“K?pp”, a K￣-meson nuclear bound state, observed in 3He(K?,Λp)n reactions

摘要

We observed a distinct peak in the Λpinvariant mass spectrum ofHe3(K?,Λp)n, well belowmK+2mp,i.e., the mass threshold of theK?to be bound to two protons. By selecting a relatively large momentum-transfer regionq=350～650MeV/c, one can kinematically separate the peak from the quasi-free process,K￣N→K￣Nfollowed by the non-resonant absorption by the two spectator-nucleonsK￣NN→ΛN. We found that the simplest fit to the observed peak gives us a Breit–Wigner pole position atBKpp=47±3(stat.)?6+3(sys.)MeV having a widthΓKpp=115±7(stat.)?20+10(sys.)MeV, and theS-wave Gaussian reaction form-factor parameterQKpp=381±14(stat.)?0+57(sys.)MeV/c, as a new form of the nuclear bound system with strangeness – “K?pp”.