You need a good foundation in quantum field theory to be able to read Elvang and Huang effectively. 'Peskin and Schroeder' and Srednicki are good standard introductions to QFT. If you understood around half of either of those text books then I think you would be in a good position.
QFT itself has many prerequisites; you need to be well versed in special relativity, electromagnetism, classical field theory and quantum mechanics to get a hold on it. Each of these areas have their own mathematical prerequisites, which include algebra and calculus, multivariable calculus, pdes, complex analysis and fourier analysis.
General relativity and string theory are also comparative to QFT in scope, prerequisite knowledge and utility in other areas of mathematical physics. You would need to learn these if you wanted to start reading a bit more broadly in scattering amplitudes or in another part of math phys, but you can go quite a long way in scattering amplitudes without too much of them.
Thank you. My formal physics education ends with QM, so that seems very much within my reach of intellectual curiosity.
As someone who has taught myself significantly more than formal education could, I know how valuable it is to have this sort of knowledge trail accessible. Much appreciated.
