Evaluating the price of tiny kinetic mixing

We consider both ``bottom-up'' and ``top-down'' approaches to the origin of gauge kinetic mixing. We focus on the possibilities for obtaining kinetic mixings ε which are consistent with experimental constraints and are much smaller than the naive estimates (ε~10^-2--10^-1) at the one-loop level. In the bottom-up approach, we consider the possible suppression from multiloop processes. Indeed we argue that kinetic mixing through gravity alone, requires at least six loops and could be as large as ~10^-13. In the top-down approach we consider embedding the Standard Model and a U(1)X in a single grand-unified gauge group as well as the mixing between Abelian and non-Abelian gauge sectors.