Design of a new experiment for measuring the β-decay rate of Λ in hypernucleus

The baryon properties may be modified in nuclear matter. We consider that the baryon modification can be clarified by measuring the β-decay rate of Λ hypernucleus(Γ_β). According to the QMC model, Γ_β decreases by 20% at max in nuclear matter.

This experiment is planned at the J-PARC K1.1 beam line. We will produce ⁵ΛHe and measure Γ_β within 4.5% accuracy . The branching ratio of the Λ’s β-decay (~8.0×10^-4) is much smaller than that of the Λ’s main decay (Λ→pπ^- and Λ→nπ⁰ and nonmesonic decay), and therefore huge background will be a severe problem. To reduce this, the target is surrounded by a 4π BGO calorimeter, and the β-decay electron is identified by the number of hit clusters in the BGO. The β-decay electron produces one-cluster hit in the BGO. The background π^- stops in BGO and emits several neutrons after nuclear absorption. Therefore, we expect that it produces two or more hit clusters. The Background reduction is estimated by GEANT4 simulation. But there is no accurate data for stopped π^- absorption in BGO nuclei. Therefore, we need to measure neutron data from stopped π^- absorption in BGO.

To measure the data, we will use π^- from K^- beam decay at the J-PARC. In this talk, how to conduct this measurement and simulation results of this measurement will be reported.