Top-Quark FCNC Decay t-cgg in Topcolor-assisted Technicolor(5)

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

KUR= 0.4

t→cggTC

Branching Ratiot→cg

10-3

200Rb Bound250300350400450500

MTC(GeV)

FIG.3:Branchingratiosoft→cggandt→cgversusMTCinTC2model.

tocalculatetheboxdiagramsshowninFig.2(d)andthetriangletop-quarkloopdiagramshowninFig.2(e).ThecalculationsarestraightforwardandtheresultsaregivenintheAppendix.

Numericalresults:Nowwearereadytogivesomenumericalresults.First,wetakealookattheinvolvedparameters.Theparametersinourcalculationsarethemassesofthe

TCtop-pionsandtop-Higgs,KUR,andthetop-piondecayconstantFt.Inourcalculationswe

takemt=170.9GeV[10]andFt=50GeV.Aboutthetop-pionandtop-Higgsmasses,inouranalysisweassume

0=m±=mh0≡MTCmπtπtt(5)

Inour guresofnumericalresultswewillshowaboundofabout250GeVontop-pionmass,whichisfromRbconstraintonthechargedtop-pion[11].NotethatsuchaboundisnotsorobustsinceinTC2modelthesizablecorrectionstoRbcanalsocomefromtheextendedtechnicolorgaugebosons.

InournumericalresultswegivethebranchingratiowiththetopwidthtakentobeΓt=1.55GeV[6].Tomakeourpredictionsmorerealistic,weapplysomekinematiccutsasin[4],e.g.,werequiretheenergyofeachdecayproductbelargerthan15GeVinthetop

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