Dark Z implications for parity violation, rare meson decays, and Higgs physics

General consequences of mass mixing between the ordinary Z boson and a relatively light Z(d) boson, the dark Z, arising from a U(1)(d) gauge symmetry, associated with a hidden sector such as dark matter, are examined. New effects beyond kinetic mixing are emphasized. Z-Z(d) mixing introduces a new source of low energy parity violation well explored by possible future atomic parity violation and planned polarized electron scattering experiments. Rare K(B) meson decays into pi(K) l(+)l(-) (l = e, mu) and pi(K) v (v) over bar are found to already place tight constraints on the size of Z-Z(d) mixing. Those sensitivities can be further improved with future dedicated searches at K and B factories as well as binned studies of existing data. Z-Z(d) mixing can also lead to the Higgs decay H -> ZZ(d), followed by Z -> l(1)(+)l(1)(-) and Zd -> l(2)(+)l(2)(-) or missing energy, providing a potential hidden sector discovery channel at the Large Hadron Collider. An illustrative realization of these effects in a 2 Higgs doublet model is presented.