TemporalMomentum[p] is the head of the temporal component of a 4-momentum p^0. The internal representation of the temporal component p^0 is TemporalMomentum[p].
TemporalMomentum may appear only inside TemporalPairs.
Overview, TemporalPair, ExplicitLorentzIndex.
TemporalMomentum[p]p
TemporalMomentum[-q] // StandardForm
(*-TemporalMomentum[q]*)TemporalMomentum[p + q]p+q
TemporalMomentum[p + q] // MomentumExpand // StandardForm
(*TemporalMomentum[p] + TemporalMomentum[q]*)TemporalMomentum[p + q] // MomentumExpand // MomentumCombine // StandardForm
(*TemporalMomentum[p + q]*)