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