We study gauge mediation of supersymmetry breaking in SU(5) supersymmetric grand unified theory with gauge fields as messengers. The generated soft supersymmetry breaking parameters lead to close to maximal mixing scenario for the Higgs mass and highly red
isasizablemixtureofbino,winoandHiggsino.Sizablemixtureofbinowithhiggsino/winocangivetherightamountofthermalrelicdensityfordarkmatterwhenµ,M1andM2areoforder100GeV.Inaddition,thecrosssectionforthedirectdetectionislargercomparedtobinoLSPwhichgivesabetterchancetoobserveit.
Inmostregionofallowedparameterspace,sneutrino/stauorstopisNLSPandtheLSPisthegravitino.GravitinoLSPscenariowiththeright-handedstauNLSPhasbeenstudiedintheframeworkofsupergravity[36][37][38][39][40][41].ThelifetimeofthestauNLSPisfrom106secto1010secandwemightbeabletodetectitusingastopper.Asweprovideaspeci cmodelinwhichthegravitinoLSPisveryplausible,wecangetamoreconcretepredictiononNLSPlifetimeandgravitinorelicdensity.SimilaranalysisshouldbedoneforthestopNLSP.
>Thegaugemessengermodelconsideredheregenerallypredictsaverylightstop,mt 1～150GeVintheleast netunedparameterspace.TheFig.4showsthatstopbecomes
NLSPifcHu 30.StopNLSPhasbeenstudiedin[35]intheframeworkoflowscalegauge
mediation.Whengravitinoisverylight,thedecayofstopNLSPcanhappenquickly,withinaminute,andthesearchforapossiblecollidersignalcanbedone.Ifgravitinomassisataroundtheweakscale,stopdecayslongafterthebigbangnucleosynthesis(BBN).UsuallydecaysofparticleshavinghadronicchannelsdestroythesuccessfulagreementofBBNandsuchscenariosarenotconsidered.6NonethelessthedetailedanalysisofstopdecayafterBBNshouldbedonetoclearupthisissue.IfstopNLSPwithweakscalegravitinomassisconsistentwithBBN,morenaturalparameterspaceisallowed.
5.Conclusions
InthispaperwestudiedgaugemediationofsupersymmetrybreakinginSU(5)SUSYGUTwithheavygauge eldsasmessengers.WewereledtoconsiderthisgaugemessengermodelbyrecentlydiscussedpossibilityofgeneratingthemaximalmixingscenariofortheHiggsmassradiatively[7].Intheoptimalscenariocoloredparticlesdonotcontributetotherenormalizationgrouprunningofthem2Huwhichinprinciplecaneliminate netuningofEWSB.Thegaugemessengermodeldoesnotleadtotheoptimalscenario(onlyclosetoit),sincestopmassesarenotnegativeenoughattheGUTscale.However,itstillhighlyreducesthe netuningofEWSBandhasmanyinterestingfeatures.
InthisscenarionegativescalarmassessquaredattheGUTscale,withsquarksmorenegativethansleptons,togetherwithnon-universalgauginomasses,withM1>M2>M3,leadtoaviablespectrumattheEWscaleinlargeportionofparameterspace.NoneofthesoftSUSYbreakingparametersattheGUTscalehastobelargerthan400GeVandnoneofthesuperpartnerhastobeheavierthan400GeVtosatisfyallthelimitsfromdirectsearchesforSUSYparticlesandalsothelimitontheHiggsmass.Thereisnootherexistingscenariowithsimilarfeatures.Andyet,justlikeanomalymediationortheusualgaugemediation,alsothisscenarioisgovernedbyasingleparameter-theSUSYbreakingscaleMSUSY.Theratiosofdi erentsoftSUSYbreakingparametersareentirely