MomentumExpand[expr]
expands Momentum[a+b+ ...]
in expr
into Momentum[a] + Momentum[b] + ...
.
Overview, ExpandScalarProduct, MomentumCombine.
[Momentum[p + q]] // StandardForm
MomentumExpand
(*Momentum[p] + Momentum[q]*)
[p + q, r] ScalarProduct
[p + q, r] // StandardForm
ScalarProduct
(*Pair[Momentum[p + q], Momentum[r]]*)
= MomentumExpand[ScalarProduct[p + q, r]] ex
// StandardForm
ex
(*Pair[Momentum[p] + Momentum[q], Momentum[r]]*)
= MomentumExpand[ScalarProduct[p + q, r - p]] ex
// StandardForm
ex
(*Pair[Momentum[p] + Momentum[q], -Momentum[p] + Momentum[r]]*)