High-Resolution Decay-Pion Spectroscopy of $_\Lambda ^4\text{H}$ Hypernuclei

The first observation of \({}_{\Lambda }^{4}\text{H}\) fragments by means of decay-pion spectroscopy with a high resolution magnetic spectrometer was achieved in 2012 at the Mainz Microtron MAMI. The extracted Λ binding energy was consistent with older nuclear emulsion data, but almost one order of magnitude higher in precision, while being limited by systematic uncertainties. This paper gives details on the kaon tagging at 0° forward angle which was indispensable for the success of the experiment. In addition, a re-analysis of the data collected in 2012 is presented, including an improved a posteriori spectrometer calibration and a more robust fitting of the decay-pion peak in the momentum spectrum.