TemporalPair[ExplicitLorentzIndex[0], TemporalMomentum[p]]
is a special pairing used in the internal representation to denote p^0, the temporal component of a 4-momentum
p.
Overview, TemporalPair, TC, ExplicitLorentzIndex.
[ExplicitLorentzIndex[0], TemporalMomentum[p]] TemporalPair
p^0
[ExplicitLorentzIndex[0], TemporalMomentum[p + q]]
TemporalPair
% // ExpandScalarProduct
(p+q)^0
p^0+q^0