“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 ³He(K⁻, Λp)n, well below the mass threshold of m_K + 2m_p. By selecting a relatively large momentum-transfer region q = 350 ∼ 650 MeV/c, one can kinematically separate the peak from the quasi-free process, KN → KN followed by the non-resonant absorption by the two spectator-nucleons KNN → ΛN. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at B_Kpp = 47 ± 3 (stat.) ₋⁺³₆ (sys.) MeV having a width Γ_Kpp = 115 ± 7 (stat.) ₋⁺¹⁰₂₀ (sys.) MeV, and the S-wave Gaussian reaction form-factor parameter Q_Kpp = 381 ± 14 (stat.) ₋⁺⁵⁷₀ (sys.) MeV/c, as a new form of the nuclear bound system with strangeness - “K⁻pp”.