Hidden sector monopole, vector dark matter and dark radiation with Higgs portal

Abstract

We show that the 't Hooft-Polyakov monopole model in the hidden sector with\nHiggs portal interaction makes a viable dark matter model, where monopole and\nmassive vector dark matter (VDM) are stable due to topological conservation and\nthe unbroken subgroup $U(1)_X$. We show that, even though observed CMB data\nrequires the dark gauge coupling to be quite small, a right amount of VDM\nthermal relic can be obtained via $s$-channel resonant annihilation for the\nmass of VDM close to or smaller than the half of SM higgs mass, thanks to Higgs\nportal interaction. Monopole relic density turns out to be several orders of\nmagnitude smaller than observed dark matter relic density. Direct detection\nexperiments, particularly, the projected XENON1T experiment, may probe the\nparameter space where the dark Higgs is lighter than $\\lesssim 60 {\\rm GeV}$.\nIn addition, the dark photon associated with unbroken $U(1)_X$ contributes to\nthe radiation energy density at present, giving $\\Delta N_{\\rm eff}^\\nu \\sim\n0.1$ as the extra relativistic neutrino species.\n