At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a 9 Be target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at zero degree forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. In coincidence, negatively charged decay-pions were detected by two independent high-resolution spectrometers. About 10 3 pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at p π ≈ 133 MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen 4 Λ H → 4 He + π − , stopped inside the target. Its Λ binding energy was determined to be B Λ = 2.12 ± 0.01 (stat.) ± 0.09 (syst.) MeV with respect to the 3 H + Λ mass.

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