Dark Matter Freeze-In with a Heavy Mediator: Beyond the EFT Approach

TL;DR

This paper investigates the limitations of the effective field theory approach in dark matter freeze-in models with heavy mediators, emphasizing the importance of including mediator effects for accurate predictions and collider phenomenology.

Contribution

It demonstrates the breakdown of EFT in certain mass regimes and discusses the implications for collider searches and model testing in heavy mediator freeze-in scenarios.

Findings

EFT breaks down when mediator mass is 10-100 times TRH.

Resonance and decay effects are significant in these regimes.

Collider signals differ qualitatively from standard freeze-in models.

Abstract

We study dark matter freeze-in scenarios where the mass of the mediator particle that couples dark matter to the Standard Model is larger than the reheat temperature, TRH, in the early Universe. In such setups, the standard approach is to work with an effective field theory (EFT) where the mediator is integrated out. We examine the validity of this approach in various generic s- and t-channel mediator frameworks. We find that the EFT approach breaks down when the mediator mass is between one to two orders of magnitude larger than TRH due to various effects such as s-channel resonance, a small thermally-suppressed abundance of the mediator, or decays of Standard Model particles through loops induced by the mediator. This highlights the necessity of including these contributions in such dark matter freeze-in studies. We also discuss the collider phenomenology of the heavy mediators, which is qualitatively different from standard freeze-in scenarios. We highlight that, due to the low TRH, the Standard Model-dark matter coupling in these scenarios can be relatively larger than in standard freeze-in scenarios, improving the testability prospects of these setups.