleptons[2],grandunifiedtheoriessuchasSU(5)orE6[3],andcompositemodels[4].Moreover,insupersymmetrictheoriessquarkscoupletolepton–quarkpairsiftheR-symmetryisbrokeninthetrilinearcouplingsofthesuperfields[5,6].Vectorlep-toquarksingrandunifiedtheorieswithbothlepton-quarkanddiquarkcouplingsmustbeveryheavytosuppressprotondecay;certainscalarleptoquarksinGUTmultipletscouldneverthelessberelativelylight[7](disregardingthenotorioushierarchyproblemforthetimebeing).SquarksinsupersymmetrictheoriesshouldnaturallybeexpectedinthemassrangeofafewhundredGeV.Ageneralclassificationofthesenovelstates1hasbeenpresentedinRef.[8].Inthisanalysisthecouplingsofleptoquarkstolepton–quarkpairsareassumedtobebaryon-andlepton-numberconservinginordertoavoidrapidprotondecay,familydiagonaltoexcludeFCNCprocessesbeyondtheCKMmixing,andchiraltopreservethehelicitysuppressioninleptonicpiondecay.Moreover,thecouplingsaretakendimensionlessandallinteractionsareassumedtorespecttheSU(3)C×SU(2)L×U(1)YsymmetryoftheStandardModel.Theallowedstatescanbeclassifiedaccordingtospin,weakisospinandfermionnumber.WeadoptthenotationofRefs.[9]toconformwiththenotationgenerallyemployedinexperimentalpapers:vectorleptoquarksaredenotedbyVI,scalarleptoquarksbySI;isomultipletswithdifferenthyperchargesaredistinguishedbyatilde.ForconveniencetheninepossiblestatesofscalarandvectorleptoquarksarelistedinTable1.Leptoquarksintheupper(lower)partofTable1carryfermionnumberF=2(F=0).ThecouplingsaredenotedgenericallybygRorgLwithR,Lreferingtothechiralityofthelepton.Eachstatecancouplewithdifferentstrength;forsimplicity,theadditionalindicesaresuppressed.InprinciplethetwoscalarstatesS0andS1/2andthetwovectorstatesV0andV1/2couldhavebothchiralgRandgLcouplingsatthesametime;however,sincetheproductofthetwocouplingsisconstrainedverystronglybyraredecays[10,11,12,13],weassumeonlyoneofthetwocouplingstobenon-zero.Thespecialtypeofleptoquarkthatdoesnotinduceprotondecay(asaresultofitsquantumnumbers)andiscompatiblewiththerenormalizationoftheelectroweakmixinganglefromthesymmetryvalue3/8attheGUTscaledowntosin2ΘW∼=0.23attheelectroweakscale[7],ismarkedbyanasterisk.OnlyasmallsubsetofallthesestatesisrealizedinsupersymmetrictheorieswithR-paritybreaking.Moreover,supersymmetryrequiresthesestatestohaveuniversalleft-handedcouplingstoleptons.IfleptoquarksorsquarksinR-paritybreakingsupersymmetrictheoriesexist,alarge1WeshallgenericallydenoteleptoquarksandsquarksinR-paritybreakingscenariosbyLQ.2LQQDecayBRCouplingLimitsHERAe−ie+→qk¯qModee±jRef.[11,12]estimatesikS0˜dR−1/3eLuνLdeRu121gL−gLgRgL0.06gR0.10.400.28LL−RR˜S0−4/3eRd1gRgR0.10.30RR+2/3νLu0√2gL−−S1−1/3νLdeLu12−gL−gLgL0.090.40−LL−4/3eLd1−√2gL0.21LL−1/3νLdeRu01gLgRgL0.09−0.30−RLV1/2−4/3eLdeRd1gLgRgR0.050.320.32LRRL+2/3νLu0gL−−˜V1/2−1/3eLu1gLgL0.090.32LR−2/3νL¯ueR¯d01gL−gRgL0.1−0.052−RLS1/2−5/3eL¯ueR¯u1gLgRgR0.090.0260.026LRRL˜dL+1/3νL¯d0gL−−˜S1/2∗˜uL−2/3eL¯d1gLgL0.10.052LRV0−2/3eL¯dνL¯ueR¯d121gLgLgRgL0.05gR0.090.0800.056LL−RR˜V0−5/3eR¯u1gRgR0.090.027RR+1/3νL¯d0√2gL−−V1−2/3eL¯dνL¯u12−gLgLgL0.040.080LL−−5/3eL¯u1√2gL0.019LLTable1:Scalar(S)andvector(V)leptoquarks/squarkswithelectriccharges(Q),decaymodes,branchingratiosforchargedlepton+jetchannelswitheitherLorRcouplings,andtheYukawacouplings(gR,L)withthemoststringentlimitsfromraredecaysandestimatesfromtherecentHERAdata(seetext).Thehelicitycombinationsik(=L,R)contributingtotheprocesse−ie+→qk¯qaregiveninthelastcolumn.Alsoshownarepossiblesquarkassignmentsofleptoquark-typestates;thespecialleptoquarksingledoutinRef.[7]ismarkedbyanasterisk.3varietyofphenomenaareexpectedtobeobservedexperimentally.Inelectron/positron–protoncollisions,theseparticlesareproducedassingleresonances,withtheratedeterminedbythestrengthoftheLQ-l-qYukawacouplings[8,14].Pairproductionisanimportantproductionmechanismforleptoquarksi