"${\text{K}}^{-}\text{p}\text{p}$", a $\overline{K}$-Meson Nuclear Bound State, Observed in ${}^3\mathrm{He}(K^{-},\hat{I}›p)n$ Reactions

Abstract

We observed a distinct peak in the $\hat{I}›p$ invariant mass spectrum of ${}^3\mathrm{He}(K^{-},\hat{I}›p)n$, well below the mass threshold of $m_K+2m_p$. 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, $\overline{K}N\to \overline{K}N$ followed by the non-resonant absorption by the two spectator-nucleons $\overline{K}NN\to \hat{I}›N$. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at $B_{Kpp}=47\pm 3(stat.){}_{-6}^{+3}(sys.)$ MeV having a width $\hat{I}{“}_{Kpp}=115\pm 7(stat.){}_{-9}^{+10}(sys.)$ MeV, and the $S$-wave Gaussian reaction form-factor parameter $Q_{Kpp}=381\pm 14(stat.){}_{-0}^{+57}(sys.)$ MeV/$c$, as a new form of the nuclear bound system with strangeness -- "$K^{-}pp$".

17 pages, 4 figures