"${\mathit{\text{K}}}^{-}\mathit{\text{pp}}$", a $\bar{K}$-Meson Nuclear Bound State, Observed in ${}^3\mathrm{H}\mathrm{e}(K^{-},\mathrm{\Lambda }p)n$ Reactions

We observed a distinct peak in the $\mathrm{\Lambda }p$ invariant mass spectrum of ${}^3\mathrm{H}\mathrm{e}(K^{-},\mathrm{\Lambda }p)n$, well below the mass threshold of $m_K + 2 m_p$\mathrm{.}Byselectingarelativelylargemomentum-transferregion$q = 350 \sim 650$MeV\mathrm{/}$c$,onecanclearlyseparatethepeakfromthequasi-freeprocess,$\overline{K}N \rightarrow \overline{K}N$ followed by the non-resonant absorption by the two spectator-nucleons $\bar{K}NN\to \mathrm{\Lambda }N$. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at $B_{\rm {\it Kpp}} = 47 \pm 3 , (stat.) ,^{+3}{-6} ,(sys.)$MeVhavingawidth$\Gamma{\rm {\it Kpp}} = 115 \pm 7 , (stat.) ,^{+10}_{-9} ,(sys.)$MeV,andthe$S$-wave Gaussian reaction form-factor parameter $Q_{Kpp}=381\pm 14(stat\mathrm{.}){}_{-0}^{+57}(sys\mathrm{.})$ MeV/$c$, as a new form of the nuclear bound system with strangeness – “$K^{-}pp$”.