Optimal $β$-beam at the CERN-SPS

Abstract A β -beam with maximum γ = 150 (for 6 He ions) or γ = 250 (for 18 Ne) could be achieved at the CERN-SPS. We study the sensitivity to θ 13 and δ of such a beam as function of γ , optimizing with the baseline constrained to CERN–Frejus (130 km), and also with simultaneous variation of the baseline, for a fixed ion flux. These results are compared to the standard scenario previously considered, with lower γ = 60 / 100 , and also with a higher γ ∼ 350 option that requires a more powerful accelerator. We conclude that the sensitivity to CP violation and θ 13 increases significantly with γ if the baseline is increased proportionally, while for the CERN–Frejus scenario the dependence on γ is mild provided γ is above 100.