With each external line one has to assoicate the following factors.

**Spin 1/2**: For each fermion or antifermion line entering the graph
include the spinor

(7.372) |

and for spin-1/2 particles leaving the graph, the spinor

(7.373) |

**Spin 0**: For each boson assign a factor

(7.374) |

**Photons**: For each photon entering the graph include a polarization
vector

(7.375) |

and for photons leaving the graph the vector

(7.376) |

2004-03-18