DiracChainJoin[exp] joins chains of Dirac matrices with explicit Dirac indices wrapped with a head DiracChain. Notice that DiracChainJoin is not suitable for creating closed Dirac chains out of the FeynArts output with explicit Dirac indices, e.g. when the model contains 4-fermion operators. Use FCFADiracChainJoin for that.
Overview, DiracChain, DCHN, DiracIndex, DiracIndexDelta, DIDelta, DiracChainCombine, DiracChainExpand, DiracChainFactor, FCFADiracChainJoin.
DCHN[SpinorUBar[p1, m1], i] DCHN[GAD[\[Mu]] . GAD[\[Nu]], i, j] DCHN[j, SpinorV[p2, m2]]
DiracChainJoin[%](v(\text{p2},\text{m2}))_j \left(\gamma ^{\mu }.\gamma ^{\nu }\right){}_{ij} \left(\bar{u}(\text{p1},\text{m1})\right)_i
\left(\varphi (\overline{\text{p1}},\text{m1})\right).\gamma ^{\mu }.\gamma ^{\nu }.\left(\varphi (-\overline{\text{p2}},\text{m2})\right)