Muon Anomaly and Dark Parity Violation

The muon anomalous magnetic moment exhibits a 3.6σ discrepancy between experiment and theory. One explanation requires the existence of a light vector boson, Zd (the dark Z), with mass 10--500 MeV that couples weakly to the electromagnetic current through kinetic mixing. Support for such a solution also comes from astrophysics conjectures regarding the utility of a U(1)d gauge symmetry in the dark matter sector. In that scenario, we show that mass mixing between the Zd and ordinary Z boson introduces a new source of ``dark'' parity violation, which is potentially observable in atomic and polarized electron scattering experiments. Restrictive bounds on the mixing (m_Zd/mZ)δ are found from existing atomic parity violation results, δ²<2×10^-5. Combined with future planned and proposed polarized electron scattering experiments, a sensitivity of δ²~10^-6 is expected to be reached, thereby complementing direct searches for the Zd boson.