Precise Mass Measurement of Hypertriton via Decay Pion Spectroscopy

Decay pion spectroscopy provides a direct and model-independent approach to the precise mass measurement of light hypernuclei, such as the hypertriton (3ΛH), through the monochromatic momentum of pions emitted in two-body weak decays. In this experiment at the Mainz Microtron (MAMI), decay pions from stopped hypernuclei produced in the p(e,e’K+)Λ reaction on a natural lithium target were measured with Spectrometer A (SpekA). Since the achievable precision of the hypertriton mass is directly determined by the accuracy of the pion momentum, this proceedings focuses on the momentum calibration essential for decay pion spectroscopy. A comprehensive calibration based on elastic electron scattering was performed using 181Ta and 12C targets. The reconstructed scattering angle, momentum linearity over the full spectrometer acceptance, and the dependence on the reaction point along the beam axis were systematically evaluated and corrected. These procedures constrained the systematic uncertainty of the elastic-scattering-based momentum calibration to below 10 keV, excluding the beam-energy contribution. The established calibration provides the foundation for a high-precision determination of the hypertriton mass using decay pion spectroscopy.