On April 2015, the J-PARC E56 (JSNS2: J-PARC Sterile Neutrino Search using neutrinos from J-PARC Spallation Neutron Source) experiment officially obtained stage-1 approval from J-PARC. We have since started to perform liquid scintillator R&D for impr
oving energy resolution and fast neutron rejection. Also, we are studying Avalanche Photo-Diodes (SiPM) inside the liquid scintillator. In addition to the R&D work, a background measurement for the proton beam bunch timing using a small liquid scintillator volume was planned, and the safety discussions for the measurement have been done. This report describes the status of the R&D work and the background measurements, in addition to the milestones required before stage-2 approval.
At the 17th J-PARC PAC, which was held on September 2013, we proposed the sterile neutrino search at J-PARC MLF. After reviewing the proposal, PAC recommended to have a background measurement at the detectors candidate site location in their report t
o investigate whether the background rates can be manageable for the real experiment or not. Therefore, we have performed the background measurements (MLF; 2013BU1301 test experiment) during the summer of 2014, also following the 18th J-PARC PAC recommendations, and the measurements results are described here.
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 (D
AR) 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.
