Neutral top-pion and the rare top decays $tto c l_{i} l_{j}$(2)

Tocalculatethebranchingratiosoftheraretopdecayst→clilj,weassumethatthe

totaldecaywidthΓtofthetopquarkisdominatedbythedecaychannelt→wb,whichhasbeentakenΓ(t→wb)=1.56GeV.Inourcalculation,wehavetakenmt=175GeV,mc=1.2GeV,mτ=1.78GeV,mµ=0.105GeV,andαe=1

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

0hundredGeVdependingonthemodels.Asnumericalestimation,wetaketheπtmass

mπttovaryfrom150GeVto400GeV.OurnumericalresultsaresummarizedinFig.2～

Fig.4.

-8) 10*

Br(t->c150200250300350400

m (GeV)

Figure3:SameasFig.2butfortheraretopdecayt→cµµ.

0ThebranchingratiosBr(t→cττ)andBr(t→cµµ)areplottedasfunctionsoftheπt

massmπtforthreevaluesofthefreeparameterεinFig.2andFig.3,respectively.From

these gures,wecanseethatthevaluesofthesebranchingratiosincreaseasεincreasing.InalloftheparameterspaceoftheTC2models,thevaluesofbranchingratioBr(t→cττ)arelargerthanthoseofBr(t→cµµ).Forexample,formπt=200GeVandε=0.05,the

valuesofBr(t→cττ)andBr(t→cµµ)are8.2×10 8and5.6×10 9,respectively.This

0isbecausetheone-loopcontributionsofπtexchangetothebranchingratiosBr(t→cττ),

Br(t→cµµ),andBr(t→cee)areapproximatelyequaltoeachother,whicharesmaller

0than5×10 9.However,thetree-levelcontributionsofπtexchangetothebranching

m2lratiosBr(t→cll)areproportionalto

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

magnitude.Fortheprocesst→cµµ,twokindsofthecontributionsarecomparable.Fortheprocesst→cee,thefactorm2e

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

cancontributethisprocessviatheon-shellphotonpenguindiagramsgeneratedbythe

00FCscalarcouplingπtτµandπtτe.Thus,usingthepresentexperimentalboundonthe

LFVprocessµ→eγ,i.e.Br(µ→eγ)≤1.1×10 11,wecangivetheconstraintsonthe avormixingfactorKfor150GeV≤mπt≤400GeV,inwhichwehaveassumed

K=Kτµ=Kτe.Ifwetakethatmπtissmallerthan400GeV,thantheremustbeK≤

0.2[8].Takingintoaccounttheconstraintsonthe avormixingfactorK,thebranchingratioBr(t→cτµ)≈Br(t→cτe)isplottedasafunctionofmπtforthreevaluesofthe

parameterεinFig.4.Onecansee,fromFig.4,thatthevalueofBr(t→cτµ)increasesasmπtdecreasingandεincreasing.For0.02≤ε≤0.08and150GeV≤mπt≤400GeV,

thevalueofBr(t→cτµ)≈Br(t→cτe)isintherangeof1.8×10 10～1.7×10 8.

TheCERNLHCwillallowtoprobethecouplingsofthetopquarktobothknownandnewparticlesinvolvedinpossibletopdecaychannelsdi erentfromthemaint→wb.ItisveryusefultoanalyzetheraretopdecaysfordetectingthenewphysicsbeyondtheSM[1,2].Ifweassumethattheproductioncrosssectionofthetopquarkpairsisabout8×105fbattheLHCwithayearlyintegratedluminosityof£=100fb 1,thenwecanonlyobtain8cττeventsperyearformπt=150GeVandε=0.08.

Thesignaloftheraretopdecayt→cττistheisolatedτleptonswithac-quarkjet.Ifwewanttodetectthissignal,wemustconsiderthee ciencyproblemsinmeasuringtheτleptonandidentifyingac-quarkjet.Furthermore,wewillhavetotakethesuitablekinematicalcutstoextractthesignalfromapossiblelargereduciblebackground,whichwillfurtherdegradethesignal.Thus,itisverydi culttodetectthepossiblesignalsof

0theneutraltop-pionπtbytheprocesst→cττatthefutureLHCexperiments.However,

ifthetopcolorscenarioorothernewphysicsbeyondtheSMcanpredicttheexistenceofalightneutralscalar,forexamplems<100GeV,whichcaninducetheFCscalarcouplings,itssignalsmightbedetectedviatheprocesst→cττatthefutureLHCexperiments.Evenifwecannotobservetheraretopdecayt→cττinducedbythelightneutralscalar,wecanobtainexperimentalboundsonthemassesofthiskindofnewparticles[14].

Thepossibilityofdetectingthetop-pionsviatheFCprocessesattheLHCexper-imentshasbeenextensivelystudiedinRef.[7,13].Theyhaveshownthatthesignalsof

9

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

top-pionsshouldbeobservableattheLHCinasizableregionoftheparameterspace.Therefore,wehavetosaythatthetop-pionsaremoreeasydetectedusingtheseFCpro-cessesthanusingtheraretopdecayst→clilj.However,ourworkprovidesonepossiblemethodtostudythesignalsoftheneutralscalarattheLHCexperiments.

Acknowledgments

ChongxingYuewouldliketothanktheAbdusSalamInternationalCentreforTheoreticalPhysics(ICTP)forpartialsupport.ThisworkwassupportedinpartbytheNationalNatureScienceFoundationofChina(90203005)andtheNaturalScienceFoundationoftheLiaoningScienti cCommittee(20032101).

10

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

References

[1]Forreviewssee:M.Benekeetal.(conveners),A.Ahmadovetal.,hep-ph/0003033.[2]Forreviewssee:B.Mele,hep-ph/0003064;S.Bejar,J.Guasch,andJ.Sola,

hep-ph/0101294.

[3]E.O.IltanandI.Turan,Phys.Rev.D67(2003)015004;E.O.Iltan,hep-ph/0401229.

[4]C.T.HillandE.H.Simmons,Phys.Rep.381(2003)235,[Erratum-ibid,

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