Reading a 2 flavor neutrino oscillation derivation I saw the following approximation being made regarding energy and momentum: E-p ≃ m²/2E
I can't see how this step is taken, and what is being assumed. I would appreciate it if someone could help me understand this.
The approximation is made on page 3-4 of this pdf: http://www.hep.phy.cam.ac.uk/~thomson/partIIIparticles/handouts/Handout_11_2011.pdf
The same approximation is stated at wikipedia: https://en.wikipedia.org/wiki/Ultrarelativistic_limit
But neither explains how it is done.