content_image/318778.jpg content_image/318779.jpg content_image/318780.jpg
potential in the rest frame. For t < 0 we pick up only the k · p = 0 pole. Then 

<img src='content_image/318778.jpg'>

Let’s now examine the t > 0 solution. Again we’ll ignore the uninteresting k · p ′ = 0 Yukawa pole contribution. We find that 

<img src='content_image/318779.jpg'>

We can therefore write A µ Proca in the form of Eq. (F.39) with 

<img src='content_image/318780.jpg'>

where, from now on, we will implicitly take k 0 = k 0 $_{+}$. However because k $^{µ}$k$_{µ}$ = µ 2 instead of 0 as in the E&M case we cannot immediately write | ⃗ E | $^{2}$+ | ⃗ B | 2 as ⃗ E · ⃗ E ∗ as we did before. Additionally, since we do not have gauge invariance like in the E&M case, we do not have transversality of the fields; there are three 

