Evidence for Normal Neutrino Ordering with Combined Atmospheric Neutrino Experiments

Abstract: We present a combined analysis of the latest publicly available atmospheric neutrino data from Super-Kamiokande including all data-taking stages previous to the gadolinium phase, IceCube-DeepCore, and the six-detection-unit configuration of KM3NeT/ORCA. Our fit uses all event samples simultaneously and treats the systematic uncertainties shared among experiments as fully correlated, while detector-response uncertainties are modeled independently for each experiment. %We measure The combined data constrain the atmospheric oscillation parameters to $Δm^2_{31} = 2.44^{+0.06}_{-0.04} \times 10^{-3}\,\mathrm{eV}^2$ and $\sin^2θ_{23} = 0.52^{+0.03}_{-0.04}$, with a precision of $2.2\%$ and $6.1\%$, respectively, which is competitive with current accelerator-based measurements, and independently constrain the $CP$-violating phase to $δ_{CP} = 1.30^{+0.24}_{-0.28}~π$. Finally, comparing the orderings of the neutrino mass spectrum, we find a preference for the normal ordering with $Δχ^2 = 11.0$; further calibrated with pseudoexperiments, we exclude the inverted ordering at $99.95\%$ confidence level ($3.3σ$).