Updated constraints on non-standard neutrino interactions from Planck

TL;DR

This work uses Planck and auxiliary cosmological data to constrain non-standard neutrino interactions, focusing on a Fermi-like 4-point contact and a light pseudoscalar mediator. By solving Boltzmann equations with both collisionless and collisional treatments and performing Bayesian MCMC analyses, the authors derive robust bounds: G_X ≤ (0.06 GeV)^{-2} for the 4-point case and g_{ii} ≤ 1.2×10^{-7}, g_{ij} ≤ 2.3×10^{-11}(m/0.05 eV)^{-2} for pseudoscalar interactions, with corresponding lifetime limits for decays. The results show that neutrinos cannot be strongly self-interacting across all cosmic history, while allowing early decoupling or late recoupling scenarios; the standard ΛCDM framework with free-streaming neutrinos remains favored. Overall, the paper tightens cosmological constraints on beyond-Standard-Model neutrino interactions and clarifies the viability of decoupling/recoupling scenarios in light of Planck data.

Abstract

We provide updated bounds on non-standard neutrino interactions based on data from the Planck satellite as well as auxiliary cosmological measurements. Two types of models are studied - A Fermi-like 4-point interaction and an interaction mediated by a light pseudoscalar - and we show that these two models are representative of models in which neutrinos either decouple or recouple in the early Universe. Current cosmological data constrain the effective 4-point coupling to be $G_X \leq \left(0.06 \, {\rm GeV}\right)^{-2}$, corresponding to $G_X \leq 2.5 \times 10^7 G_F$. For non-standard pseudoscalar interactions we set a limit on the diagonal elements of the dimensionless coupling matrix, $g_{ij}$, of $g_{ii} \leq 1.2 \times 10^{-7}$. For the off-diagonal elements which induce neutrino decay the bound is significantly stronger, corresponding to $g_{ij} \leq 2.3 \times 10^{-11}(m/0.05 \, {\rm eV})^{-2}$, or a lifetime constraint of $τ\geq 1.2 \times 10^{9} \, {\rm s} \, (m/0.05 \, {\rm eV})^{3} \,$. This is currently the strongest known bound on this particular type of neutrino decay. We finally note that extremely strong neutrino self-interactions which completely suppress anisotropic stress over all of cosmic history are very highly disfavored by current data ($Δχ^2 \sim 10^4$).

Figures (9)

• Figure 1: case A ( Top panel) CMB temperature angular power spectra. The black line shows the Lambda Mixed (Cold + Hot) Dark Matter ($\Lambda$MDM) model with $\omega_{\rm cdm}=0.099$ and $\omega_\nu = 0.013$ (corresponding to $\Sigma m_\nu=1.2$ eV). The blue and green lines depict the theoretical spectrum obtained if massive neutrinos decouple at redshift $z_i=50000$ and $z_i=15000$, respectively. The red and purple lines represent the corresponding power spectrum obtained by plugging into the Boltzmann equations collisional term related to the $G_X$ value corresponding to the decoupling redshift found from Eq. (\ref{['eq:gtozcaseA']}): $G_X=2.20\times10^{-3}\, {\rm MeV}^{-2}$ for a decoupling redshift $z_i=50000$ and $G_X=1.34\times10^{-2}\, {\rm MeV}^{-2}$ for a decoupling redshift $z_i=15000$. ( Bottom panel) Percentage error introduced by the approximations of switching off the hierarchy at $z<z_i$ instead of plugging into the equations the correct expression of $G_X$. The grey band defines the cosmic variance.
• Figure 2: case B ( Top panel) CMB temperature angular power spectra. The black line shows the $\Lambda$MDM model with $\omega_{\rm cdm}=0.099$ and $\omega_\nu = 0.013$ (corresponding to $\Sigma m_\nu=1.2$ eV). The blue and green lines depict the theoretical spectrum obtained if massive neutrinos recouple through the interactions with a pseudoscalar at redshift $z_i=1500$ and $z_i=2000$, respectively. The red and purple lines represent the corresponding power spectrum obtained by plugging into the Boltzmann equations the collisional term related to the $g$ value corresponding to the recoupling redshift found from Eq. (\ref{['eq:gtozcaseB']}): $g<1.17\times10^{-7}$ when $z_i=1500$ and $g<1.21\times10^{-7}$ when $z_i=2000$. ( Bottom panel) Percentage error introduced by the approximations of switching off the hierarchy at $z>z_i$ instead of plugging into the equations the correct expression of $g$. The grey band defines the cosmic variance.
• Figure 3: case A CMB temperature angular power spectrum for three different cosmological models: the red line shows the $\Lambda$CDM model with $\omega_{\rm cdm}=0.112$, while the black line is the $\Lambda$MDM model with $\omega_{\rm cdm}=0.099$ and $\omega_\nu = 0.013$ (corresponding to $\Sigma m_\nu=1.2$ eV). The blue and green lines depict the spectra obtained if Fermi like 4-point interactions prevent massive neutrinos from free-streaming until redshift $z_i=15000$ (blue line, upper panel), $z_i=50000$ (green line, upper panel), $z_i=1\times10^5$ (blue line, bottom panel), $z_i=3\times10^5$ (green line, bottom panel).
• Figure 4: case A Ratio between the CMB temperature power spectra accounting for the Fermi like 4-point interactions at different redshift (as described in Fig. \ref{['fig:clscaseA']}) and the $\Lambda$MDM spectrum.
• Figure 5: case B CMB temperature angular power spectrum for three different cosmological models: the red line shows the $\Lambda$CDM model with $\omega_{\rm cdm}=0.112$, while the black line is the $\Lambda$MDM model with $\omega_{\rm cdm}=0.099$ and $\omega_\nu = 0.013$ (corresponding to $\Sigma m_\nu=1.2$ eV). The blue and green lines depict the theoretical spectra obtained if massive neutrinos recouple through the interactions with a light pseudoscalar degree of freedom at redshift $z_i=1500$ or $z_i=3000$, respectively.