The topcolor-assisted technicolor (TC2) model predicts several pseudo-scalars called top-pions and at loop level they can induce the FCNC top quark decay t->cgg which is extremely suppressed in the Standard Model (SM). We find that in the allowed parameter
10-2
MTC = 300 GeV
tg→cg
Branching Ratiot→10-3cg
10-4
0.10.150.20.250.30.350.4
KUR
TCinTC2model.FIG.4:Branchingratiosoft→cggandt→cgversusKURTC
quarkrestframe.
InFig.3weshowthebranchingratiosoft→cggandt→cgversusMTC.Weseethatthecontributionsoftop-pionscansigni cantlyenhancesuchraredecaysandintheallowedparameterspacethebranchingratiocanbeupto10 3,whichismuchlargerthanthepredictionsintheSM(10 9)[2]andintheMSSM(10 4)[4].
AsshowninFig.3,thebranchingratiooft→cggislargerthant→cg,whichisalsoobservedintheSM[2]andtheMSSM[4].Asdiscussedin[2,4],thereasonforthisbehavioristhattheformfactorF1inEq.(4),whichmakesimportantcontributiontot→cgg,doesnotcontributetot→cg.
TCNotethattheTC2contributionsaresensitivetotheparameterKURwhichis xedto
TC0.4inFig.3.InFig.4weshowthedependenceonKURfor xedtop-pionmass.Weseethat
TCthebranchingratiosincreasedrasticallyasKURgetslarge.
Finally,inTable1wesummarizetheTC2predictionsfortheFCNCtopquarkdecayswithcomparisontothepredictionsintheSMandMSSM.TheTC2predictionsaretaken
TCfromFig.3forMTC=300GeVandKUR=0.4.WeseethatforeachdecaymodetheTC2
modelallowsamuchlargerbranchingratiothantheothertwomodels.