Neutrino masses and mixings: Status of known and unknown $3\nu$ parameters

TL;DR

This paper provides an updated global analysis of neutrino oscillation data as of 2016, refining known parameters and assessing the status of unknowns like mass hierarchy, CP violation, and theta_23 octant, with implications for neutrino mass scale measurements.

Contribution

It offers the latest comprehensive global fit of neutrino oscillation parameters, incorporating new experimental results and reanalyzed data, and evaluates the status of unknown parameters and their implications.

Findings

Slight decrease in delta m^2 and sin^2theta_12 from KamLAND reanalysis.

Slight increase in |Delta m^2| from accelerator and atmospheric data.

Preference for negative sin(delta) around -0.9, no significant hierarchy or octant indication.

Abstract

Within the standard 3nu 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 anti-nu and NOvA nu runs in both appearance and disappearance mode), 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 (delta m^2, |Delta m^2|, sin^2theta_12, sin^2theta_13, sin^2theta_23), and the status of the three remaining unknown parameters: the mass hierarchy, the theta_23 octant, and the possible CP-violating phase delta. With respect to previous global fits, we find that the reanalysis of KamLAND data induces a slight decrease of both delta m^2 and sin^2theta_12, while the latest accelerator and atmospheric data induce a slight increase of |Delta m^2|. Concerning the unknown parameters, we confirm the previous intriguing preference for negative values of sin(delta) [with best-fit values around sin(delta) ~ -0.9], but we find no statistically significant indication about the theta_23 octant or the mass hierarchy (normal or inverted). Assuming an alternative (so-called LEM) analysis of NOvA data, some delta ranges can be excluded at >3 sigma, 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 nu mass scale: the sum of nu masses, the effective nu_e mass, and the effective Majorana mass.