We have observed a "K-pp" -like structure in the d(π+,K+) reaction at 1.69 GeV/c. In this reaction, a Λ (1405) hyperon resonance is expected to be produced as a doorway to form K-pp through the Λ ∗p→ K-pp process. However, most of the Λ (1405) produced would escape from the deuteron without secondary reactions. Therefore, coincidence of high-momentum (>250MeV/c) proton(s) at large emission angles (39° < θlab. < 122°) was requested to enhance the signal-to-background ratio. A broad enhancement in the proton coincidence spectra is observed around the missing mass of 2.27 GeV/c2, which corresponds to the K-pp binding energy of 95+18-17 (stat.) +30-21 (syst.) MeV and the width of 162+87-45 (stat.) +66-78 (syst.) MeV.
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