Neutrino masses and mixings: Status of known and unknown 3ν parameters

Abstract

Within the standard 3ν mass–mixing framework, we present an up-to-date global analysis of neutrino oscillation data (as of January 2016), including the latest available results from experiments with atmospheric neutrinos (Super-Kamiokande and IceCube DeepCore), at accelerators (first T2K ν‾ and NOνAν runs in both appearance and disappearance modes), and at short-baseline reactors (Daya Bay and RENO far/near spectral ratios), as well as a reanalysis of older KamLAND data in the light of the “bump” feature recently observed in reactor spectra. We discuss improved constraints on the five known oscillation parameters (δm2, |Δm2|, sin2⁡θ12, sin2⁡θ13, sin2⁡θ23), and the status of the three remaining unknown parameters: the mass hierarchy [sign(±Δm2)], the θ23 octant [sign(sin2⁡θ23−1/2)], and the possible CP-violating phase δ. With respect to previous global fits, we find that the reanalysis of KamLAND data induces a slight decrease of both δm2 and sin2⁡θ12, while the latest accelerator and atmospheric data induce a slight increase of |Δm2|. Concerning the unknown parameters, we confirm the previous intriguing preference for negative values of sin⁡δ (with best-fit values around sin⁡δ≃−0.9), but we find no statistically significant indication about the θ23 octant or the mass hierarchy (normal or inverted). Assuming an alternative (so-called LEM) analysis of NOνA data, some δ ranges can be excluded at >3σ, and the normal mass hierarchy appears to be slightly favored at ∼90% C.L. We also describe in detail the covariances of selected pairs of oscillation parameters. Finally, we briefly discuss the implications of the above results on the three non-oscillation observables sensitive to the (unknown) absolute ν mass scale: the sum of ν masses Σ (in cosmology), the effective νe mass mβ (in beta decay), and the effective Majorana mass mββ (in neutrinoless double beta decay).