Non-Unitary Neutrino Mixing in the NOνA Near Detector Data

The ν_µ → ν_e oscillation probability over a short baseline (≲1 km) would be negligible for the case when the mixing matrix for three active neutrinos is unitary. However, in the case of a non-unitary mixing of three neutrinos, this probability would be non-negligible due to the so-called “zero distance” effect. Hence, the near detector of accelerator experiments such as NOνA can provide strong constraints on the parameters of the non-unitary mixing with very large statistics. By analyzing the NOνA near-detector data, we find that the non-unitary mixing does not improve fits to the ν_e or ν_µ events over the standard unitary mixing. This leads to constraints on the non-unitary parameters: α₀₀ > 0.911, |α₁₀ | < 0.020, and α₁₁ > 0.952 at 90% C.L. A combined analysis with the near-and far-detector data does not change these constraints significantly.