Confusion about the Z factor(Renormalization factor)

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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 irrelevant for calculations at the leading order of perturbation theory. Thanks.

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 irrelevant for calculations at the leading order of perturbation theory. Thanks.

Z is the field-strength renormalization factor

DarMM
@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.