Updated fit to three neutrino mixing: exploring the accelerator-reactor complementarity

TL;DR

This work presents a comprehensive global fit of three-neutrino oscillations using data available through fall 2016, incorporating solar, atmospheric, reactor, and accelerator experiments to update the six oscillation parameters. It emphasizes the accelerator–reactor complementarity and evaluates confidence levels for the less precisely known parameters $\theta_{23}$, $\delta_{CP}$, and the mass ordering using Monte Carlo methods, finding mass ordering and the $\theta_{23}$ octant sensitivity to be below $1\sigma$. The best-fit $\delta_{CP}$ lies near $270^\circ$, with CP-conserving values disfavored at around $1\sigma$ (NO) and more for IO, while maximal $\theta_{23}$ mixing is only weakly disfavored. The analysis also discusses tensions in $\Delta m^2_{21}$ between solar and KamLAND data, the importance of consistently including reactor $\Delta m^2_{3\ell}$ information, and the limited sensitivity to mass ordering provided by current data. The conclusions highlight that, although hints of CP violation and non-maximal $\theta_{23}$ exist, definitive conclusions require future, higher-statistics measurements.

Abstract

We perform a combined fit to global neutrino oscillation data available as of fall 2016 in the scenario of three-neutrino oscillations and present updated allowed ranges of the six oscillation parameters. We discuss the differences arising between the consistent combination of the data samples from accelerator and reactor experiments compared to partial combinations. We quantify the confidence in the determination of the less precisely known parameters $θ_{23}$, $δ_\text{CP}$, and the neutrino mass ordering by performing a Monte Carlo study of the long baseline accelerator and reactor data. We find that the sensitivity to the mass ordering and the $θ_{23}$ octant is below $1σ$. Maximal $θ_{23}$ mixing is allowed at slightly more than 90% CL. The best fit for the CP violating phase is around $270^\circ$, CP conservation is allowed at slightly above $1σ$, and values of $δ_\text{CP} \simeq 90^\circ$ are disfavored at around 99% CL for normal ordering and higher CL for inverted ordering.

Figures (13)

• Figure 1: Global $3\nu$ oscillation analysis. Each panel shows the two-dimensional projection of the allowed six-dimensional region after marginalization with respect to the undisplayed parameters. The different contours correspond to $1\sigma$, 90%, $2\sigma$, 99%, $3\sigma$ CL (2 dof). The normalization of reactor fluxes is left free and data from short-baseline reactor experiments are included as explained in the text. Note that as atmospheric mass-squared splitting we use $\Delta m^2_{31}$ for NO and $\Delta m^2_{32}$ for IO. The regions in the four lower panels are obtained from $\Delta\chi^2$ minimized with respect to the mass ordering.
• Figure 2: Global $3\nu$ oscillation analysis. The red (blue) curves correspond to Normal (Inverted) Ordering. The normalization of reactor fluxes is left free and data from short-baseline reactor experiments are included. Note that as atmospheric mass-squared splitting we use $\Delta m^2_{31}$ for NO and $\Delta m^2_{32}$ for IO.
• Figure 3: Dependence of the global $\Delta\chi^2$ function on the Jarlskog invariant. The red (blue) curves are for NO (IO).
• Figure 4: Leptonic unitarity triangle for the first and third columns of the mixing matrix. After scaling and rotating the triangle so that two of its vertices always coincide with $(0,0)$ and $(1,0)$ we plot the $1\sigma$, 90%, $2\sigma$, 99%, $3\sigma$ CL (2 dof) allowed regions of the third vertex. Note that in the construction of the triangle the unitarity of the $U$ matrix is always explicitly imposed. The regions for both orderings are defined with respect to the common global minimum which is in NO.
• Figure 5: Left: Allowed parameter regions (at $1\sigma$, 90%, $2\sigma$, 99% and $3\sigma$ CL for 2 dof) from the combined analysis of solar data for GS98 model (full regions with best fit marked by black star) and AGSS09 model (dashed void contours with best fit marked by a white dot), and for the analysis of KamLAND data (solid green contours with best fit marked by a green star) for fixed $\theta_{13}=8.5^\circ$. Right: $\Delta\chi^2$ dependence on $\Delta m^2_{21}$ for the same three analyses after marginalizing over $\theta_{12}$.