New ambiguity in probing CP violation in neutrino oscillations

If neutrinos get mass via the seesaw mechanism the mixing matrix describing neutrino oscillations can be effectively nonunitary. We show that in this case the neutrino appearance probabilities involve a new $CP$ phase $\ensuremath{\phi}$ associated with nonunitarity. This leads to an ambiguity in extracting the ``standard'' three-neutrino phase ${\ensuremath{\delta}}_{CP}$, 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 ${\ensuremath{\delta}}_{CP}$.