Understanding neutrinos: the absolute mass scale

Neutrino oscillation experiments have firmly established that at least two out of the three active neutrinos have a non-zero mass, by measuring the neutrino mass splittings with great accuracy. However, no currently available experimental avenue has been able to determine the neutrino absolute mass scale to date. Cosmology currently represents a promising scenario. The goal of Simons Observatory is to measure the total mass in the three neutrino species with σ(Σmν)=0.04 eV. If Σmν ≥ 0.1 eV, such a measurement could give a clear indication of a non-zero mass sum. Simons Observatory plans to achieve this measurement through three different methods: (i) CMB lensing from Simons Observatory combined with new BAO measurements from DESI; (ii) SZ cluster counts from Simons Observatory calibrated with weak lensing measurements from LSST; and (iii) thermal SZ distortion maps from Simons Observatory combined with BAO measurements from DESI. The legacy Simons Observatory dataset could be used in combination with a future cosmic variance-limited measurement of the optical depth to reionization which would enable a 3σ detection of the minimal mass sum within the normal hierarchy (Σmν=0.06eV).

Fig.1. Summary of current limits on the neutrino mass scale, Σmν , and forecast sensitivity, from cosmological probes and laboratory searches.

Figure (1) shows a summary of current limits on the neutrino mass scale, Σmν , and forecast sensitivity, from cosmological probes and laboratory searches. The mass sum is shown as a function of the mass of the lightest neutrino eigenstate, mlight, for the normal and inverted hierarchy. Current cosmological bounds (TT,TE,EE+lowE+lensing+BAO) exclude at 95% confidence the region above the horizontal brown dashed line. The 1σ sensitivity for SO combined with large scale structure measurements (LSS) is shown for two example cases, enabling a 3σ measurement of Σmν for the minimal mass scenario for the inverted ordering. The expected sensitivity from the β-decay experiment KATRIN is indicated with a vertical yellow band on the right.

Fig.2. SO projections for neutrino physics (68% limits): the sum of neutrino masses (Σmν ) and the effective number of relativistic species (Neff ). SO should give a clear indication of a non- zero mass sum in the inverted hierarchy (where Σmν ≥ 0.1 eV), and should measure the minimal normal hierarchy at 3 − 4σ when combined with LiteBIRD (Σmν ≥ 0.06 eV, indicated).

Figure (2) shows a summary of the forecast sensitivity of SO-Nominal and SO-enhanced to neutrino physics. Simons Observatory should be able to test the presence of additional non-scalar light relics coupled to the thermal plasma at any time back to reheating. In addition, it would provide a >3sigma measurement of the sum of the neutrino masses when combined with cosmic-variance-limited measurements of the optical depth to reionization.