Electron-positron pair annihilation

[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 $k_2$?

 

[AndyW]

I would label the incoming momentum of the positron as $p^{\mu}$, as it is a common notation in particle physics to use $p^{\mu}$ to represent the four-momentum of a particle. However, it is important to note that this choice is arbitrary and does not affect the physical interpretation of the diagram. Both $p^{\mu}$ and $p^{\nu}$ represent the same quantity, the four-momentum of the positron, and can be used interchangeably in calculations. Therefore, whether we associate the incoming positron with photon $k_1$ or photon $k_2$ does not change the physics of the process being described. It is simply a matter of convention and personal preference.