K- pp, a (K)over-bar-meson nuclear bound state, observed in 3He(K-, Λp)n reactions

We observed a distinct peak in the Ap invariant mass spectrum of He-3(K-, Lambda p)n, well below m(K) + 2m(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 similar to 650 MeV/c, one can kinematically separate the peak from the quasi-free process, (K) over barN -> (K) over barN followed by the non-resonant absorption by the two spectator-nucleons (K) over bar NN -> Lambda N. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at B-Kpp = 47 +/- 3 (stat.)(-6)(+3)(sys.) MeV having a width Gamma(Kpp) = 115 +/- 7 (stat.)(-20)(+10)(sys.) MeV, and the S-wave Gaussian reaction form-factor parameter Q(Kpp) = 381 +/- 14 (stat.)(-0)(+57)(sys.) MeV/c, as a new form of the nuclear bound system with strangeness - K pp. C) 2018 Published by Elsevier B.V.