Cosmological bounds on the "millicharges" of mirror particles

TL;DR

This paper revises cosmological constraints on the kinetic mixing parameter between ordinary and mirror photons, establishing upper limits on 'millicharges' of mirror particles, which impacts their viability as dark matter candidates.

Contribution

It provides updated cosmological bounds on the kinetic mixing parameter, especially under the assumption of exact and asymmetric mirror parity.

Findings

Upper limit of ε < 3 x 10^{-10} for exact mirror parity

Weaker bounds for asymmetric mirror sectors with higher electroweak scale

Mirror particles can acquire nanocharges affecting dark matter models

Abstract

Mirror world, a parallel hidden sector with microphysics identical to ordinary particle physics, can have several interesting phenomenological and astrophysical implications and mirror matter can be a natural candidate for dark matter in the universe. If the ordinary and the mirror photons have a kinetic mixing due to the Lagrangian term $(\epsilon/2) F_{\mu\nu} F'^{\mu\nu}$, then mirror particles effectively acquire the electric charges $\sim \epsilon$ with respect to the ordinary photon, so that they become a sort of particles historically coined as "millicharged" though nowadays they must be called more appropriately as "nanocharged". In this paper we revise the cosmological bounds on the kinetic mixing parameter and in the case of exact mirror parity set an upper limit $\epsilon < 3 \times 10^{-10}$. Much weaker limit can be obtained in the case of asymmetric mirror sector, with an electroweak symmetry breaking scale larger than the ordinary electroweak scale.