Neutrino lines and photon continua from cascade dark matter decay

Technical Summary: Neutrino Lines and Photon Continua from Cascade Dark Matter Decay

Problem Statement
The paper investigates the indirect detection signatures of sub-GeV decaying dark matter (DDM) where the decay proceeds via a two-body cascade: $\chi \to X + \nu$, followed by the decay of a light mediator $X$ into Standard Model photons ($X \to \gamma\gamma$ or $X \to 3\gamma$). A central challenge in this scenario is the "MeV gap" in gamma-ray observations and the potential suppression of photon signals if the mediator $X$ is long-lived. If $X$ has a macroscopic decay length, secondary photons may not trace the original dark matter distribution, weakening constraints from gamma-ray telescopes. Conversely, the neutrino produced in the primary decay is prompt and propagates without attenuation, offering a monochromatic line signature. The authors aim to systematically compare the sensitivity of neutrino-line searches against photon-continuum searches across different mediator lifetimes and model parameters.

Methodology
The authors analyze two benchmark models for the mediator $X$:

1. Axion-Like Particle (ALP): A scalar mediator decaying via $a \to \gamma\gamma$.
2. Kinetically Mixed Dark Vector: A vector mediator ($A'$) with mass $m_{A'} < 2m_e$, decaying via the loop-induced process $A' \to 3\gamma$.

The analysis incorporates the following technical components:

• Kinematics and Spectra: The neutrino energy is fixed by two-body kinematics ($E_\nu = \frac{m_\chi}{2}(1 - 1/k^2)$, where $k = m_\chi/m_X$). The photon spectra are derived from the boosted decay of the mediator. The ALP yields a "box-like" spectrum, while the dark vector yields a smoother continuum due to the three-body phase space.
• Propagation Effects: The study explicitly models the suppression of the photon signal based on the mediator's decay length ($\lambda_X = \beta_X \gamma_X \tau_X$).
  • Galactic Component: A "Galactic-collinear" benchmark is adopted, assuming the mediator travels along the line of sight. The flux is weighted by the probability of decay within the region of interest (ROI).
  • Extragalactic (EG) Component: A "strict delayed" EG component is calculated, accounting for the fact that mediators produced at high redshift ($z_p$) may decay at a lower redshift ($z_d$). This component scales linearly with the mediator width ($\Gamma_X$) in the long-lived limit.
• Constraints:
  • Gamma-rays: Constraints are derived from current data (COMPTEL, INTEGRAL/SPI) and projected sensitivities (AMEGO-X) using a conservative, background-agnostic recasting method.
  • Neutrinos: Limits are derived from the monochromatic neutrino line using data and projections from Borexino, Super-Kamiokande, JUNO, and Hyper-Kamiokande.
• Robustness Checks: The authors validate their results against two conservative scenarios: (1) using only the EG component (removing the Galactic collinear approximation) and (2) applying a simplified spatial-smearing treatment to account for non-local photon redistribution in the Galactic halo.

Key Contributions and Results

• Neutrino-Dominated Regions: The primary result is the identification of parameter regions where neutrino-line searches provide the leading constraints on the dark matter lifetime ($\tau_\chi$), surpassing gamma-ray limits. This occurs specifically when the mediator is long-lived.
• Mechanism of Dominance:
  • For short-lived mediators, photons are produced promptly, and gamma-ray constraints dominate.
  • As the mediator lifetime increases (due to smaller couplings or higher boosts), the Galactic photon flux is suppressed because mediators decay outside the ROI. While the EG component also scales with the width, it can become comparable to or dominate the Galactic component in the long-lived limit, yet the combined photon sensitivity generally weakens relative to the neutrino channel.
  • The neutrino line remains robust because it is produced at the primary vertex and is insensitive to the subsequent propagation or decay of $X$.
• Dependence on Parameters:
  • ALP Model: Increasing the mass ratio $k$ (making the ALP lighter and more boosted) or decreasing the coupling $g_{a\gamma\gamma}$ increases the decay length, expanding the neutrino-dominated region.
  • Dark Vector Model: Decreasing the kinetic mixing $\epsilon$ or the mediator mass $m_{A'}$ (which suppresses the loop-induced width $\Gamma_{A' \to 3\gamma} \propto \epsilon^2 m_{A'}^9$) similarly favors neutrino dominance.
• Robustness: The classification of "neutrino-dominated" vs. "photon-dominated" regions remains stable even when the Galactic-collinear approximation is removed (using EG-only limits) or when spatial smearing is applied. The spatial smearing analysis shows that while it can deplete compact ROIs (like the Galactic Center) or enhance wider ROIs, it does not qualitatively alter the dominance boundaries.

Significance
The paper claims that neutrino telescopes offer a unique and often leading probe for cascade dark matter decay scenarios involving long-lived mediators. While gamma-ray searches are powerful for prompt decays, they are susceptible to signal suppression when the mediator propagates macroscopic distances. In contrast, the monochromatic neutrino line provides a mediator-lifetime-independent constraint on the primary decay vertex. The authors conclude that future neutrino experiments, particularly JUNO and Hyper-Kamiokande, will be crucial for testing sub-GeV cascade decays in the long-lived mediator regime, providing a complementary and sometimes superior test to next-generation MeV gamma-ray missions like AMEGO-X.