Ground-state binding energy of HΛ4 from high-resolution decay-pion spectroscopy

A systematic study on the Λ ground state binding energy of hyperhydrogen HΛ4 measured at the Mainz Microtron MAMI is presented. The energy was deduced from the spectroscopy of mono-energetic pions from the two-body decays of hyperfragments, which were produced and stopped in a 9Be target. First data, taken in the year 2012 with a high resolution magnetic spectrometer, demonstrated an almost one order of magnitude higher precision than emulsion data, while being limited by systematic uncertainties. In 2014 an extended measurement campaign was performed with improved control over systematic effects, increasing the yield of hypernuclei and confirming the observation with two independent spectrometers and two targets of different thicknesses. The analysis of these data is in agreement with the previously published value for the HΛ4 binding energy as well as with a consistent re-analysis of the 2012 data. When compared to the HeΛ4 binding energy from emulsion data, a large charge symmetry breaking effect in the A=4A=4 hypernuclear system is confirmed.