$mathbf{{textit K^-}{textit p}{textit p}}$, a ${overline{K}}$-Meson Nuclear Bound State, Observed in $^{3}{rm He}({K^-}, {Lambda} p)n$ Reactions

We observed a distinct peak in the $Lambda p$ invariant mass spectrum of $^{3}{rm He}(K^-, , Lambda p)n$, well below the mass threshold of $m_K + 2 m_p$. By selecting a relatively large momentum-transfer region $q = 350 sim 650$ MeV/$c$, one can clearly separate the peak from the quasi-free process, $overline{K}N rightarrow overline{K}N$ followed by the non-resonant absorption by the two spectator-nucleons $overline{K}NN rightarrow Lambda N $. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at $B_{rm {it Kpp}} = 47 pm 3 , (stat.) ,^{+3}_{-6} ,(sys.)$ MeV having a width $Gamma_{rm {it Kpp}} = 115 pm 7 , (stat.) ,^{+10}_{-9} ,(sys.)$ MeV, and the $S$-wave Gaussian reaction form-factor parameter $Q_{rm {it Kpp}} = 381 pm 14 , (stat.),^{+57}_{-0} ,(sys.)$ MeV/$c$, as a new form of the nuclear bound system with strangeness -- $K^-pp$.