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

We have observed a "K- pp"-like structure in the d( pi(+), K+) reaction at 1.69 GeV/c. In this reaction, a Lambda (1405) hyperon resonance is expected to be produced as a doorway to form K- pp through the Lambda * p -> K pp process. However, most of the Lambda (1405) produced would escape from the deuteron without secondary reactions. Therefore, coincidence of high-momentum (> 250MeV/c) proton(s) at large emission angles (39 degrees < theta(lab). < 122 degrees) 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/c(2), 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.