Observation of the "K- pp"-like structure in the d( +, K+) reaction at 1.69 GeV/c

Abstract

We have observed a "$K^-pp$"-like structure in the $d(\\pi^+,K^+)$ reaction at\n1.69 GeV/$c$. In this reaction $\\Lambda(1405)$ hyperon resonance is expected to\nbe produced as a doorway to form the $K^-pp$ through the $\\Lambda^*p\\rightarrow\nK^-pp$ process. However, most of the produced $\\Lambda(1405)$'s would escape\nfrom deuteron without secondary reactions. Therefore, coincidence of\nhigh-momentum ($>$ 250~MeV/$c$) proton(s) in large emission angles\n($39^\\circ<\\theta_{lab.}<122^\\circ$) was requested to enhance the\nsignal-to-background ratio. A broad enhancement in the proton coincidence\nspectra are observed around the missing-mass of 2.27 GeV/$c^2$, which\ncorresponds to the $K^-pp$ binding energy of 95 $^{+18}_{-17}$ (stat.)\n$^{+30}_{-21}$ (syst.) MeV and the width of 162 $^{+87}_{-45}$ (stat.)\n$^{+66}_{-78}$ (syst.) MeV.\n