Circumscribing Late Dark Matter Decays Model Independently

TL;DR

This work derives model-independent constraints on late-decaying dark matter that produce monoenergetic photons, by combining gamma-ray line limits from the Galactic Center with measurements of the isotropic diffuse photon background. The analysis expresses the decay signal in terms of the key plane $m_\chi \tau$ versus $\varepsilon$, showing that GC line limits set a jagged lower bound while the iDPB imposes an upper exclusion region; together they rule out substantial contributions from such decays to the MeV background. Through representative models—sterile neutrinos, mUED-like decays, and gravitino scenarios—the authors illustrate how the GC and iDPB constraints translate into strict limits on the energy carried by photons and the corresponding lifetimes, often forcing $\Delta m$ and $\varepsilon$ to be very small. The results imply that late decays cannot easily explain the iDPB in the MeV range, emphasizing the robustness of gamma-ray observations in probing particle-cosmology connections and guiding future high-resolution gamma-ray experiments.

Abstract

A number of theories, spanning a wide range of mass scales, predict dark matter candidates that have lifetimes much longer than the age of the universe, yet may produce a significant flux of gamma rays in their decays today. We constrain such late decaying dark matter scenarios model-independently by utilizing gamma-ray line emission limits from the Galactic Center region obtained with the SPI spectrometer on INTEGRAL, and the determination of the isotropic diffuse photon background by SPI, COMPTEL and EGRET observations. We show that no more than ~5% of the unexplained MeV background can be produced by late dark matter decays either in the Galactic halo or cosmological sources.

Figures (4)

• Figure 1: Model-independent constraints on the product of mass and lifetime, $m_\chi \tau$, versus the energy carried away by the monochromatic photon emission, $\varepsilon$, for a generic late-decaying dark matter model: $\chi \rightarrow \chi' + \gamma$. Regions excluded by either the gamma-ray line emission limits from the Galactic Center region or overproduction of the isotropic diffuse photon background are shown, together with preferred ranges of parameters from three well-studied models.
• Figure 2: Top: Limits on the diffuse gamma-ray line emission from the Galactic Center region (an angular region within a $13^{\circ}$ radius) as adopted from Ref. Teegarden. Bottom: Representative measurements of the diffuse photon background from SPI Churazov:2006bk, COMPTEL Weidenspointner and EGRET Strong:2004ry in the energy range around 0.01 MeV--100 GeV. The thick solid line, summarizing the overall trend of the data, is to be compared to predictions of decaying dark matter scenarios.
• Figure 3: Photon spectrum from isotropic Galactic halo decays (dotted line) for $\varepsilon=1$ MeV, with $m_\chi \tau \simeq 7 \times 10^{24}$ GeV s chosen from Fig. \ref{['fig:constraints']} such that the line emission bounds from the Galactic Center region are saturated. Also displayed are the spectra from cosmological decays (dashed line) and the total spectrum (solid line), which falls well below the isotropic diffuse photon background (thick solid line).
• Figure 4: Similar to Fig. \ref{['fig:constraints']}, focusing upon the MeV range of $\varepsilon$. The contribution of late dark matter decays to the isotropic diffuse photon background is 10% or more in the diagonal band. SUSY and mUED inspired decaying dark matter models of Ref. Cembranos:2007fj cannot make significant contribution to the iDPB while abiding by the gamma-ray line emission limits from the Galactic Center region.