H. Noumi

The serious inconsistency problem between the values of experimental and theoretical Γn /Γp ratio of the nonmesonic weak decay of Λ hypernuclei has been resolved recently. We have shown that the reason behind the problem was the quenching of the nucleon yields which in turn was due to the contribution of the three-body process. We have measured that the branching ratio of the three-body process in nonmesonic weak decay, 0.29±0.13, is so large that the absolute values of Γn and Γp must be determined taking account of the contribution of the 3-body process. In this paper, the recent studies toward the finding of the 3-body process via the exclusive coincidence experiments are presented.

More than 50 radiation-resistant electromagnets were constructed for the primary proton and the secondary particle beam lines of the Hadron Experimental Hall of Japan Proton Accelerator Research Complex (J-PARC). The main radiation-resistant technologies we employed were the Polyimide-resin Insulation conductor for magnets at the relatively low radiation exposure and the Mineral Insulation Cable for magnets at the seriously high radiation environment. The remote handling and maintenance scheme of radiation-resistant magnets for seriously high radiation environment was developed also based on the Chimney magnet technology and applied to magnets near the production target in the Hadron Hall. On January 27th 2009, the first proton beam was successfully introduced to the Hadron Hall from the main accelerator of J-PARC, i.e. 50 GeV Proton Synchrotron. On February 10th, the secondary particles were extracted to the experimental area of the Hadron Hall through the secondary particle beam line. No serious problem happened on magnets of both primary proton and the secondary particle beam lines until the end of the beam operation scheduled on February 26th .

The high intensity proton accelerator facility J-PARC is now in its final stage of construction. Soon intense primary proton beams will be slowly extracted to an experimental hall (HD-hall) and transported to a production target. Since the radiation level around the target will be very high, the beam line tunnel needs to be surrounded by 2.5 m-5 m thick concrete shields in the HD-hall. Radiation resistant magnets for handling high intensity beams are buried inside these thick concrete shields. This structure, however, brought us a problem of leading electric power lines to the magnets. For a solution to this problem, we have developed water cooled bus ducts penetrating radiation shield. These bus ducts enable us to connect the power supplies placed outside the shields to the magnets in the shortest power line length without any deterioration of protection ability of radiation shielding. The maximum power capacity of the bus duct is over 5000 A. The bus ducts plays an important part in our radiation resistant magnet system.

We would like to point out that the (K^-,N) reactions are particularly useful for the study of the K-nucleus interaction. We have measured the missing mass spectra derived from the momenta of both neutrons and protons from the ¹² C(K^-,N) reactions. It was found that an appreciable amount of strength were below the K-nucleus threshold which indicates that the K-nuclear potential is strongly attractive. The missing mass spectra are compared with theoretical calculations. It is found that a potential depth of around -190 MeV reproduces well the spectrum of the ¹²C(K^-,n) reaction and a less deep potential of around -160 MeV reproduces well that of the ¹²C(K^-,p) reaction. Our data show that the K-nucleus potential is very deep to realize kaon condensation in the core of neutron stars.

We made a series of surveys to obtain neutrino beam line direction toward SuperKamiokande (SK) at a distance of 250 km for the long-baseline neutrino oscillation experiment at KEK. We found that the beam line is directed to SK within 0.03 mr and 0.09 mr (in sigma) in the horizontal and vertical directions, respectively. During beam operation, we monitored the muon distribution from secondary pions produced at the target and collected by a magnetic horn system. We found that the horn system functions like a lens of a point-to-parallel optics with magnification of approximately -100 and the focal length of 2.3 m. Namely, a small displacement of the primary beam position at the target is magnified about a factor -100 at the muon centroid, while the centroid position is almost stable against a change of the incident angle of the primary beam. Therefore, the muon centroid can be a useful monitor of the neutrino beam direction. We could determine the muon centroid within 6 mm and 12 mm in horizontal and vertical directions, respectively. This means that the neutrino beam direction could be controlled within 0.03 mr and 0.06 mr (in sigma) in the horizontal and vertical directions, respectively. We confirmed the beam direction with the neutrino distribution reconstructed at the near detector in KEK.