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Very recently, we have performed a couple of experiments, {it{KEK PS-E549/E570}}, for the detailed study of the strange tribaryon $S^0(3115)$ obtained in {it{KEK PS-E471}}. These experiments were performed to accumulate much higher statistics with improved experimental apparatusespecially for the better proton spectroscopy of the $^4$He({it{stopped K}}$^-$, {it{N}}) reaction. In contrast to the previous proton spectrum, no narrow ($sim$ 20 MeV) peak structure was found either in the inclusive $^4$He({it{stopped K}}$^-$, {it{p}}) or in the semi-inclusive $^4$He({it{stopped K}}$^-$, {it{p}}$X^pm$) reaction channel, which is equivalent to the previous $E471$ event trigger condition. Detailed analysis of the present data and simulation shows that the peak, corresponding to $S^0(3115)$, has been an experimental artifact. Present analysis does not exclude the possible existence of a much wider structure. To be sensitive to such structure and for better understanding of the non-mesonic $K^-$ absorption reaction channel, detailed analysis of the data is in progress.
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We have measured proton and neutron energy spectra by stopping negative kaons on liquid helium4. Two distinct peak structures were found on both spectra, which were assigned to the formation of new kinds of strange stribaryons. In this paper, we summarize both results.
We have investigated correlations of coincident $Lambda N$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $Lambda p$ and $Lambda n$ branches of the two-nucleon absorption process in the $Lambda N$ invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.
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 -- $K^-pp$.
The search of nuclear bound states of anti-K in few-body nuclear systems such as K-pp, can be extended from the nuclear medium to the vacuum, using the glue-rich Y(1S) decays at B-factories. Here the possibility for such a measurement at the future SuperB factory is discussed.
We have performed an exclusive measurement of the $K^{-}+! ~^{3}{rm He} to Lambda pn$ reaction at an incident kaon momentum of $1 {rm GeV}/c$.In the $Lambda p$ invariant mass spectrum, a clear peak was observed below the mass threshold of $bar{K}!+!N!+!N$, as a signal of the kaonic nuclear bound state, $bar{K}NN$.The binding energy, decay width, and $S$-wave Gaussian reaction form-factor of this state were observed to be $B_{K} = 42pm3({rm stat.})^{+3}_{-4}({rm syst.}) {rm MeV}$, $Gamma_{K} = 100pm7({rm stat.})^{+19}_{-9}({rm syst.}) {rm MeV}$, and $Q_{K} = 383pm11({rm stat.})^{+4}_{-1}({rm syst.}) {rm MeV}/c$, respectively. The total production cross-section of $bar{K}NN$, determined by its $Lambda p$ decay mode, was $sigma^{tot}_{K} cdot BR_{Lambda p} = 9.3pm0.8({rm stat.})^{+1.4}_{-1.0}({rm syst.}) mu{rm b}$.We estimated the branching ratio of the $bar{K}NN$ state to the $Lambda p$ and $Sigma^{0}p$ decay modes as $BR_{Lambda p}/BR_{Sigma^{0}p} sim 1.7$, by assuming that the physical processes leading to the $Sigma N!N$ final states are analogous to those of $Lambda pn$.
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