Measuring the leptonic CP phase in neutrino oscillations with non-unitary mixing

Abstract

Non-unitary neutrino mixing implies an extra CP violating phase that can fake the leptonic Dirac CP phase δCP of the simplest three-neutrino mixing benchmark scheme. This would hinder the possibility of probing for CP violation in accelerator-type experiments. We take T2K and T2HK as examples to demonstrate the degeneracy between the "standard" (or "non-unitary") and "nonunitary" CP phases. We find, under the assumption of non-unitary mixing, that their CP sensitivities severely deteriorate. Fortunately, the Tokai'N Toyama to Kamioka (TNT2K) proposal of supplementing T2(H)K with a μDAR source for better measurement of δCP can partially break the CP degeneracy by probing both cosδCP and sinδCP dependences in the wide spectrum of the μDAR flux. We also show that the further addition of a near detector to the μDAR setup can eliminate the degeneracy completely. This summary is based on ref [1].