Constraints on the lepton asymmetry from DESI DR2 BAO data

TL;DR

This work constrains the lepton asymmetry in the neutrino sector by measuring the degeneracy parameter $ξ$ using Planck 2018 CMB and DESI DR2 BAO data. It analyzes two cosmological model classes, ΛCDM+$∑m_ν$+$ξ$ and CPL+$∑m_ν$+$ξ$, including a ΔN_eff extension, and treats massive neutrinos with full Boltzmann dynamics, allowing for mass-hierarchy variations (NH, IH, DH). The main results show upper bounds of $ξ<0.56$ (NH), $0.62$ (IH), $0.58$ (DH) in ΛCDM, and $ξ<0.47$ (NH), $0.48$ (IH), $0.49$ (DH) in CPL, with ΔN_eff providing a further ~10% tightening and allowing $ξ_μ$ and $ξ_τ$ to vary independently weakening the bound by ~30%. The findings suggest the ξ bounds are robust to model choice and hierarchy, and that dynamical dark energy can modestly tighten constraints; future CMB B-modes and 21 cm observations could further improve sensitivity to neutrino lepton asymmetry.

Abstract

It is important to explore the potential existence of lepton asymmetry in the neutrino sector. Conducting a joint analysis of DESI DR2 BAO data and \emph{Planck} 2018 CMB data, we obtain the upper limits on the neutrino degeneracy parameter, i.e., $ξ<0.56$ for the normal mass hierarchy while $ξ<0.62$ for the inverted mass hierarchy, at 95\% confidence level. Considering the influence of the dynamical dark energy, we find that these upper limits remain to be robust. This work may provide helpful implications for model buildings of the matter-antimatter asymmetry in the universe.

Figures (3)

• Figure 1: One-dimensional posterior distributions of $\xi$. Results are shown for $\Lambda\mathrm{CDM}$$+\Sigma m_\nu + \xi$ and CPL$+\Sigma m_\nu + \xi$, each assuming either NH or IH.
• Figure 2: Two-dimensional posterior distributions for $\xi$ versus $\sum m_\nu$ (upper panels), $w_0$ (bottom left panel), and $w_a$ (bottom right panel). Light and dark shaded regions, respectively, stand for 68% C.L. and 95% C.L.. Results are shown for $\Lambda\mathrm{CDM}$$+\Sigma m_\nu + \xi$ and CPL$+\Sigma m_\nu + \xi$, each assuming either NH or IH.
• Figure 3: Triangle plot for the parameters $\sum m_\nu,\xi,\Delta N_{\rm eff},{\rm and~ }h$. Light and dark shaded regions, respectively, stand for 68% C.L. and 95% C.L.. Results are shown for$\Lambda$CDM$+\sum m_\nu+\xi+\Delta N_{\rm eff}$ and CPL$+\sum m_\nu+\xi+\Delta N_{\rm eff}$, each assuming either NH or IH.