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 $\Delta m_{21}^2$ 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 ${\delta }_{\mathrm{CP}}=\pm \pi /2$, the framework yields testable predictions for ${\theta }_{12}$ that can be probed by JUNO. Two viable scenarios arise, one predicting ${\sin }^2{\theta }_{12}\text{gsim}0.335$, 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.

Comments: 20 pages, 5 figures, 1 table. Updated to match the published version in PRD