Hi I'm studying electron-muon scattering and now considering the Lorentz invariant integration measure. The textbook introduced it, which use dirac delta function to show that d3p/E is a Lorentz scalar. I understood it but I wanted to find other way and tried like this: I need a hint on the underlined equation. The primed one is lorentz transformed one. How can I show that d(p^2) is Lorentz invariant? Just calculate it hard ?