content_image/317290.jpg content_image/317292.jpg content_image/317291.jpg content_image/317294.jpg content_image/317295.jpg content_image/317297.jpg content_image/317293.jpg content_image/317296.jpg content_image/317298.jpg
where u and v are positive and negative energy solutions of Dirac equation. 

Momentum 

Polarization vector 

<img src='content_image/317290.jpg'>

<img src='content_image/317292.jpg'>

<img src='content_image/317291.jpg'>

where q is reference momentum, reflecting the freedom of on-shell gauge transformation, k is the vector boson momentum. 

Useful properties: 

antisymmetry 

Fierz rearrangement 

charge conjugation 

eikonal identity 

in an N -point amplitude, momentum conservation imply 

<img src='content_image/317294.jpg'>

<img src='content_image/317295.jpg'>

<img src='content_image/317297.jpg'>

<img src='content_image/317293.jpg'>

<img src='content_image/317296.jpg'>

<img src='content_image/317298.jpg'>

