Structure near K−K^-+pp+pp threshold in the in-flight 3^3He(K−,Λp)n(K^-,\Lambda p)n reaction

To search for an S= -1 di-baryonic state which decays to $\Lambda p$, the ${\rm{}^3He}(K^-,\Lambda p)n_{missing}$ reaction was studied at 1.0 GeV/$c$. Unobserved neutrons were kinematically identified from the missing mass $M_X$ of the ${\rm{}^3He}(K^-,\Lambda p)X$ reaction in order to have a large acceptance for the $\Lambda pn$ final state. The observed $\Lambda p n$ events, distributed widely over the kinematically allowed region of the Dalitz plot, establish that the major component comes from a three nucleon absorption process. A concentration of events at a specific neutron kinetic energy was observed in a region of low momentum transfer to the $\Lambda p$. To account for the observed peak structure, the simplest S-wave pole was assumed to exist in the reaction channel, having Breit-Wigner form in energy and with a Gaussian form-factor. A minimum $\chi^2$ method was applied to deduce its mass $M_X\ =$ 2355 $^{+ 6}_{ - 8}$ (stat.) $\pm 12$ (syst.) MeV/c$^2$, and decay-width $\Gamma_X\ =$ 110 $^{+ 19}_{ - 17}$ (stat.) $\pm 27$ (syst.) MeV/c$^2$, respectively. The form factor parameter $Q_X \sim$ 400 MeV/$c$ implies that the range of interaction is about 0.5Comment: 12pages, 8 figure

Observation of a K̄ NN bound state in the He 3 (K-, Λp)n reaction OBSERVATION of A K̄ NN BOUND STATE ... T. YAMAGA et al.

We have performed an exclusive measurement of the K-+He3→Λpn reaction at an incident kaon momentum of 1GeV/c. In the Λp invariant mass spectrum, a clear peak was observed below the mass threshold of K̄+N+N, as a signal of the kaonic nuclear bound state, K̄NN. The binding energy, decay width, and S-wave Gaussian reaction form factor of this state were observed to be BK=42±3(stat.)-4+3(syst.)MeV, ΓK=100±7(stat.)-9+19(syst.)MeV, and QK=383±11(stat.)-1+4(syst.)MeV/c, respectively. The total production cross section of K̄NN, determined by its Λp decay mode, was σKtotBRΛp=9.3±0.8(stat.)-1.0+1.4(syst.)μb. We estimated the branching ratio of the K̄NN state to the Λp and ς0p decay modes as BRΛp/BRς0p∼1.7, by assuming that the physical processes leading to the ςNN final states are analogous to those of Λpn. © 2020 American Physical Society

“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 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 – “K−pp”