Dark matter decaying into millicharged particles as a solution to AMS-02 positron excess

The positron excess observed by PAMELA and then confirmed by AMS 02 has intrigued the particle physics community since 2008. Various dark matter decay and annihilation models have been built to explain the excess. However, the bounds from isotropic gamma ray disfavor the canonical dark matter decay scenario. We propose a solution to this excess based on the decay of dark matter particles into intermediate millicharged particles which can be trapped by the galactic magnetic field. The subsequent decay of the millicharged particles to electron positron in our vicinity can explain the excess. Since these particles diffuse out of halo before decay, their contribution to the isotropic gamma ray background is expected to be much smaller than that in canonic dark matter decay scenarios. We show that the model is testable by direct dark matter search experiments.