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 $\delta m_{13}^2$, 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 $\delta m_{13}^2$ can be determined for $4|U_{e3}{|}^2(1-|U_{e3}{|}^2)\equiv {\sin }^{2}2{\theta }_{RCT}\text{gsim}0.008$. That of ${\sin }^2{\theta }_{ATM}-1/2$ can be resolved for ${\sin }^{2}2{\theta }_{ATM}=0.96$ when ${\sin }^{2}2{\theta }_{RCT}\text{gsim}0.06$. The CP-violating phase ${\delta }_{MNS}$ can be uniquely constrained for ${\sin }^{2}2{\theta }_{RCT}\text{gsim}0.02$ if its true value is around $90^{\circ }$ or $270^{\circ }$, while it can be constrained for ${\sin }^{2}2{\theta }_{RCT}\text{gsim}0.03$ if its true value is around $0^{\circ }$ or $180^{\circ }$.

Comments: 15 pages,6 figures, More discussions added together with some figures. References added