The ESSnuSB project proposes to base a neutrino “Super Beam” of unprecedented luminosity at the European Spallation Source. The original proposal identified the second peak of the oscillation probability as the optimal to maximize the discovery potential to leptonic CP violation. However this choice reduces the statistics at the detector and penalizes other complementary searches such as the determination of the atmospheric oscillation parameters, particularly the octant of θ23 as well as the neutrino mass ordering. We find that including the atmospheric sample, the combination
not only improves very significantly these drawbacks, but also enhances both the CP violation discovery potential and the precision in the measurement of the CP violating phase, for which the facility was originally optimized. We then reassess the optimization of the ESSnuSB setup when the atmospheric neutrino sample is considered, with an emphasis in performing a measurement of the CP violating phase as precise as possible. We find that for values of δ near to maximal CP violation, shorter baselines like that with the Zinkgruvan detector site (360km) would be optimal.
In these conditions, a measurement better than 8◦ would be achievable for any value of the θ23 octant and the mass ordering. Conversely, if present and next generation facilities were not able to discover CP violation, longer baselines like that with the Garpenberg detector site (540 km) and more even splitting between neutrino and neutrino modes would be preferable.
