Phases of Dark Matter from Inverse Decays (2504.16981v1)

Abstract: Inverse decays are an interesting avenue for producing dark matter in the early universe. We study in detail various phases of dark matter parameter space where inverse decays control its abundance, expanding on our work of INDY dark matter and going beyond. The role of initial conditions and the impact of departure from kinetic equilibrium are investigated as well. We show how these inverse decay phases can arise in theories of a kinetically mixed dark photon and dark Higgs, with promising prospects for detection at upcoming experiments.

Phases of Dark Matter from Inverse Decays

The investigation of dark matter (DM) remains a central pursuit within astrophysics and cosmology due to its fundamental role in shaping the universe. The paper "Phases of Dark Matter from Inverse Decays" provides a comprehensive exploration into a novel approach for determining the DM relic abundance via inverse decays, expanding on the concept of 'INDY' (INverse DecaY) dark matter. This paper provides a detailed classification of the phases that DM can undergo and addresses various theoretical and practical implications.

Abstract and Mechanism Overview

The manuscript begins by analyzing inverse decays as a mechanism for the production of DM in the early universe. It extends previous work on INDY dark matter by exploring parameter spaces where inverse decays primarily dictate DM abundance. The processes studied are based on decays and inverse decays of a dark matter candidate (denoted as $\chi$) from an unstable particle ($\psi$), along with $\psi$'s self-annihilation into standard model (SM)-linked particles.

Theoretical Framework

The analysis involves detailed exploration of the Boltzmann equations governing the number densities of particles, considering both annihilations and decays. The equations are parameterized using decay width and cross-section values, leading to diverse 'phases' of the DM abundance scenario, driven by varying decay and annihilation coupling constants. Two key dimensions are examined: the coupling strength and the mass splitting between $\chi$ and $\psi$.

Key Phases and Their Implications

1. Coannihilating via Decays: In this phase observed at high decay coupling values, the annihilation of $\psi$ predominantly sets the relic abundance of $\chi$, keeping the two species in chemical equilibrium (CE). This scenario mimics conventional coannihilation processes, albeit, reaching equilibrium through decays and inverse decays rather than direct annihilations.
2. INDY Dark Matter Phases: For significant annihilation coupling values, where annihilations happen frequently enough to ensure $\psi$ stays in equilibrium, two sub-cases of INDY DM arise:

- In Chemical Equilibrium (CE): The DM abundance is largely replicated through $\psi$ decays. The decay coupling adheres to a power-law relationship with respect to the mass scale.

- Out of Chemical Equilibrium (NCE): Here, the initial conditions vastly influence the observed abundance. For small mass splittings, the inverse decay coupling and thus the $\chi$ relic density are sensitive to initial $\chi$ distributions.

1. Freeze-In (FI) and Freeze-In-Freeze-Out (FIFO): Examined under the assumption that DM initially has an insignificant abundance. Analyzing different couplings highlights alternative scenarios where either $\chi$ is never in CE, or initially is, then drops out as decays slow.
2. Freezeout and Decay: Similar to classic WIMP (Weakly Interacting Massive Particle) scenarios but pertinent to conditions where only subsequent decays of a frozen-out particle contribute to the DM population.

Model Implementation and Predictions

The paper proposes a minimal renormalizable model that implements these dynamics through a vector mediator (dark photon) connecting the dark sector to the SM via kinetic mixing. The parameter space supports wide-ranging masses and couplings, constrained by cosmological and terrestrial observations. While typical collider searches might not detect $\chi$ due to weak interactions, the model's dark photon could be detected in future experimental setups exploring weak force interactions.

Conclusions

This comprehensive investigation into DM production mechanisms via inverse decays opens up rich zones of parameter space previously unexplored. The results further imply new potential paths for detecting dark sector particles, urging future empirical studies to concentrate on the proposed light mediator scenarios. The insights also align with and support the viability of non-WIMP cosmos possibilities for achieving precise cosmological models, motivating existing and next-generation dark matter detection experiments.

These explorations invite substantial future work, particularly in model-specific predictions and constraints, offering fertile ground for experimental verification and theoretical advancements in DM research.