Neutrinos, WMAP, and BBN

TL;DR

This work analyzes whether anomalous hints for the primordial helium abundance $Y_p$ and the effective number of relativistic species $N_{eff}$ can be explained by new neutrino physics. It studies two main avenues: neutrino degeneracy through the electron-neutrino degeneracy parameter $\xi_{\nu_e}$ and additional light species such as sterile or right-handed neutrinos, employing a modified BBN code to map the joint impact on $Y_p$ and $N_{eff}$ and to derive the relation $\Delta N_{eff} = \sum_{\alpha=e,\mu,\tau} \left[ \frac{30}{7} (\frac{\xi_{\nu_\alpha}}{\pi})^2 + \frac{15}{7} (\frac{\xi_{\nu_\alpha}}{\pi})^4 \right]$. The analysis finds that allowing $N_{eff}$ around 4 relaxes the bound on $\xi_{\nu_e}$ to roughly $-0.14 \lesssim \xi_{\nu_e} \lesssim 0.12$ and identifies sterile-neutrino regions with $N_{eff} \approx 4$ and $Y_p \ge 0.258$, with lepton asymmetries capable of suppressing sterile production. It further discusses experimental and astrophysical tests (beta decay, $0\nu\beta\beta$, reactor and beam experiments, atmospheric neutrinos, IceCube, and galactic supernova observations) and the role of future cosmological data (e.g., Planck, SPT) in constraining these scenarios, highlighting the potential for new neutrino-sector physics if the hints persist.

Abstract

New data from WMAP have appeared, related to both the fractional energy density in relativistic species at decoupling and also the primordial helium abundance, at the same time as other independent observational estimates suggest a higher value of the latter than previously estimated. All the data are consistent with the possibility that the effective number of relativistic species in the radiation gas at the time of Big Bang Nucleosynthesis may exceed the value of 3, as expected from a CP-symmetric population of the known neutrino species. Here we explore the possibility that new neutrino physics accounts for such an excess. We explore different realizations, including neutrino asymmetry and new neutrino species, as well as their combination, and describe how existing constraints on neutrino physics would need to be relaxed as a result of the new data, as well as possible experimental tests of these possibilities.

Figures (2)

• Figure 1: $Y_p$ contours in the $\xi_{\nu_e}$ and $N_{eff}$ parameter space assuming neutrino flavor equilibration ($\xi_{\nu_e} = \xi_{\nu_\mu} = \xi_{\nu_\tau}$). The horizontal light (yellow) band corresponds to the 1$\sigma$ WMAP 7 year result. The black contours show a range of calculated values of $Y_p$ given model independent inputs of $\xi_{\nu_e}$ and $N_{\rm eff}$. The shaded (colored) vertical bands mark the Izotov and Thuan 1$\sigma$, 2$\sigma$, and 3$\sigma$ ranges of $Y_p$. The bottom black curve shows the contribution to $N_{eff}$ from neutrino asymmetries alone.
• Figure 2: