Forbidden dark matter at the weak scale via the top portal

Abstract

At the tail of its velocity distribution, cold dark matter (DM) can annihilate at finite temperature to states heavier than itself. We explore the possibility that DM freeze-out is dictated by these ``forbidden annihilations'' at the electroweak scale. Demanding that annihilation products be Standard Model particles, we find that for the forbidden mechanism to primarily set the DM relic abundance, DM must couple predominantly, if not solely, to top quarks. This can be arranged by invoking a nontrivial flavor structure such as minimal flavor violation. We avail two avenues to achieve the correct thermal cross section, requiring a mediator exchanged in the $s$- or $t$-channel. These simplified models submit easily to direct detection and collider searches, and necessarily hide from indirect detection signals. Viable supersymmetric spectra involving the forbidden mechanism may be found if combined with coannihilation.