Exploring neutrino oscillations with superbeams

We consider the medium- and long-baseline oscillation physics capabilities of intense muon-neutrino and muon-antineutrino beams produced using future upgraded megawatt-scale high-energy proton beams. In particular we consider the potential of these conventional neutrino “superbeams” for observing ${\nu }_{\mu }\to {\nu }_e$ oscillations, determining the hierarchy of neutrino mass eigenstates, and measuring $\mathrm{CP}$ violation in the lepton sector. The physics capabilities of superbeams are explored as a function of the beam energy, baseline, and the detector parameters (fiducial mass, background rates, and systematic uncertainties on the backgrounds). The trade-offs between very large detectors with poor background rejection and smaller detectors with excellent background rejection are illustrated. We find that, with an aggressive set of detector parameters, it may be possible to observe ${\nu }_{\mu }\to {\nu }_e$ oscillations with a superbeam provided that the amplitude parameter ${\sin }^{2}2{\theta }_{13}$ is larger than a few $\times {10}^{-3}.$ If ${\sin }^{2}2{\theta }_{13}$ is of order ${10}^{-2}$ or larger, then the neutrino mass hierarchy can be determined in long-baseline experiments, and if in addition the large mixing angle MSW solution describes the solar neutrino deficit, then there is a small region of parameter space within which maximal $\mathrm{CP}$ violation in the lepton sector would be observable (with a significance of a few standard deviations) in a low-energy medium-baseline experiment. We illustrate our results by explicitly considering massive water Cherenkov and liquid argon detectors at superbeams with neutrino energies ranging from 1 GeV to 15 GeV, and baselines ranging from 295 km to 9300 km. Finally, we compare the oscillation physics prospects at superbeams with the corresponding prospects at neutrino factories. The sensitivity at a neutrino factory to $\mathrm{CP}$ violation and the neutrino mass hierarchy extends to values of the amplitude parameter ${\sin }^{2}2{\theta }_{13}$ that are one to two orders of magnitude lower than at a superbeam.