Couplings and W boson
The example of the electron-neutrino pair shows how the weak interaction change the nature of a particle, by emitting an intermediate particle called W boson. This boson carries a positive or negative elementary charge (NB: The red arrows refer to initial particles, the green to those in the final state). In (A), an electron loses its electric charge by emitting a W boson and becomes a neutrino; (B) represents the reverse reaction, the capture of the W boson by a neutrino; in (C), the disintegration of a W-minus is obtained from (B) by replacing the incoming neutrino, with an outgoing antineutrino. In the fourth diagram (D), the emission of a W-plus boson by a positron, is obtained from (C) applying the same rule. We changed the direction of two arrows and particles were changed in antiparticles.