Cosmogenic neutrinos as probes of new physics

TL;DR

This work assesses how GRAND's radio-based detection of Earth-skimming $\nu_\tau$ can probe Beyond the Standard Model physics using cosmogenic neutrinos. The authors model the cosmogenic flux with a minimal three-parameter astrophysical framework and propagate it through detector response, then analyze three BSM scenarios—neutrino self-interactions, pseudo-Dirac neutrinos, and neutrinos scattering on ultra-light dark matter—each producing distinctive spectral features or flux suppression. They forecast GRAND's sensitivity by profiling over astrophysical uncertainties and show complementary reach to IceCube-Gen2, identifying new regions of parameter space that GRAND can explore (e.g., $g_{\tau\tau}$ in $[10^{-2},10^{-1}]$ for $m_φ$ around $10^8$ eV, $\delta m^2$ in $[10^{-15},10^{-13}]$ eV$^2$, and notable νDM constraints in the heavy mediator regime). These results highlight the potential of cosmogenic neutrinos to test new physics at EeV energies, even with current flux uncertainties, and emphasize the practical significance of combining spectral features with flux-level observations for future high-energy neutrino astronomy.

Abstract

The scattering of extremely energetic cosmic rays with both cosmic microwave background and extragalactic background light, can produce $\mathcal{O}(10^{18} \,{\rm eV})$ neutrinos, known as cosmogenic neutrinos. These neutrinos are the only messengers from the extreme cosmic accelerators that can reveal the origin of the most energetic cosmic rays. Consequently, much effort is being devoted to achieving their detection. In particular, the GRAND project aims to observe the $ν_τ$ and $\bar ν_τ$ components of the cosmogenic neutrino flux in the near future using radio antennas. In this work, we investigate how the detection of cosmogenic neutrinos by GRAND can be used to probe beyond the Standard Model physics. We identify three well-motivated scenarios which induce distinct features in the cosmogenic neutrino spectrum at Earth: neutrino self-interactions mediated by a light scalar ($ν$SI), pseudo-Dirac neutrinos (PD$ν$) and neutrinos scattering on ultra-light Dark Matter ($ν$DM). We show these scenarios can be tested by GRAND, using 10 years of cosmogenic neutrino data, in a region of parameter space complementary to current experiments. For the $ν$SI model,, we find that GRAND can constrain the coupling to $ν_τ$ in the range [$10^{-2},10^{-1}$] for a scalar with mass in the range 0.1 to 1 GeV. For PD$ν$, we find that GRAND is sensitive to sterile-active mass squared splitting in the range [$10^{-15},10^{-13}$] ${\rm eV}^2$. Finally, for the $ν$DM model, assuming a heavy mediator, GRAND can do substantially better than the current limits from other available data. These results rely on the fact that the actual cosmogenic flux is around the corner, not far from the current IceCube limit.

Figures (10)

• Figure 1: Simulated single-flavor cosmogenic neutrino spectra assuming the flavor ratio at the Earth to be $\nu_e:\nu_\mu:\nu_\tau=1:1:1$, for different values of the spectral index $\gamma$, maximum energy $E_{\text{max}}$, source evolution parameter $m$, and in the last panel, besides protons, we consider a fraction $\alpha_S$ of iron. Except for the last panel (bottom-right) all cases are for a pure-proton scenario.
• Figure 2: Direction-averaged effective area (left panel) and effect of the energy resolution in the distribution of events (right panel). The effective area has been taken from Ref. GRAND:2018iaj, while for the energy resolution we demonstrate its effect by means of a mock flux in which a spectral feature (a dip) has been artificially added so as to mimic the possible signatures of the BSM models under consideration.
• Figure 3: On the left panel we present the single-flavor cosmogenic neutrino flux at the Earth for $\gamma=2.5, m=3$ and $E_{\rm max }=250$ EeV without $\nu$SI (blue) and with $\nu$SI for $g_{\tau\tau}=0.03$ and $m_\phi =0.2$ GeV (orange). On the right panel we have the corresponding event distributions at GRAND (10-year exposure).
• Figure 4: On the left panel we show how $L_{\rm eff}$ changes as a function of the redshift $z_p$ of the production point. On the right panel we can see for different values of the source evolution parameter $m=-3$ (dotted green), $m=0$ (dashed orange) and $m=3$ (solid blue) the distribution of events as a function of $L_{\rm eff}$, where we fixed $\gamma = 2.5$ and $E_{\rm max} = 250$ EeV.
• Figure 5: On the left panel we present the single-flavor cosmogenic neutrino flux at the Earth for $\gamma =2.5, m=3$ and $E_{\rm max}= 250$ EeV without PD$\nu$ (blue) and with PD$\nu$ for $\delta m^2 = 10^{-14}$ eV$^{2}$ (orange). On the right panel we have the corresponding event distributions at GRAND (10-year exposure).