Neutral top-pion and the rare top decays $tto c l_{i} l_{j}$

We study the rare top decays $t\to c l_{i} l_{j}(l=\tau,\mu,or e)$ in the framework of topcolor-assisted technicolor($TC2$) models. We find that the neutral top-pion $\pi_{t}^{0}$ can produce significant contributions to these processes via the flavor chan

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aNeutraltop-pionandtheraretopdecayst→cliljChongxingYue,LeiWang,DongqiYuDepartmentofPhysics,LiaoningNormalUniversity,Dalian116029.P.R.China February2,2008AbstractWestudytheraretopdecayst→clilj(l=τ,µ,ore)intheframeworkoftopcolor-assistedtechnicolor(TC2)models.We ndthattheneutraltop-pionπ0tcanproducesigni cantcontributionstotheseprocessesviathe avorchangingcouplingsπ0tt¯candπ0tlilj.Fortheπ0tmassmπt=150GeVandtheparameterε=0.08,thebranchingratioBr(t→cττ)canreach7.1×10 7.Takingintoaccounttheconstraintsofthepresentexperimentallimitoftheprocessµ→eγonthefreeparametersofTC2models,we ndthatthevalueofBr(t→cτµ)≈Br(t→cτe)isintherangeof1.8×10 10～1.7×10 8.

PACSnumber(s):14.65.Ha,14.80.Cp,

We study the rare top decays $t\to c l_{i} l_{j}(l=\tau,\mu,or e)$ in the framework of topcolor-assisted technicolor($TC2$) models. We find that the neutral top-pion $\pi_{t}^{0}$ can produce significant contributions to these processes via the flavor chan

I.Introduction

Itiswellknownthat,inthestandardmodel(SM),the avorchangingneutralcurrent(FCNC)isabsentattree-levelandatone-loopleveltheyareGIMsuppressed.TheSMresultsoftheraredecays,whichareinducedbyFCNC,areverysmallandcannotbedetectedinthecurrentorfuturehighenergycolliderexperiments.Thus,raredecaysprovideaverysensitiveprobeofnewphysicsbeyondtheSM.Detectionofraredecaysatvisiblelevelsbyanyofthefuturecolliderswouldbeinstantevidenceofnewphysics.Searchingforraredecaysisoneofthemajorgoalsofthenextgenerationofhighenergycolliderexperiments.

Thelargevalueofthetopquarkmasso ersthepossibilitythatitcanbesingledouttoplayakeyroleinprobingthenewphysicsbeyondtheSM.Thepropertiesofthetopquarkcouldrevealinformationregarding avorphysics,electroweaksymmetrybreaking(EWSB)mechanism,aswellasnewphysicsbeyondtheSM[1].Oneoftheseconsequencesisthatraretopdecayscanbeusedtodetectnewphysics.Muchofthetheoreticalactivityinvolvingraretopdecayshasoccurredwithinsomespeci cmodelsbeyondtheSM[2].IthasbeenshownthatthevaluesofthebranchingratiosBr(t→cv)(v=g,γ,orz)orBr(t→cvv)(v=g,γ,orw)canindeedbeenhancedbyseveralordersofmagnitude,whichmaybewithintheobservablethresholdofnear-future-experiments.Recently,theraretopdecayst→clilj(l=τ,µ,ore)havebeenstudiedintheframeworkofthegeneraltwoHiggsdoubletmodel[3].TheyhaveshownthattheneutralHiggsbosonsh0andA0canmakethebranchingratiosBr(t→clilj)reach1×10 8.

0,±Thepresenceofthetop-pionsπtinlow-energyspectrumisaninevitablefeatureof

topcolorscenario[4].ThesenewparticleshavelargeYukawacouplingstothethirdfamilyquarksandcaninducethetree-levelFCcouplings,whichhavesigni cantcontributionstotheraretopdecayst→cγ(g,z)andt→cww[5].Inthispaper,wewillstudy

0thecontributionsoftheneutraltop-pionπttotheraretopdecayst→cliljinthe

contextoftopcolor-assistedtechnicolor(TC2)models[6].Theseraredecayprocessescan

0¯occuratthetree-levelorone-looplevel,whichareinducedbytheFCcouplingsπttc

00orπtlilj(i=j).Ournumericalresultsshowthattheneutraltop-pionπtcangenerate

2

We study the rare top decays $t\to c l_{i} l_{j}(l=\tau,\mu,or e)$ in the framework of topcolor-assisted technicolor($TC2$) models. We find that the neutral top-pion $\pi_{t}^{0}$ can produce significant contributions to these processes via the flavor chan

signi cantcontributionstotheseprocesses.Inalloftheparameterspace,thevaluesofthebranchingratioBr(t→cττ)arelargerthanthoseofBr(t→cµµ)orBr(t→cee).Formπt=150GeVandtheparameterε=0.08,wehaveBr(t→cττ)=7.1×10 7.Taking

intoaccounttheconstraintsofthepresentexperimentallimitoftheprocessµ→eγonthe

0 avormixingparameterKij,wefurthercalculatethecontributionsofπttotheraretop

decayst→cτµandcτe.We ndthatthebranchingratioBr(t→cτµ)isapproximatelyequaltothebranchingratioBr(t→cτe).Inmostoftheparameterspace,thevalueofBr(t→cτµ)isintherangeof1.8×10 10～1.7×10 8.

Thepaperisorganizedasfollows:Insection2,wewillpresenttheexpressionsofthe

0branchingratiosBr(t→clilj)contributedbytheneutraltop-pionπt.Ournumerical

resultsandconclusionsaregiveninsection3.

0II.Theneutraltop-pionπtandthebranchingratiosBr(t→clilj)

ForTC2models[4,6],theunderlyinginteractions,topcolorinteractions,areassumedtobechiralcriticallystrongatthescaleabout1TeVandcoupledpreferentiallytothethirdgeneration,andthereforedonotpossesGIMmechanism.Thenon-universaltopcolorinteractionsresultintheFCcouplingverticeswhenonewritestheinteractionsinthemasseigenbasis.Thus,thetop-pionscaninducethenewFCscalarcouplingvertices.

0Thecouplingsoftheneutraltop-pionπttoordinaryfermions,whicharerelatedtothe

raretopdecayst→clilj,canbewrittenas[7,8]:

mt

2Ft tttt¯50tctt¯0[KURKULtγtπt+KURKULtLcRπt]+

νw

√ml¯50mτ0lγlπt+Kτiτ¯γ5liπt.(1)2νw2νw

Whereνw=ν/

2ε ε2.

tcInthefollowingcalculation,wewilltakeKUR=(2)√

We study the rare top decays $t\to c l_{i} l_{j}(l=\tau,\mu,or e)$ in the framework of topcolor-assisted technicolor($TC2$) models. We find that the neutral top-pion $\pi_{t}^{0}$ can produce significant contributions to these processes via the flavor chan

leptone(µ),Kτiisthe avormixingfactor,whichisthefreeparameter.Certainly,there

0isalsotheFCscalarcouplingπtµ¯e.However,thetopcolorinteractionsonlycontactwith

thethirdgenerationfermions.The avormixingbetweenthe rst …… 此处隐藏：5368字，全部文档内容请下载后查看。喜欢就下载吧 ……