Non-Unitarity Effects and Fake CP Violation in Neutrino Oscillation Experiments

Abstract

Future long-baseline neutrino oscillation experiments aim to establish leptonic CP violation and determine the neutrino mass ordering with unprecedented precision. However, these measurements can be significantly affected by possible deviations from the unitarity of the PMNS mixing matrix, which introduce additional CP-violating phases capable of generating fake CP-violating signals. We investigate the impact of non-unitary leptonic mixing on CP-violation and mass-ordering measurements at DUNE and Hyper-Kamiokande using GLoBES simulations with a custom non-unitary probability engine. We analyze the energy dependence of the neutrino--antineutrino CP asymmetry, quantify the fake-to-genuine CP asymmetry ratio, evaluate the CP violation discovery sensitivity, and study the hierarchy--CP--non-unitarity degeneracies in the $(δ_{CP},ϕ_{21})$ parameter space. We demonstrate that non-unitary mixing can generate sizeable CP asymmetries even for CP-conserving values of the standard Dirac phase, thereby mimicking genuine leptonic CP violation. While the fake contribution remains below $5\%$ of the genuine signal near each experiment's oscillation maximum, it exceeds the genuine signal in specific intermediate energy windows ($\sim130\%$ at $1.5$--$1.6$~GeV), demonstrating that fake CP violation can dominate over the genuine contribution in these regions. The combined DUNE and Hyper-Kamiokande analysis reduces the allowed $(δ_{CP},φ_{21})$ parameter space by a factor of $7$ at $1σ$ and $\sim16$ at $2σ$--$3σ$, substantially suppressing the degeneracy that neither experiment resolves individually and providing a robust strategy for distinguishing genuine from fake CP violation while improving sensitivity to the neutrino mass ordering.