We observed a distinct peak in the Λp invariant mass spectrum of He3(K,Λp)n, well below mK+2mp, i.e., the mass threshold of the K to be bound to two protons. By selecting a relatively large momentum-transfer region q=350∼650 MeV/c, one can kinematically separate the peak from the quasi-free process, K‾N→K‾N followed by the non-resonant absorption by the two spectator-nucleons K‾NN→ΛN. We found that the simplest fit to the observed peak gives us a Breit–Wigner pole position at BKpp=47±3(stat.)−6 +3(sys.) MeV having a width ΓKpp=115±7(stat.)−20 +10(sys.) MeV, and the S-wave Gaussian reaction form-factor parameter QKpp=381±14(stat.)−0 +57(sys.) MeV/c, as a new form of the nuclear bound system with strangeness – “Kpp”.

Original languageEnglish
Pages (from-to)620-625
Number of pages6
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
Publication statusPublished - 2019 Feb 10


  • Kaon
  • Mesonic nuclear bound state
  • Strangeness


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