Lessons from the first JUNO results

A bstract First results from the JUNO reactor neutrino experiment already determine with world-leading precision the small neutrino squared-mass splitting $$ \Delta {m}_{21}^2 $$ ∆ m 21 2 and the mixing angle θ 12 . 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, $$ \Delta {m}_{3\ell}^2 $$ ∆ m 3 ℓ 2 . 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 $$ \Delta {m}_{3\ell}^2 $$ ∆ m 3 ℓ 2 -determinations gives a slight preference for Normal Ordering, with a p -value for Inverted Ordering of 2%–2.6% (2 . 2 σ –2 . 3 σ ). 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 ∆ χ 2 = 4.6 and 9.4 without and with Super-K and IceCube-24 atmospheric neutrino data, respectively.