Dark-Matter-Enhanced Probe of Relic Neutrino Clustering

TL;DR

This work investigates using ultrahigh-energy (UHE) neutrinos as probes of the cosmic neutrino background (CνB) by leveraging a novel source: heavy neutrinophilic dark matter (DM) decays that produce UHE neutrinos. The authors model off-shell $Z$-mediated neutrino–neutrino scattering with the CνB, propagating fluxes through a cosmological transport equation that accounts for energy loss, absorption, and (subdominant) reinjection, and they include both DM-produced and astrophysical/cosmogenic neutrino components. They quantify IceCube-Gen2 radio’s sensitivity to local CνB overdensities, parameterized by $\xi$, and find that probing overdensities as small as $\xi\sim10^6$ is possible in favorable DM-mass and lifetime regimes (especially when relaxing certain $\gamma$-ray constraints). When the astrophysical and cosmogenic fluxes are included, the sensitivity generally lies in the range $\xi\sim10^8$–$10^{10}$, with the DM-induced component dominating in some parameter spaces. Overall, this approach provides a complementary and potentially powerful avenue to test CνB clustering and to explore the properties of heavy DM, illustrating a bridge between high-energy neutrino astronomy and cosmological neutrino physics.

Abstract

We propose heavy decaying dark matter (DM) as a new probe of the cosmic neutrino background (C$ν$B). Heavy DM, with mass $\gtrsim 10^9$ GeV, decaying into neutrinos can be a new source of ultrahigh-energy (UHE) neutrinos. Including this contribution along with the measured astrophysical and predicted cosmogenic neutrino fluxes, we study the scattering of UHE neutrinos with the C$ν$B via standard weak interactions mediated by the $Z$ boson. We solve the complete neutrino transport equation, taking into account both absorption and reinjection effects, to calculate the expected spectrum of UHE neutrino flux at future neutrino telescopes, such as the IceCube-Gen2 radio. We argue that such observations can be used to probe the C$ν$B properties and, in particular, local C$ν$B clustering. We find that, depending on the absolute neutrino mass and the DM mass and lifetime, a local C$ν$B overdensity $\gtrsim 10^6$ can be probed at the IceCube-Gen2 radio within ten years of data taking.

Figures (5)

• Figure 1: The prompt energy spectra of (anti)neutrinos resulting from DM decay into neutrino-antineutrino pairs summed over all flavors. We present results for three different DM masses: $m_\mathrm{DM}=10^9$ (red), $10^{12}$ (blue), and $10^{15}\;\mathrm{GeV}$ (orange). The spectra are generated using HDMSpectraBauer:2020jay.
• Figure 2: Comparison of neutrino fluxes, summed over all flavors, at Earth without (solid) and with (dashed) scattering off the C$\nu$B cloud with overdensity $\xi=10^{11}$ for normal neutrino mass ordering with lightest neutrino mass fixed at $0.01$ eV. For DM-induced flux component, we show two benchmark points for $m_\text{DM}=10^{11}$ GeV, one that saturates the lifetime constraint from $\gamma$ rays (green) and another one that saturates the constraint from neutrinos (orange) --see \ref{['fig:tauConstraints']}. The gray hatched band illustrates the uncertainty in the prediction of the cosmogenic neutrino fluxes (taken from Ref. Muzio:2023skc), whereas the astrophysical flux was taken from Ref. IceCube:2025ewu. The light yellow band shows the astrophysical plus cosmogenic flux after scattering with the C$\nu$B cloud. The shaded regions in the upper half-plane are the current exclusion limits from Auger PierreAuger:2019ens and IceCube IceCubeCollaborationSS:2025jbi, while the other dot-dashed curves are the sensitivities of future experiments (taken from Ref. Ackermann:2022rqc).
• Figure 3: Left: projected IceCube-Gen2 radio sensitivity at 90% C.L. on the C$\nu$B overdensity $\xi$ as a function of the lightest neutrino mass for two specific values of DM mass. Here we have considered the neutrino flux from DM decay only. Solid (dashed) lines correspond to cases where the DM-induced neutrino flux is computed for the DM parameters saturating the $\gamma$-ray (neutrino) constraints on DM decay. The horizontal purple line is the current 95% C.L. KATRIN upper limit on the local C$\nu$B overdensity KATRIN:2022kkv, and the vertical brown line is the 90% C.L. KATRIN upper limit on the absolute neutrino mass KATRIN:2024cdt. The vertical gray line represents the cosmological upper limit from Planck Planck:2018vyg. Right: projected IceCube-Gen2 radio sensitivity at 90% C.L. on the C$\nu$B overdensity $\xi$ as a function of the DM mass for four specific values of the lightest neutrino mass. Here the DM-induced neutrino flux corresponds to the DM parameters saturating the neutrino constraint.
• Figure 4: Left: projected IceCube-Gen2 radio sensitivity at 90% C.L. on the C$\nu$B overdensity $\xi$ as a function of the lightest neutrino mass, for the case where the neutrino flux includes contributions from both astrophysical sources and DM decay, with the latter computed using both $\gamma$-ray and neutrino constraints. Four benchmark points are shown, corresponding to two values of DM mass and two values of the DM lifetime saturating the constraints. Right: projected IceCube-Gen2 radio sensitivity at 90% C.L. on the C$\nu$B overdensity $\xi$ as a function of the DM mass, for the case where the neutrino flux includes contributions from both astrophysical sources and DM decay, with the latter computed using both $\gamma$-ray and neutrino constraints. Four benchmark points are shown, corresponding to two values of neutrino mass and two values of the DM lifetime saturating the constraints.
• Figure 5: Existing limits in DM mass-lifetime parameter space for neutrinophilic DM. The red shaded region corresponds to the excluded parameter space from the $\gamma$-ray constraints Das:2023wtkKachelriess:2018rtyChianese:2021jkeSarmah:2024ffy that are typically the strongest. These constraints are derived based on the $\gamma$-ray data collected by experiments like Pierre Auger Observatory (PAO), KASCADE and KASCADE-GRANDE. The blue shaded region denotes the parameter space excluded by the $p-\bar{p}$ constraint which is obtained by fitting the $p-\bar{p}$ data at PAO Das:2023wtkSarmah:2024ffy. Finally, the green shaded region represents the excluded parameter space from the nonobservation of UHE neutrinos Esmaili:2012usKachelriess:2018rtyArguelles:2022nbl.