"Dark" Z implications for Parity Violation, Rare Meson Decays, and Higgs Physics

TL;DR

This paper explores the implications of a light dark Z boson mixing with the standard Z, affecting parity violation, meson decays, and Higgs processes, with potential for discovery in current and future experiments.

Contribution

It introduces new effects beyond kinetic mixing, analyzes constraints from meson decays, and proposes Higgs decay channels as a discovery avenue for the dark Z boson.

Findings

Z-Z_d mixing induces measurable parity violation effects.

Current meson decay data constrains Z-Z_d mixing strength.

Higgs decays to Z Z_d provide potential signals at the LHC.

Abstract

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) nu anti-nu 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 -> Z Z_d, followed by Z -> l_1^+ l_1^- and Z_d -> l_2^+ l_2^- or "missing energy", providing a potential hidden sector discovery channel at the LHC. An illustrative realization of these effects in a 2 Higgs doublet model is presented.