MT[mu, nu]
is the metric tensor in 4 dimensions.
Overview, FeynCalcExternal, FCE, FCI, MTD, MTE.
[\[Alpha], \[Beta]] MT
\bar{g}^{\alpha \beta }
[MT[\[Alpha], \[Beta]] MT[\[Alpha], \[Beta]]] Contract
4
[a, b] // StandardForm
MT
(*MT[a, b]*)
[MT[a, b]] // StandardForm
FCI
(*Pair[LorentzIndex[a], LorentzIndex[b]]*)
[FCI[MT[a, b]]] // StandardForm
FCE
(*MT[a, b]*)