Cosmological bounds on the "millicharges" of mirror particles

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.