Fermion flow
= Fermion flow
The arrow on a charged-fermion line in a <Feynman diagram> tracks fermion-number flow and fixes the order of the <gamma matrices> between external <Dirac spinors>. For an external particle it agrees with the particle direction; for an external antiparticle it is opposite to the antiparticle direction. It therefore need not point along the assigned four-momentum of every external leg.