New ambiguity in probing CP violation in neutrino oscillations

TL;DR

This paper reveals a new ambiguity in measuring CP violation in neutrino oscillations caused by non-unitarity in the mixing matrix, which introduces an additional CP phase and complicates experimental analysis.

Contribution

It identifies a previously unrecognized CP phase arising from non-unitarity, affecting the interpretation of CP violation measurements in neutrino experiments.

Findings

Non-unitarity introduces a new CP phase, phi.

The new phase causes ambiguity in delta_CP extraction.

This ambiguity persists even after combining neutrino and antineutrino data.

Abstract

If neutrinos get mass via the seesaw mechanism the mixing matrix describing neutrino oscillations can be effectively non-unitary. We show that in this case the neutrino appearance probabilities involve a new CP phase, phi, associated to non-unitarity. This leads to an ambiguity in extracting the "standard" three--neutrino phase delta_CP, which can survive even after neutrino and antineutrino channels are combined. Its existence should be taken into account in the planning of any oscillation experiment aiming at a robust measurement of delta_CP.