AMS02 positron excess from decaying fermion DM with local dark gauge symmetry

Positron excess observed by PAMELA, Fermi and AMS02 may be due to dark matter (DM) pair annihilation or decay dominantly into muons. In this paper, we consider a scenario with thermal fermionic DM ($\chi$) with mass $\sim O(1-2)$ TeV decaying into a dark Higgs ($\phi$) and an active neutrino ($\nu_a$) instead of the SM Higgs boson and $\nu_a$. We first present a renormalizable model for this scenario with local dark $U(1)_X$ gauge symmetry, in which the DM $\chi$ can be thermalized by Higgs portal and the gauge kinetic mixing. Assuming the dark Higgs ($\phi$) mass is in the range $2 m_\mu < m_\phi < 2 m_{\pi^0}$, the positron excess can be fit in a natural manner without conflict with constraints from antiproton and gamma ray fluxes or direct detection experiments. Also having such a light dark Higgs, the self-interaction of DM can be enhanced to some extent, and three puzzles in the CDM paradigm can be somewhat relaxed.