Measuring the Leptonic CP Phase in Neutrino Oscillations with Non-Unitary Mixing

TL;DR

This paper investigates how non-unitary neutrino mixing complicates the measurement of the leptonic CP phase in experiments like T2K and T2HK, and proposes methods to resolve the resulting degeneracies using additional sources and detectors.

Contribution

It demonstrates the degeneracy between standard and non-unitary CP phases and proposes experimental setups to break this degeneracy, improving CP violation sensitivity.

Findings

Non-unitary mixing introduces degeneracy in CP phase measurements.

Supplementing T2HK with a $d$AR source partially resolves degeneracy.

Adding a near detector fully eliminates the degeneracy.

Abstract

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