  We propose a definite search for sterile neutrinos at the J-PARC Materials
and Life Science Experimental Facility (MLF). With the 3 GeV Rapid Cycling
Synchrotron (RCS) and spallation neutron target, an intense neutrino beam from
muon decay at rest (DAR) is available. Neutrinos come from \mu+ decay, and the
oscillation to be searched for is (anti \nu \mu -> anti \nu e) which is
detected by the inverse \beta decay interaction (anti \nu e + p -> e+ + n),
followed by a gamma from neutron capture.
  The unique features of the proposed experiment, compared with the LSND and
experiments using horn focused beams, are;
  (1) The pulsed beam with about 600 ns spill width from J-PARC RCS and muon
long lifetime allow us to select neutrinos from \mu DAR only.
  (2) Due to nuclear absorption of \pi- and \mu-, neutrinos from \mu- decay are
suppressed to about the $10^{-3}$ level.
  (3) Neutrino cross sections are well known. The inverse \beta decay cross
section is known to be a few percent accuracy.
  (4) The neutrino energy can be calculated from positron energy by adding ~1.8
MeV.
  (5) The anti \nu \mu and \nu e fluxes have different and well defined
spectra. This allows us to separate oscillated signals from those due to \mu-
decay contamination.
  We propose to proceed with the oscillation search in steps since the region
of \Delta m^2 to be searched can be anywhere between sub-eV^2 to several tens
of eV^2. We start to examine the large \Delta m^2 region, which can be done
with short baseline at first. At close distance to the MLF target gives a high
neutrino flux, and allows us to use relatively small detector.
  If no definitive positive signal is found, a future option exists to cover
small \Delta m^2 region. This needs a relatively long baseline and requires a
large detector to compensate for the reduced neutrino flux.
