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 u mu -> anti u e) which is detected by the inverse beta decay interaction (anti u 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 u mu and u 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.
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