I have some problems with this integral $$ I=\int_{0}^{1}z(1-z)log(1-z(1-z)\frac{q^2}{m^2})dz $$ I see $z(1-z)$ get max value at $\frac{1}{4}$ and if $q^2>4m^2$ log function will be negative and not be defined. That's why I have branch cut for $q>2m$ but what I don't understand is my textbook come up with the imaginary part of the integral $$ Im(I(q^2))=±\pi\int_{1/2+1/2\beta}^{1/2-1/2\beta}z(1-z)dz $$ where $\beta = \sqrt{1-\frac{4m^2}{q^2}}$
My questions are:
1.How can we neglect real term? I think we do not, but it does not contribute anything. Can you show it pls?
2.How is the imaginary part of the log function $±\pi$. Can someone show it too?