Impostor Among $ν$s: Dark Radiation Masquerading as Self-Interacting Neutrinos

Abstract

Multiple cosmological observations hint at neutrino self-interactions beyond the Standard Model, yet such interactions face severe constraints from terrestrial experiments. We resolve this tension by introducing a model where active neutrinos resonantly convert to self-interacting dark radiation after BBN but before CMB epoch. This exploits the fact that cosmological observables cannot distinguish between neutrinos and dark radiation with the same abundance and free-streaming properties. Our mechanism, based on a simple type-I seesaw framework along with a keV-scale scalar mediator, achieves two objectives: (i) it produces strongly self-interacting dark radiation that imitates neutrino self-interactions favored by cosmological data, and (ii) it depletes the active neutrino energy density, relaxing cosmological neutrino mass bounds and easing the tension with neutrino oscillation data. The model naturally evades laboratory constraints through suppression of the neutrino-mediator coupling by the squared mass ratio of active and sterile neutrinos. We show that this scenario is favored over $Λ$CDM by the combined Planck and DESI data, while being consistent with all other constraints. Our mechanism is testable in future laboratory probes of absolute neutrino mass and searches for sterile neutrinos.

Figures (11)

• Figure 1: Evolution of interaction rates (top) and temperatures (bottom) for a benchmark point with degenerate neutrino masses. The resonant $\nu-\chi$ conversion occurs when $T_\nu\sim m_\phi$, leading to efficient neutrino cooling (i.e. final $N_{\rm eff}^\nu=2.28$, $N_{\rm eff}^{\rm tot}=3.04$).
• Figure 2: $\left(\,\sum m_\nu,\,m_\phi\,\right)$ plane for one $(\textbf{Left Panel})$ and two $(\textbf{Right Panel})$ flavors of DR with $\lambda_{\phi\chi}=0.003$, $M_N/\lambda_{\phi N}= 100$ MeV assuming degenerate $m_\nu$. The colored contours show $N_{\rm eff}^\nu$ at $T_\gamma\sim 10^{-4}$ MeV. The upper right corner (gray hatched) is excluded by BBN where $\Delta N_{\rm eff}^{\rm tot} > 0.4$. Terrestrial neutrino mass bounds from $\nu$ oscillation (black dashed) and KATRIN (black dot-dashed) are overlaid. Magenta contours show the marginalized $2\sigma$ upper limit on $\sum m_\nu$ as a function of $G_{\rm eff}$ from Fig. \ref{['fig:mcmc']}. $1\sigma$ cosmological preferred SI and MI modes from Table \ref{['tab:numbers']} are shown in red bands in the region not excluded by $\nu$ oscillation and cosmology. The red star is the benchmark point used in Fig. \ref{['fig:benchmark']}.
• Figure 3: 1D marginalized posterior and 2D marginalized contours for $\log_{10} G_{\rm eff}$ and $\sum m_\nu$ with Planck + DESI dataset for $0.75 N_\chi$ and $1.2 N_\chi$ scenarios. The darker and lighter bands in the 2D plot represent $1\sigma$ and $2\sigma$ allowed regions, respectively.
• Figure S1: $N_{\rm eff}$ [Eq. \ref{['eq:N_eff']}] and $N_{\rm eff}^\nu$ as functions of $N_\chi$ after $\phi$ decouples.
• Figure S2: Interaction rates over Hubble for dominant processes that keep the active neutrinos and the sterile neutrinos (or HNLs) in the thermal bath with the SM particles. The production of HNL via $\nu_l \nu_l\to \nu_l \nu_h$ (blue), $e \nu_l\to e \nu_h$ (green), and $e e \to \nu_l \nu_h$ (magenta) only depend on the mixing angle between active neutrinos and sterile neutrinos; hence, their rates scale down by $\sim m_{\nu_l}/M_{N}$ with respect to the SM $e \nu_l\to e \nu_l$ (black). HNL pair production via $Z/\phi$ as mediators (orange) has $\Gamma/H \lesssim 10^{-3}$ at all times. Therefore, requiring $\Gamma/H < 1$ for HNL production at all times implies a low-reheating temperature of $T_{\rm rh}\lesssim 1$ GeV. We have taken $M_N = 10$ MeV for this benchmark case.