Matter Profile Mismodeling as the Origin of the CP CPT Degeneracy in Long-Baseline Neutrino Oscillation

Abstract

We demonstrate that approximating the Earth's density profile with a single path averaged value instead of the spatially resolved Preliminary Reference Earth Model PREM generates a joint parameter degeneracy in the $({\delta }_{\text{CP}},{\Delta }_{\text{CPT}},{\theta }_{\text{CPT}})$ space. At long baselines ($L\ge 5000\text{ km}$), this geometric artifact becomes entirely indistinguishable from genuine Beyond the Standard Model (BSM) physics. While previous studies established individual channel biases separately, this work provides the first analytic proof that these biases form an extended two dimensional valley within the full three-parameter space. Furthermore, we rigorously demonstrate that multi channel combinations alone are insufficient to resolve this degeneracy. A Poisson loglikelihood chi square analysis confirms that DUNE like exposures utilizing a constant density approximation would erroneously report this artifact as a $\pm 3\sigma$ BSM discovery. Specifically, at $L=7000\text{ km}$, the constant-density assumption shifts the recovered ${\delta }_{\text{CP}}$ by $17.8^{\circ }$ and fabricates a spurious CPT violation at ${\Delta }_{\text{CPT}}=4.1\times 10^{-4}{\text{ eV}}^2$. Consequently, full spatially resolved PREM propagation is established as a strict mathematical requirement rather than an optional refinement for the correct physical interpretation of data from next-generation long baseline experiments, including DUNE, HyperKamiokande, and P2O.