If think I found my mistake: evaluating the previous z_1 integration with the delta function will automatically change the integration limits of the z_2 integration.
Looking at the step before the above integral, where we have the integral over z_1, from z_1=0 to z_1=\infty, we note that...
Homework Statement
We have the evaluation of the integral associated with the electron self-energy diagram and I am following Brian Hatfield's book "Quantum Field Theory of point particles and strings", and I am having problems with the integration limits after changing variables from z_2 to...