The three-body nonmesonic weak decay process of _Λ ¹²C hypernucleus and its exclusive measurement at J-PARC (E18)

In spite of the recent crucial progresses in the studies of nonmesonic weak decay (NMWD) of Λ hypernuclei, its consistent understanding is yet to be achieved. In the experimental progresses, the long standing Γ_n/Γ_p problem has finally been solved recently. And for the first time, two-nucleon induced NMWD channel has been identified experimentally and its decay width, 0.27±0.13, measured but only with a 2σ confidence level. On the other hand, the decay asymmetry remains to be understood yet and the accurate measurement of the decay width of each channel is much awaited for the consistent understanding of NMWD. In E18 of J-PARC, we are going to measure Γ_i of NMWD including Γ_2N with 10 % statistical uncertainties. Main concerns in E18 experiment such as the handling of the high beam intensity, the neutron background contamination in the neutron coincidence yields and the control of the trigger rates with the beam intensity as high as 10⁷π ⁺/spill are discussed and it is shown that they can be handled properly.