# Peskin and Schroeder - page 4 - choice of notation

1. Jan 21, 2016

### spaghetti3451

In page 4, Peskin and Schroeder has the following diagram:

The diagram shows the collision of an electron beam and a positron beam to produce a $\mu^{+}$ beam a $\mu^{-}$ beam.

My question is this:

The electron and positron beams are shown to have momenta $\textbf{p}$ and $\textbf{p}'$.

But the muon beams are shown to have momenta $\textbf{k}$ and $\textbf{k}'$.

$\textbf{p}$ is standard notation for momentum, but $\textbf{k}$ is standard notation for wavevector. But then, in natural units, $\hbar=1$.

Therefore, is the different use of symbols for momenta a matter of notational difference, or is there some underlying physics I am missing?

2. Jan 21, 2016

### Orodruin

Staff Emeritus
You can call things whatever you like as long as you define what is meant. Throughout PS you will notice p, k, and q being used to denote different momenta (and more importantly, 4-momenta) simply for the reason that the others are being used to denote something else in the particular context.

3. Jan 21, 2016

Thanks! :)