Leak-in Dark Matter

TL;DR

Leak-in dark matter proposes a new out-of-equilibrium mechanism where a hidden sector is populated through a feeble, non-adiabatic interaction with the standard model, leading to viable relic abundance and observable signals.

Contribution

This paper introduces the leak-in mechanism for dark matter production, analyzing its properties, and demonstrates its viability in a B-L vector portal model with observable consequences.

Findings

Leak-in leads to dark matter relic abundance independent of initial conditions.

Small SM couplings in leak-in models can produce detectable experimental signals.

Leak-in dark matter can have self-interactions addressing small-scale structure issues.

Abstract

We introduce leak-in dark matter, a novel out-of-equilibrium origin for the dark matter (DM) in the universe. We provide a comprehensive and unified discussion of a minimal, internally-thermalized, hidden sector populated from an out-of-equilibrium, feeble connection to the hotter standard model (SM) sector. We emphasize that when this out-of-equilibrium interaction is renormalizable, the colder sector undergoes an extended phase of non-adiabatic evolution largely independent of initial conditions, which we dub "leak-in." We discuss the leak-in phase in generality, and establish the general properties of dark matter that freezes out from a radiation bath undergoing such a leak-in phase. As a concrete example, we consider a model where the SM has an out-of-equilibrium B-L vector portal interaction with a minimal hidden sector. We discuss the interplay between leak-in and freezein processes in this theory in detail and demonstrate regions where leak-in yields the full relic abundance. We study observational prospects for B-L vector portal leak-in DM, and find that despite the requisite small coupling to the SM, a variety of experiments can serve as sensitive probes of leak-in dark matter. Additionally, regions allowed by all current constraints yield DM with self-interactions large enough to address small-scale structure anomalies.