Momentum[p] is the head of a four momentum p.
The internal representation of a 4-dimensional p is Momentum[p].
For other than 4 dimensions: Momentum[p, dim].
Momentum[p, 4] simplifies to Momentum[p].
Overview, DiracGamma, Eps, LorentzIndex, MomentumExpand.
This is a 4-dimensional momentum.
Momentum[p]\overline{p}
As an optional second argument the dimension must be specified if it is different from 4.
Momentum[p, D]p
The dimension index is suppressed in the output.
Momentum[p, d]p
Momentum[-q]-\overline{q}
Momentum[-q] // StandardForm
(*-Momentum[q]*)ex = Momentum[p - q] + Momentum[2 q]\left(\overline{p}-\overline{q}\right)+2 \overline{q}
ex // StandardForm
(*Momentum[p - q] + 2 Momentum[q]*)ex // MomentumExpand // StandardForm
(*Momentum[p] + Momentum[q]*)ex // MomentumCombine // StandardForm
(*Momentum[p + q]*)ChangeDimension[Momentum[p], d] // StandardForm
(*Momentum[p, d]*)