No room for minimal monopole dark matter

Abstract

The magnetic monopole of a dark sector has been advocated as an appealing dark matter candidate. We revisit the computation of the monopole abundance $Ω_M$, generated by a thermal phase transition in the minimal 't Hooft-Polyakov model. We explore the three regimes where the phase transition is second order, weakly first order, or supercooled, identifying the parameter space regions where $Ω_M$ can match the observed dark matter abundance. However, the dark sector necessarily contains a stable electrically-charged particle, namely a massive vector boson, with a calculable abundance $Ω_{W'}$. We show that, under minimal assumptions, $Ω_{W'}$ is always far larger than $Ω_M$: dark monopoles cannot constitute a sizeable fraction of dark matter.