Observation of Spin-Dependent Charge Symmetry Breaking in ΛN Interaction: Gamma-Ray Spectroscopy of _{Λ}^{4}He.

The energy spacing between the spin-doublet bound state of He-4(Lambda)(1(+), 0(+)) was determined to be 1406 +/- 2 +/- 2 keV, by measuring gamma rays for the 1(+) -> 0(+) transition with a high efficiency germanium detector array in coincidence with the He-4(K-, pi(-))(Lambda)(4). He reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus H-4(Lambda), the present result clearly indicates the existence of charge symmetry breaking (CSB) in Lambda N interaction. By combining the energy spacings with the known ground-state binding energies, it is also found that the CSB effect is large in the 0(+) ground state but is vanishingly small in the 1(+) excited state, demonstrating that the Lambda N CSB interaction has spin dependence.