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Register a new userOperational Status and Power Upgrade Prospects of the Neutrino Experimental Facility at J-PARC
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Taku Ishida
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In order to explore CP asymmetry in the lepton sector, a power upgrade to the neutrino experimental facility at J-PARC is a key requirement for both the Tokai to Kamioka (T2K) long-baseline neutrino oscillation experiment and a future project with Hyper-Kamiokande. Based on five years of operational experience, the facility has achieved stable operation with 230 kW beam power without significant problems on the beam-line apparatus. After successful maintenance works in 2013-2014 to replace all electromagnetic horns and a production target, the facility is now ready to accomodate a 750-kW-rated beam. Also, the possibility of achieving a few to multi-MW beam operation is discussed in detail.
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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.
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 improving 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.
The JSNS2 (J-PARC E56) experiment aims to search for sterile neutrinos at the J-PARC Materials and Life Sciences Experimental Facility (MLF).After the submission of a proposal to the J-PARC PAC, stage-1 approval was granted to the JSNS2 experiment. The approval followed a series of background measurements which were performed in 2014. Subsequent for stage-1 approval, the JSNS2 collaboration has made continuous efforts to write a Technical Design Report (TDR).This TDR will include two major items as discussed in the previous status report for the 20th J-PARC PAC: (1) A realistic detector location (2) Well understood and realistic detector performance using simulation studies, primarily in consideration of fast neutron rejection. Since August we have been in discussions with MLF staff regarding an appropriate detector location. We are also in the process of setting up a Monte Carlo (MC) simulation framework in order to study detectors performance in realistic conditions. In addition, we have pursued hardware R&D work for the liquid scintillator (LS) and to improve the dynamic range of the 10 photomultiplier tubes (PMTs). The LS R&D works includes Cherenkov studies inside the LS, and a Pulse Shape Discrimination (PSD) study with a test-beam, performed at Tohoku University. We also estimate the PSD performance of a full-sized detector using a detailed MC simulation. In this status report, we describe progress on this work.
In this document, technical details of the upgrade plan of the J-PARC neutrino beamline for the extension of the T2K experiment are described. T2K has proposed to accumulate data corresponding to $2times{}10^{22}$ protons-on-target in the next decade, aiming at an initial observation of CP violation with $3sigma$ or higher significance in the case of maximal CP violation. Methods to increase the neutrino beam intensity, which are necessary to achieve the proposed data increase, are described.
The committee for the study of the extension of the Hadron Experimental Facility was formed under the Hadron Hall Users Association in August, 2015. This document is a summary of the discussions among the committee members, and documented by a part of the members listed below.
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