Lifting degeneracies in the oscillation parameters by a neutrino factory

Abstract

We study the potential of a very long baseline neutrino oscillation experiment with a neutrino factory and a large segmented water-Cerenkov calorimeter detector in resolving the degeneracies in the neutrino oscillation parameters; the sign of the larger mass-squared difference $\delta m^2_{13}$, the sign of $|U_{\mu 3}|^2 (\equiv \sin^2\theta_{ATM)-1/2$, and a possible two-fold ambiguity in the determination of the CP phase $\delta_{MNS}$. 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 $10^{21}$ decaying $\mu^+$ and $\mu^-$ at 10 GeV and a 100 kton detector which is placed 2,100 km away and is capable of measuring the event energy and distinguishing $e^\pm$ from $\mu^\pm$, but not their charges. The sign of $\delta m^2_{13}$ can be determined for $4|U_{e3}|^2(1-|U_{e3}|^2)\equiv \sin^22\theta_{RCT}\gsim 0.008$. That of $\sin^2\theta_{ATM}-1/2$ can be resolved for $\sin^22\theta_{ATM}=0.96$ when $\sin^22\theta_{RCT}\gsim 0.06$. The CP-violating phase $\delta_{MNS}$ can be uniquely constrained for $\sin^22\theta_{RCT}\gsim 0.02$ if its true value is around $90^\circ$ or $270^\circ$, while it can be constrained for $\sin^22\theta_{RCT}\gsim 0.03$ if its true value is around $0^\circ$ or $180^\circ$.