Solving Degeneracies at a $β$-Beam by Combining Ions

Abstract

We study how the eightfold-degeneracy in the ($θ_{13},δ$) plane observed in $γ\sim 100$ $β$-beams can be reduced by periodically changing the ions in the storage ring. This ``ions cocktail'' allows to change the neutrino energy, at fixed $γ$, by choosing ions with different decay energies. We propose to combine the standard $^6$He and $^{18}$Ne beams with $^8$Li and $^8$B ones. These latter two ions have peaked $ν_e \to ν_μ$ oscillation probabilities for $γ= 100$ at a baseline $L \sim 700$ Km. At this distance and this $γ$ the oscillation probability of $^6$He and $^{18}$Ne neutrinos is at its second maximum. This setup is particularly suited for large enough values of $θ_{13}$ (within reach at T2K-I) and it allows solving most of the eightfold-degeneracy, measuring $θ_{13}, δ$ and the sign of the atmospheric mass difference for values of $θ_{13} \geq 5^\circ$.