Probing the Phenomenology of Dark Matter from Decoupled Freeze-Out

TL;DR

The paper investigates a dark matter scenario in which a scalar-pseudoscalar mediator structure enables decoupled freeze-out (DFO): the dark sector thermalizes at a temperature $T^\\\\\\\\prime$ different from the SM bath, while SM–dark-sector couplings remain feeble. Relic density is computed from a four-dimensional Boltzmann system that includes hidden-sector energy transfer, with the dominant annihilation channel $\\chi\\bar{\\chi} \\to a\\phi$ proceeding via $s$-wave, yielding potential indirect-detection signals. The study combines CMB anisotropy constraints, gamma-ray and radio indirect-detection limits, and BBN bounds to map the viable parameter space for mediator masses in the sub-TeV range; although many regions are excluded, a non-negligible region remains viable. This work highlights DFO as a robust alternative to secluded freeze-out, with distinctive multi-messenger signatures and prospects for future observations by CMB-S4, e-ASTROGAM, and SKA.

Abstract

We consider a model of dark matter where the mediator corresponds to a superposition of a scalar and pseudoscalar, and the scenario where, after reheating, the number densities of the dark sector particles, i.e. the dark matter and the mediators, are negligible. If the coupling of the mediators to the Standard Model is feeble, but the coupling to the dark matter is large enough, the dark sector may reach equilibrium at a temperature distinct from that of the thermal bath. The relic density is then said to be obtained via decoupled freeze out (DFO). We focus on the $s$-wave annihilation scenario, which particularly benefits from the DFO mechanism by evading standard CMB limits while still yielding indirect detection signals. We calculate the relic density by solving a set of four coupled Boltzmann equations for the number densities of the dark sector particles and the energy transfer from the light to dark sector. We finally perform a thorough analysis of experimental bounds on this scenario, namely from indirect detection and the CMB, as well as from BBN, and find that, while there are considerable constraints on the parameter space where the correct relic density is obtained, a viable region remains to be explored.

Figures (3)

• Figure 1: Integrated collision term for a selection of the most dominant processes $i\; j \rightarrow a \; k$ (left) and $i\; j \rightarrow \phi \; k$ (right) for the energy transfer Boltzmann equation \ref{['eq:Entransfer']}, where $i,j,k$ denote SM particles, as a function of the photon temperature $T$. We set $g_{af}=g_{\phi f}=1$ GeV$^{-1}$.
• Figure 2: Decoupled freeze-out region in the $(m_\chi,g_{a(\phi)\chi})$-plane for the three mediator masses $m_a=m_\phi= \{0.25, 3,30\}$ GeV, together with constraints from CMB, indirect detection and BBN. Black contour lines show the values for the SM-mediator couplings $g_{af}=g_{\phi f}$ that yield the observed DM relic density; the contour labels indicate $\log_{10}(g_{a(\phi)f})$. In the khaki region, there is an underproduction of DM while in the grey region the dark sector and the SM sector equilibrate. The upper black dashed line corresponds to the combination of couplings in the secluded scenario. Shaded regions show excluded parameter space, as detailed in the main text: CMB power spectrum (red), BBN (turquoise), and indirect detection constraints—including Fermi-LAT (green) and radio emission bounds (blue). For the radio constraints, the solid and dashed lines denote the upper error band and central value, respectively. The BBN exclusion contours shown correspond to a fixed value of $g_{\phi (a)f}$; they are largely insensitive to the value of $g_{\phi(a)\chi}$. Thus, each exclusion line represents the BBN bound associated with that specific $g_{\phi (a)f}$ contour. The white region represents allowed parameter space for decoupled freeze out.
• Figure 3: Constraints on the parameter space of the secluded FO scenario with $m_a=m_\phi=3\ {\rm GeV}$ and $g_{aq}=g_{\phi q}=10^{-6}$. The direct detection limit uses a combination of PandaXPandaX-4T:2021bab and LZ 2024 LZ:2024zvo.