Observation of H Λ 4 Hyperhydrogen by Decay-Pion Spectroscopy in Electron Scattering

At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a ^9Be 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 0° forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. Coincidentally, 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_ΛH → ^4He + π^−, stopped inside the target. Its Λ binding energy was determined to be B_Λ = 2.12 ± 0.01 (stat) ± 0.09 ðsystÞMeV with respect to the ^3H + Λ mass.