  We develop a phenomenological formalism for mixing effects between the
Standard Model and hidden-sector fields, motivated by dark matter in the
Universe as well as string theories. The scheme includes multiple Higgs-portal
interactions in the scalar sector as well as multiple gauge-kinetic mixings in
the abelian gauge sector. While some of the mixing effects can be cast in
closed form, other elements can be controlled analytically only by means of
perturbative expansions in the ratio of standard scales over large hidden
scales. Higgs and vector-boson masses and mixings are illustrated numerically
for characteristic processes.
