The Price of Tiny Kinetic Mixing

Abstract

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 ($ϵ\sim 10^{-2}-10^{-1}$) at the one-loop level. In the bottom-up approach, we consider the possible suppression from multi-loop processes. Indeed we argue that kinetic mixing through gravity alone, requires at least six loops and could be as large as $\sim 10^{-13}$. 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.

Comments: 20 pages + references; v2: added references, expanded discussion of non-Abelian dark sector; matches published version except for the title