At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a ${}^9\mathrm{Be}$ target in order to study the $\mathrm{\Lambda }$ 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 $p_{\pi }\approx 133\mathrm{MeV}/c$, corresponding to the unique signature for the two-body decay of hyperhydrogen ${}_{\mathrm{\Lambda }}{}^4\mathrm{H}\to {}^4\mathrm{He}+{\pi }^{-}$, stopped inside the target. Its $\mathrm{\Lambda }$ binding energy was determined to be $B_{\mathrm{\Lambda }}=2.12\pm 0.01(\text{stat})\pm 0.09(\text{syst})\mathrm{MeV}$ with respect to the ${}^3\mathrm{H}+\mathrm{\Lambda }$ mass.

• Received 27 January 2015

DOI:https://doi.org/10.1103/PhysRevLett.114.232501