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Fate of $θ_{12}$ under $μ-τ$ Reflection Symmetry in Light of the First JUNO Results

Ranjeet Kumar

TL;DR

This paper investigates whether μ−τ reflection symmetry, naturally realized through an underlying A4 flavor symmetry in a type-II seesaw framework, can accommodate the precise solar-neutrino measurements from JUNO. The model employs two SU(2)L triplets and a Z3 symmetry to enforce a μ−τ symmetric neutrino mass matrix, predicting θ23 = 45° and δCP = ±π/2, while tightly constraining θ12 via four effective parameters. Two vev-alignment scenarios (case-I and case-II) lead to distinct parameter correlations; case-I remains compatible with JUNO, whereas case-II requires sin^2 θ12 ≳ 0.335 and is strongly disfavored by the data. The results illustrate how JUNO sharpens the viability of flavor-symmetric constructions, with future long-baseline experiments like DUNE and T2HK offering complementary probes of potential deviations from exact μ−τ symmetry.

Abstract

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

Fate of $θ_{12}$ under $μ-τ$ Reflection Symmetry in Light of the First JUNO Results

TL;DR

This paper investigates whether μ−τ reflection symmetry, naturally realized through an underlying A4 flavor symmetry in a type-II seesaw framework, can accommodate the precise solar-neutrino measurements from JUNO. The model employs two SU(2)L triplets and a Z3 symmetry to enforce a μ−τ symmetric neutrino mass matrix, predicting θ23 = 45° and δCP = ±π/2, while tightly constraining θ12 via four effective parameters. Two vev-alignment scenarios (case-I and case-II) lead to distinct parameter correlations; case-I remains compatible with JUNO, whereas case-II requires sin^2 θ12 ≳ 0.335 and is strongly disfavored by the data. The results illustrate how JUNO sharpens the viability of flavor-symmetric constructions, with future long-baseline experiments like DUNE and T2HK offering complementary probes of potential deviations from exact μ−τ symmetry.

Abstract

The recent JUNO measurements of and open a new avenue for probing flavor symmetric structures in the lepton sector. Motivated by this, we study a model in which reflection symmetry naturally emerges from an underlying flavor symmetry within a type-II seesaw framework. Beyond its standard predictions of and , the framework yields testable predictions for that can be probed by JUNO. Two viable scenarios arise, one predicting , which is strongly disfavored by the latest JUNO results. Correlations between and model parameters further enhance the model's predictivity. Future measurements at DUNE and T2HK will provide complementary tests of this scenario.
Paper Structure (9 sections, 19 equations, 5 figures, 1 table)

This paper contains 9 sections, 19 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Model Framework
  3. Lagrangian and Neutrino Mass Matrix
  4. Numerical Analysis and Model Predictions
  5. Constraints on Model Parameters from the Solar Mixing Angle $\boldsymbol{\theta_{12}}$
  6. Solar Parameter Correlations in the Light of JUNO
  7. Conclusions
  8. Tensor Product Rules of $\boldsymbol{A_4}$
  9. Determination of Lepton Mixing Parameters

Figures (5)

  • Figure 1: Correlation between $\sin^2\theta_{12}$ and the model parameter ratio $r_1$. The predictions for case-I and case-II are shown in blue and green, respectively, in the left and right panels. The brown and light red bands represent the allowed $3\sigma$ ranges from JUNO and AHEP, respectively.
  • Figure 2: Correlation between $\sin^2\theta_{12}$ and the model parameter ratio $r_2$. The color code remains the same as in Fig. \ref{['fig:r1']}.
  • Figure 3: Correlation between $\sin^2\theta_{12}$ and the model parameter ratio $r_3$. The color code remains the same as in Fig. \ref{['fig:r1']}.
  • Figure 4: Correlation between the solar parameters $\sin^2\theta_{12}$ and $\Delta m^2_{21}$ predicted by the model, compared with the AHEP global fit data deSalas:2020pgw. Case-I and case-II are shown in blue and green in the left and right panels, respectively.
  • Figure 5: Correlation between the solar parameters $\sin^2\theta_{12}$ and $\Delta m^2_{21}$ predicted by the model, compared with the JUNO results deSalas:2020pgw. Case-I and case-II are shown in blue and green in the left and right panels, respectively.