Speaker
Description
We study a method to test the unitarity of the PMNS matrix by using only
the long baseline neutrino oscillation experiment, such as the combination of
the T2HK experiment and the one with the $\nu_e$ beam from a future neutrino
factory at J-PARC. Without a specific parametrization, one can directly extract
the elements of the lepton mixing matrix by observing the energy dependence of
the oscillation probabilities. A non-trivial test of the unitarity under the three-
generation assumption can thus be made possible by examining the orthogonality
in a similar manner to the unitarity triangle in the quark sector. As the first
trial, we perform the analysis based on the simplified situation where the matter
effects in the neutrino oscillation are absent. The simplification is not expected
to significantly affect the results since we use the $\nu_\mu \to \nu_e$ and $\nu_e\to\nu_\mu$ channels,
whose difference is sensitive to the CP phase, while it is insensitive to the matter
effects. Under this simplified analysis, we find that the combination of T2HK
and neutrino factory experiments can exclude, for example, four-generation based
models to explain anomalies in short baseline experiments by the unitarity test
at the $3\sigma$ level for a sufficient (but realistic) flux of the $\nu_e$ beam.
