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I'm studying ## \mu \to e~ \gamma ## decay from cheng & Lie' book " gauge theory of elementary particles ". In Equation (13.84), he wrote the neutrino propagator

## \sum_i \Big ( \frac{U^{*}_{ei} U_{\mu i}}{(p+k)^2-m_i^2} \Big), ##

(where the sum taken over neutrinos flavors) in the form:

##\sum_i U^{*}_{ei} U_{\mu i} \Big ( \frac{1}{(p+k)^2} + \frac{m_i^2}{[(p+k)^2]^2} + ...... \Big) ##

##= \sum_i \frac{U^{*}_{ei} U_{\mu i} m_i^2}{[(p+k)^2]^2} + ........##

Do any one know how did he drive this ? Then he write that:

the leading term vanishes, ## \sum_i U^{*}_{ei} U_{\mu i}##, reflecting the GIM cancellation mechanism.

I can't get this statement ..

Thanks,

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# I ## \mu~\to e~ \gamma ## decay width and neutrino propagator

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