'K−pp', a K‾-meson nuclear bound state, observed in 3He(K−,Λp)n reactions

We observed a distinct peak in the Λp invariant mass spectrum of He 3 ( K − , Λ p ) n , well below m K + 2 m p , 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 ‾ N N → Λ N . We found that the simplest fit to the observed peak gives us a Breit–Wigner pole position at B Kpp = 47 ± 3 ( s t a t . ) − 6 + 3 ( s y s . ) MeV having a width Γ Kpp = 115 ± 7 ( s t a t . ) − 20 + 10 ( s y s . ) MeV, and the S-wave Gaussian reaction form-factor parameter Q Kpp = 381 ± 14 ( s t a t . ) − 0 + 57 ( s y s . ) MeV/c, as a new form of the nuclear bound system with strangeness – “ K − p p ”.