Cosmologically varying kinetic mixing

Abstract

The portal connecting the invisible and visible sectors is one of the most natural explanations of the dark world. However, the early-time dark matter production via the portal faces extremely stringent late-time constraints. To solve such tension, we construct the scalar-controlled kinetic mixing varying with the ultralight CP-even scalar’s cosmological evolution. To realize this and eliminate the constant mixing, we couple the ultralight scalar within 10−33eV ≲ m0 ≪ eV with the heavy doubly charged messengers and impose the ℤ2 symmetry under the dark charge conjugation. Via the varying mixing, the keV – MeV dark photon dark matter is produced through the early-time freeze-in when the scalar is misaligned from the origin and free from the late-time exclusions when the scalar does the damped oscillation and dynamically sets the kinetic mixing. We also find that the scalar-photon coupling emerges from the underlying physics, which changes the cosmological history and provides the experimental targets based on the fine-structure constant variation and the equivalence principle violation. To ensure the scalar naturalness, we discretely re-establish the broken shift symmetry by embedding the minimal model into the ℤN-protected model. When N ~ 10, the scalar’s mass quantum correction can be suppressed much below 10−33eV.

Sub-GeV millicharge dark matter from the U(1) X hidden sector

“…While the freeze-in mechanism demands a feeble coupling between the SM and the dark matter particle, posing a puzzle that why such tiny coupling might emerge in the theory. The kinetic mixing precisely provide such smallness for the coupling constant between the SM and the hidden sector [43][44][45][46][47][48][49][50][51][52][53].…”