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

TL;DR

The paper presents a comprehensive global analysis of neutrino oscillations in the 3ν framework using updated data through January 2016, including atmospheric (SK, IceCube-DC), long-baseline (T2K, NOvA), and short-baseline reactor (Daya Bay, RENO) inputs, plus a KamLAND reanalysis that accounts for a reactor-spectrum bump. It delivers precise constraints on the five known oscillation parameters ($\delta m^2$, $|\Delta m^2|$, $\sin^2\theta_{12}$, $\sin^2\theta_{13}$, $\sin^2\theta_{23}$) and investigates the three unknowns ($\Delta\mathrm{hierarchy}$, $\theta_{23}$ octant, $\delta$), including their covariances. A persistent preference for $\sin\delta<0$ (best around $\sin\delta\approx -0.9$) is reported, while no definitive evidence for the mass hierarchy or the $\theta_{23}$ octant is found; however, using the NOvA $\mathrm{LEM}$ data strengthens $\delta$ constraints and yields a mild normal-hierarchy hint. The study also discusses the implications for absolute neutrino masses via $m_\beta$, $m_{\beta\beta}$, and $\Sigma$, noting that cosmological bounds on $\Sigma$ favor the normal hierarchy and that future data are needed to resolve remaining ambiguities.

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.

Figures (9)

• Figure 1: Global analysis of neutrino oscillation data. Bounds on the mass-mixing parameters are given in terms of standard deviations $N_\sigma$ from the best fit, for either NH (solid lines) or IH (dashed lines). Bounds on $(\delta m^2,\,\sin^2\theta_{12})$ are hierarchy-independent. Horizontal dotted lines mark the 1, 2, and $3\sigma$ levels for each parameter.
• Figure 2: As in Fig. 1 (rightmost panels) but with NO$\nu$A LEM data replacing LID data. See the text for details.
• Figure 3: Bounds at $N_\sigma=1$, 2 and 3 on each pair of parameters chosen among ($\delta m^2,\,\sin^2\theta_{12},\,\sin^2\theta_{13})$, as derived by our analysis of Solar+KL data (solid lines) and of all data (dashed lines). The dots mark the best-fit points. The bounds refer to NH case, and are very similar for IH case (not shown).
• Figure 4: Covariance plot for the $(\Delta m^2,\,\sin^2\theta_{13})$ parameters. From left to right, the regions allowed at $N_\sigma=1$, 2 and 3 refer to the analysis of LBL Acc+Solar+KL data (left panels), plus SBL reactor data (middle panels), plus Atmospheric data (right panels), with best fits marked by dots. The three upper (lower) panels refer to NH (IH).
• Figure 5: As in Fig. 4, but for the $(\sin^2\theta_{23},\,\sin^2\theta_{13})$ parameters.