Measuring the leptonic CP phase in neutrino oscillations with nonunitary mixing
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Abstract
Non-unitary neutrino mixing implies an extra $CP$ violating phase that can fake the leptonic Dirac $CP$ phase ${\ensuremath{\delta}}_{CP}$ 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 ``unitary'') and ``nonunitary'' $CP$ phases. We find, under the assumption of nonunitary mixing, that their $CP$ sensitivities severely deteriorate. Fortunately, the TNT2K proposal of supplementing T2(H)K with a $\ensuremath{\mu}\mathrm{DAR}$ source for better measurement of ${\ensuremath{\delta}}_{CP}$ can partially break the $CP$ degeneracy by probing both $\mathrm{cos}{\ensuremath{\delta}}_{CP}$ and $\mathrm{sin}{\ensuremath{\delta}}_{CP}$ dependences in the wide spectrum of the $\ensuremath{\mu}\mathrm{DAR}$ flux. We also show that the further addition of a near detector to the $\ensuremath{\mu}\mathrm{DAR}$ setup can eliminate the degeneracy completely.