Design of a weak decay π^- detector for ⁴H γ-ray spectroscopy

The charge symmetry breaking (CSB) effect in the A=4 hypernuclei is a hot topic in strangeness nuclear physics. Recently, we observed a γ transition for ⁴_ΛHe(1⁺→0⁺) at 1.406 ± 0.004 MeV in γ-ray spectroscopy experiment (J-PARC E13) with Ge detectors. Conversely, the energy splitting of ⁴_ΛH(1⁺→0⁺) measured by NaI detectors is 1.09 ± 0.02 MeV. These results indicated existence of CSB in the ΛN interaction. We are planning to measure the energy of ⁴_ΛH(1⁺→0⁺) more precisely with Ge detectors in the next experiment (J-PARC E63) to understand the origin of CSB.

The ⁴_ΛH(1⁺) state is produced via ⁷Li(K^-,π^-) reaction as a hyperfragment from highly excited unbound ⁷_ΛLi states and the γ rays from ⁴_ΛH(1⁺) is detected by Ge detectors. Moreover, we will install range counters around the target to detect the π^- mesons from two-body weak decay of the ⁴_ΛH(0⁺). Using the ⁷_ΛLi missing mass and the π^- energy, we can accurately identify the γ rays from ⁴_ΛH(1⁺) among various hyperfragments such as ³_ΛH, ⁴_ΛHe and so on. Furthermore, we are considering lifetime measurement of light hypernuclei by installing the timing and the tracking detectors additionally.

We are simulating decay π^- detection to design the counters. In this talk, we will report the simulation result.