Fate of $\theta_{12}$ under $\mu-\tau$ Reflection Symmetry in Light of the First JUNO Results

Abstract

The recent Jiangmen Underground Neutrino Observatory (JUNO) measurements of $\theta_{12}$ and $\Delta m^2_{21}$ open a new avenue for probing flavor symmetric structures in the lepton sector. Motivated by this, we study a model in which $\mu-\tau$ reflection symmetry naturally emerges from an underlying $A_4$ flavor symmetry within a type-II seesaw framework. Beyond its standard predictions of $\theta_{23}=45^{\circ}$ and $\delta_{\rm CP}=\pm \pi/2$, the framework yields testable predictions for $\theta_{12}$ that can be probed by JUNO. Two viable scenarios arise, one predicting $\sin^2\theta_{12} \gsim 0.335$, which is strongly disfavored by the latest JUNO results. Correlations between $\theta_{12}$ and model parameters further enhance the model's predictivity. Future measurements at DUNE and T2HK will provide complementary tests of this scenario.