Vacuum Magnetic Birefringence Experiment as a probe of Dark Sector

Abstract

Vacuum magnetic birefringence (VMB) is a nonlinear electromagnetic effect predicted by QED. In addition to the effect from QED, the effect also has a possibility to probe the dark sector. The effect predicted by QED is parity conservative, but the effect from the dark sector can induce parity violation. To pursue this possibility, we calculated the effect from the dark sector with the generalized Heisenberg-Euler effective Lagrangian that is applicable to parity-violating theories. Various dark sector models exist, among which the contribution of the dark sector neutrinos to the VMB experiment is examined. The contribution comes from the mixing of photon with the dark sector Z boson and violates parity, thus inducing a parity-violating electromagnetic interaction. If the polarization vector is denoted by $ε(θ_i)$ when its direction is rotated by an angle $θ_i$ from the direction of the applied magnetic field, the change of the polarization from $ε(45^\circ)$ to $ε(-45^\circ)$ and vice versa are examined. The contribution from the dark sector modifies the magnitude of the polarization change, so it can be detected by measuring the magnitude precisely. In addition to the change in the magnitude, the dark sector also induces parity-violating effects. We also propose a new scheme to measure the effect of parity violation directly. By measuring the change of polarization from $ε(0^{\circ})$ to $ε(90^\circ)$, and vice versa, with a ring Fabry-Pérot resonator, one can search for the effect directly. The signal that appears in this scheme is evidence of parity violation from beyond standard model theories.