Precise lifetime measurement of ${}_{\mathrm{\Lambda }}^4$H hypernucleus using in-flight ${}^4$He$(K^{-},{\pi }^0{)}_{\mathrm{\Lambda }}^4$H reaction

We present a new measurement of the ${}_{\mathrm{\Lambda }}^4$H hypernuclear lifetime using in-flight $K^{-}$ + ${}^4$He $\to$ ${}_{\mathrm{\Lambda }}^4$H + ${\pi }^0$ reaction at the J-PARC hadron facility. We demonstrate, for the first time, the effective selection of the hypernuclear bound state using only the $\gamma$-ray energy decayed from ${\pi }^0$. This opens the possibility for a systematic study of isospin partner hypernuclei through comparison with data from ($K^{-}$, ${\pi }^{-}$) reaction. As the first application of this method, our result for the ${}_{\mathrm{\Lambda }}^4$H lifetime, $\tau {(}_{\mathrm{\Lambda }}^4\mathrm{H})=206\pm 8(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 12(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{s}$, is one of the most precise measurements to date. We are also preparing to measure the lifetime of the hypertriton (${}_{\mathrm{\Lambda }}^3$H) using the same setup in the near future.