One-loop flavor changing electromagnetic transitions

We discuss the effect of the external fermion masses in the flavor-changing radiative transitions of a heavy fermion (quark or lepton) to a lighter fermion at the one-loop level, and point out an often overlooked crucial difference in the sign of a charge factor between transitions of the down type $s\to dγ$ and the up type $c\to uγ$. We give formulas for the $F\to fγ$ effective vertex in various approximations and the exact formula for $t\to cγ$ and $τ\to μγ$.

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