Recent content by phylz

Thank you. ##1.## Now I understand that the ##t-## and ##u-## channel diagrams(tree level diagrams) have no branch cut singularities for ##s>4m^2##. But at the loop level, why are the ##t-## and ##u-## diagrams proportional to ##\sqrt{1-4m^2/q^2}## ? ##2.## Yeah, it is better. ##3.## Yes, you...

Thank you. ##1.## You mean that the Mandelstam variables ##t## and ##u## correspond to the branch cut singularities of ##t##- and ##u##-channel diagrams respectively? Why? ##2.## I can't understand your last sentence. ##3.## ##t=-2\mathbf p^2 (1- \cos \theta)^2, \,\,\,\text{} \,\,\,\,u =...

In Peskin's textbook section 7.3 The Optical Theorem for Feynman Diagrams(Page233), he said it is easy to check that the corresponding t- and u-channel diagrams have no branch cut singularities for s above threshold. But I can't figure out how to prove it. Can angone help me? Thanks!

@DarMM thank you. Now I got it, for electron, $Z_2^(-1)=1-\frac{d\Sigma}{d\slashed{p}}|_{\slashed{p}=m}$, at the leading-order contribution, electron self-energy is 0, so Z=1.

Z is the field-strength renormalization factor

In Peskin's textbook chapter 7 Radiative Corrections: Some formal developments (page 229), he said the Z factors are irrelevant for calculations at the leading order of perturbation theory, but are important in the calculation or higher-order corrections. My question is how can the Z factor be...