Precise measurement of the $\mathrm{\Lambda }$-binding energy difference between ${}_{\mathrm{\Lambda }}^3$H and ${}_{\mathrm{\Lambda }}^4$H via decay-pion spectroscopy at MAMI

We performed high-precision decay-pion spectroscopy of light $\mathrm{\Lambda }$ hypernuclei at the Mainz Microtron (MAMI) using the A1 spectrometer facility. By measuring the monochromatic ${\pi }^{-}$ momentum from the two-body weak decay ${}_{\mathrm{\Lambda }}^3\mathrm{H}\to {}^3\mathrm{H}\mathrm{e}+{\pi }^{-}$ and referencing it to the ${}_{\mathrm{\Lambda }}^4\mathrm{H}\to {}^4\mathrm{H}\mathrm{e}+{\pi }^{-}$ decay, we determined the $\mathrm{\Lambda }$ binding energy of ${}_{\mathrm{\Lambda }}^3\mathrm{H}$ with unprecedented accuracy. The obtained value, $B_{\mathrm{\Lambda }}{(}_{\mathrm{\Lambda }}^3\mathrm{H})=0.523\pm 0.013(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.075(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$~MeV, is consistent with the STAR result, but indicates a significantly deeper binding than inferred from earlier measurements. This result implies a stronger $\mathrm{\Lambda }$-deuteron interaction and provides stringent constraints on hyperon-nucleon interactions.