# A Electron-positron pair annihilation

1. Nov 6, 2016

### spaghetti3451

Consider the following u-channel diagram (on the right) in the link below.

https://www.physicsforums.com/attachments/diphoton-png.95147/

Let the polarization vectors of $k_1$ and $k_2$ be $(\epsilon^{\mu}_{1})^{*}$ and $(\epsilon^{\mu}_{2})^{*}$ respectively.

Do you label the incoming momentum of the positron as $p^{\mu}$ or $p^{\nu}$? In other words, do you associate the incoming positron with the photon $k_1$ or photon $k_2$?

2. Nov 11, 2016