Lessons from the first JUNO results

First results from the JUNO reactor neutrino experiment already determine with world-leading precision the small neutrino squared-mass splitting ∆m$^2_{2l}$ and the mixing angle θ12. In this article we perform an exploratory study beyond these, taking advantage of the first JUNO data release to discuss its sensitivity to the large squared-mass splitting, ∆m$^2_{3l}$. When combined with constraints from global oscillation data, this may already contain some information on the neutrino mass ordering. Indeed, we find that the combination of the complementary ∆m$^2_{3l}$-determinations gives a slight preference for Normal Ordering, with a p-value for Inverted Ordering of 2%–2.6% (2.2σ–2.3σ). We study the robustness of this result with respect to potential systematic uncertainties and statistical fluctuations. Taken at face value, a full global analysis of oscillation data including the publicly available JUNO information and data leads to a preference for Normal Ordering with ∆χ2 = 4.6 and 9.4 without and with Super-K and IceCube-24 atmospheric neutrino data, respectively.