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 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. The trade-offs between very large detectors with poor background rejection and smaller detectors with excellent background rejection are illustrated. We find that 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 CP-violation in the lepton sector would be observable in a low-energy medium-baseline experiment. We explicitly consider massive water Cherenkov and liquid argon detectors at superbeams with neutrino energies ranging from 1 GeV to 15 GeV, and baselines 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 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.Comment: Revtex (singlespaced), 41 pages, uses epsf.sty, 12 postscript figures. Minor corrections and notation changes, expanded discussions, x-axis numbers added to Fig.9(a),(c). To be published in Phys. Rev.