Constraining the Neutrino Mixing Matrix via Single-Sector Charged-Lepton Rotations in the JUNO Precision Era

The unprecedented precision now being achieved in the measurement of the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) lepton mixing matrix opens a new window onto the underlying structure of the neutrino mass matrix and the possibly associated flavor symmetries. In this work, we investigate the constraints imposed on the unitary matrix Uν that diagonalizes the neutrino mass matrix, under the hypothesis that the charged-lepton mixing matrix Ul consists of a single two-by-two rotation in one of the three sectors: (1,2), (1,3), or (2,3). For this analysis, we considered the latest global fit, which incorporates the precision measurement of θ12 from the JUNO experiment. For each scenario, we also derive analytical expressions for the entries of Uν in terms of the measured PMNS parameters to obtain compact sum-rule-like formulae.