Lifting degeneracies in the oscillation parameters by a neutrino factory

We study the potential of a very long baseline neutrino oscillation experiment with a neutrino factory and a large segmented water-Čerenkov calorimeter detector in resolving the degeneracies in the neutrino oscillation parameters; the sign of the larger mass-squared difference δm132, the sign of |Uμ3|2(≡sin2θATM)−1/2, and a possible two-fold ambiguity in the determination of the CP phase δMNS. We find that the above problems can be resolved even if the particle charges are not measured. The following results are obtained in our exploratory study for a neutrino factory which delivers 1021 decaying μ+ and μ− at 10 GeV and a 100 kton detector which is placed 2100 km away and is capable of measuring the event energy and distinguishing e± from μ±, but not their charges. The sign of δm132 can be determined for 4|Ue3|2(1−|Ue3|2)≡sin22θRCT≳0.008. That of sin2θATM−1/2 can be resolved for sin22θATM=0.96 when sin22θRCT≳0.06. The CP-violating phase δMNS can be uniquely constrained for sin22θRCT≳0.02 if its true value is around 90° or 270°, while it can be constrained for sin22θRCT≳0.03 if its true value is around 0° or 180°.