Lepton Mixing from a Lattice Flavon Model: A Two-Branch Octant-delta Prediction

We extend the single-flavon $B$-lattice Froggatt-Nielsen (FN) framework – previously successful for quark masses and Cabibbo-Kobayashi-Maskawa (CKM) mixing – to the lepton sector. The same $B$-lattice power structure ($\epsilon \equiv 1\mathrm{/}B\simeq 0.19$) generates charged-lepton mass hierarchies and a normal-ordered neutrino spectrum; large neutrino mixing angles require an additional approximate mu-tau symmetry, broken at $\mathcal{O}(\epsilon )$ to generate a nonzero reactor angle and CP-violating phase. The Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix factorizes as $U_{\mathrm{P}\mathrm{M}\mathrm{N}\mathrm{S}}=U_e^{†}{U}_{\nu }$, with near-tribimaximal $U_{\nu }$ corrected by small charged-lepton rotations whose phases are naturally aligned by the single-flavon origin of the Yukawa textures. This alignment produces a two-branch prediction in the $({\theta }_{23},\delta )$ plane: a lower-octant solution with ${\theta }_{23}\approx {43}^{\circ }$, $\delta \approx {286}^{\circ }$, and an upper-octant solution with ${\theta }_{23}\approx {46}^{\circ }$, $\delta \approx {304}^{\circ }$. The lower octant is favored by a $\sim \text{\hspace{0.17em}⁣}4:1$ theoretical prior. The Jarlskog invariant $J_{\mathrm{C}\mathrm{P}}\simeq -0.027$ is nearly branch-independent; only precision measurements of the atmospheric octant and Dirac phase at DUNE, Hyper-Kamiokande, IceCube, and JUNO can distinguish the two solutions.