  As long-baseline neutrino experiments enter the precision era, the
difficulties associated with understanding neutrino interaction cross sections
on atomic nuclei are expected to limit experimental sensitivities to
oscillation parameters. In particular, the ability to relate experimental
observables to neutrino energy in previous experiments has relied solely on
theoretical models of neutrino-nucleus interactions, which currently suffer
from very large theoretical uncertainties.
  By observing charged current $\nu_\mu$ interactions over a continuous range
of off-axis angles from 1 to 4 degrees, the nuPRISM water Cherenkov detector
can provide a direct measurement of the far detector lepton kinematics for any
given set of oscillation parameters, which largely removes neutrino interaction
modeling uncertainties from T2K oscillation measurements. This naturally
provides a direct constraint on the relationship between lepton kinematics and
neutrino energy. In addition, nuPRISM is a sensitive probe of sterile neutrino
oscillations with multiple energy spectra, which provides unique constraints on
possible background-related explanations of the MiniBooNE anomaly. Finally,
high-precision measurements of neutrino cross sections on water are possible,
including $\nu_e$ measurements and the first ever measurements of neutral
current interactions as a function of neutrino energy.
  The nuPRISM detector also benefits the proposed Hyper-Kamiokande project. A
demonstration that neutrino interaction uncertainties can be controlled will be
important to understanding the physics reach of Hyper-K. In addition, nuPRISM
will provide an easily accessible prototype detector for many of the new
hardware components currently under consideration for Hyper-K. The following
document presents the configuration, physics impact, and preliminary cost
estimates for a nuPRISM detector in the J-PARC neutrino beamline.
