Freezing-in dark matter through a heavy invisible $Z'$

TL;DR

This paper explores a new mechanism for dark matter production in a $U(1)'$ extended Standard Model, where heavy fermions generate generalized Chern-Simons couplings that facilitate dark matter creation via a heavy $Z'$ gauge boson, with implications for early universe cosmology.

Contribution

It introduces a novel dark matter production mechanism mediated by GCS couplings in a $U(1)'$ extension, linking anomaly cancellation to dark matter relic abundance.

Findings

GCS couplings enable sufficient DM production at intermediate $Z'$ scales

High reheating temperatures ($T_ h angle 10^{10}$ GeV) are compatible with the scenario

The UV completion involves heavy fermions generating GCS-like terms, affecting relic abundance dependence

Abstract

We demonstrate that in a class of the $U(1)'$ extension of the Standard Model (SM), under which all the Standard Model matter fields are uncharged and the additional neutral gauge boson $Z'$ couples to a set of heavy nonstandard fermions, dark matter (DM) production mediated by $Z'$ can proceed through the generation of generalized Chern-Simons (GCS) couplings. The origin of the GCS terms is intimately connected to the cancellation of gauge anomalies. We show that the DM production cross section triggered by GCS couplings is sufficient even for an intermediate scale $Z'$ . A large range of DM and Z masses is then allowed for reasonably high reheating temperature ($T_\rh \gtrsim 10^{10}~\text{GeV}$). This type of scenario opens up a new paradigm for unified models. We also study the UV completion of such effective field theory constructions, augmenting it by a heavy fermionic spectrum. The latter, when integrated out, generates the GCS-like terms and provides a new portal to the dark sector. The presence of a number of derivative couplings in the GCS-like operators induces a high temperature dependence to the DM production rate. The mechanism has novel consequences and leads to a new reheating dependence of the relic abundance.