The $ν$ generation: present and future constraints on neutrino masses from cosmology and laboratory experiments

TL;DR

This study develops a Bayesian joint framework to constrain the absolute neutrino mass scale by combining oscillation data, direct beta-decay limits, cosmology, and neutrinoless double beta decay, while marginalizing over nuclear-modeling uncertainties with a nuisance parameter $\xi$. Using Planck 2015 and oscillation inputs, the authors demonstrate that current limits are dominated by cosmology and oscillations, yielding $M_\nu < 0.19$–$0.21$ eV and $m_{\beta\beta} < 0.045$ eV (NH) or $<0.066$ eV (IH); $m_\beta$ is constrained to $<0.06$–$0.08$ eV. For forthcoming data, constraints remain similar in spirit, with next-generation results (Next-generation II) offering potential measurements of non-minimal mass parameters especially for NH, around $M_\nu \sim 0.1$ eV, $m_{\beta\beta}$ ~20 meV, and $m_\beta$ ~30 meV, contingent on controlling nuclear uncertainties. The authors emphasize that improved nuclear matrix-element modeling could sharpen $m_{\beta\beta}$ by up to a factor of four and that future experiments could also probe the Majorana phase $\alpha_{21}$, making a combined cosmology-lab approach a powerful path to unveiling the neutrino mass hierarchy and the nature of neutrinos.

Abstract

We perform a joint analysis of current data from cosmology and laboratory experiments to constrain the neutrino mass parameters in the framework of bayesian statistics, also accounting for uncertainties in nuclear modeling, relevant for neutrinoless double $β$ decay ($0\nu2β$) searches. We find that a combination of current oscillation, cosmological and $0\nu2β$ data constrains $m_{ββ}~<~0.045\,\mathrm{eV}$ ($0.014 \, \mathrm{eV} < m_{ββ} < 0.066 \,\mathrm{eV}$) at 95\% C.L. for normal (inverted) hierarchy. This result is in practice dominated by the cosmological and oscillation data, so it is not affected by uncertainties related to the interpretation of $0\nu2β$ data, like nuclear modeling, or the exact particle physics mechanism underlying the process. We then perform forecasts for forthcoming and next-generation experiments, and find that in the case of normal hierarchy, given a total mass of $0.1\,$ eV, and assuming a factor-of-two uncertainty in the modeling of the relevant nuclear matrix elements, it will be possible to measure the total mass itself, the effective Majorana mass and the effective electron mass with an accuracy (at 95\% C.L.) of $0.05$, $0.015$, $0.02\,\mathrm{eV}$ respectively, as well as to be sensitive to one of the Majorana phases. This assumes that neutrinos are Majorana particles and that the mass mechanism gives the dominant contribution to $0\nu2β$ decay. We argue that more precise nuclear modeling will be crucial to improve these sensitivities.

Figures (3)

• Figure 1: Posterior distributions for the neutrino mass parameters, for NH (top row) and IH (bottom row). Solid (dashed) curves correspond to marginalization over nuclear uncertainties (fixed fiducial values for nuclear parameters). Note that the "Next-generation I" and "Next-generation II" forecasted results have been derived by assuming a fiducial value of $M_{\nu}=(0.10\pm0.06)\mathrm{eV}$.
• Figure 2: Two-dimensional posterior distributions for the neutrino mass parameters in the $m_{\beta\beta}-m_{\beta}$ plane, for NH (red) and IH (blue), from the combination of oscillation and "Planck 2015" datasets. Contours correspond to 95% C.L.. The dashed lines are the 95% C.L. upper limits on $m_{\beta\beta}$ from GERDA phase 1 within the range $\xi=[0.5-2]$ (vertical) and on $m_\beta$ from KATRIN (horizontal).
• Figure 3: Posterior distribution for $\alpha_{21}$ from the next-generation II dataset. Solid (dashed) lines are for $\xi$ marginalized over ($\xi=1$).