Session EA: Strangeness in Nuclear Physics

Study of spectroscopy of $\Lambda $-hypernuclei using the (e,e'K$^{+})$ reaction at JLAB

Electroproduction using high precision continuous-wave electron beam, such as available at Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson National Laboratory (JLAB), has proved to be an effective mechanism to study the spectroscopy of $\Lambda $-hypernuclei. In comparison to the production using secondary meson beams, the present use of precision electron beam improves the energy resolution by more than a factor of two, reaching sub-MeV level while approximately preserving the yield. In addition, the (e,e'K$^{+})$ reaction acts on protons, dominantly producing high spin stretched, spin-flip states of neutron rich hypernuclei which are in complementary to that produced by the (K$^{-}$, $\pi ^{-})$ and ($\pi ^{+}$, K$^{+})$ reactions. The precision and power of the beam enables a detailed spectroscopic study of $\Lambda $-hypernuclei for a wide range of target masses and selected isotopes. The program will provide new information on the $\Lambda $N interaction, SU(3)-flavor symmetry in the nuclei, and the single particle nature of a $\Lambda$ in the nuclear mean-field. This presentation will give an overview of the spectroscopy programs carried out in both Hall A and C with their presently achieved results.   

Hyperon Interactions from Lattice QCD

Current status of the the full QCD simulations of the hyperon(Y)-nucleon(N) and hyperon(Y)-hyperon(Y) interactions on the lattice is reviewed.   

Study of the $K^-pp$ bound state in the FINUDA experiment

The possible existence of antikaon-nuclear bound states has been suggested by many theoretical studies in this decade, after the first quantitative calculations on few-body systems by Akaishi and Yamazaki [Phys. Rev. C 65, 044005 (2002)]. Experimental searches, including reanalyses of old experiments, on such a bound state has been carried out in several institutes up to now. The FINUDA experiment also investigated the existence of light kaonic nuclei, produced by stopped $K^-$ absorption. It was carried at a $\phi$-factory DA$\Phi$NE at INFN-LNF (Italy), which supplies very slow kaons ($\sim 16\,\mathrm{MeV}$) as decay particles of $\phi$ mesons produced by the electron-positron collision. In 2005, we reported the first result on the invariant mass spectrum of back-to-back $\Lambda$-p pairs, emitted from light nuclear targets [Phys. Rev. Lett. 94, 212303 (2005)]. Their invariant mass distributes far below the $K^-+p+p$ threshold around $2.37\,\mathrm{GeV}/c^2$, and we proposed a possibility that a $K^-pp$ bound state with its binding energy $\sim 115\,\mathrm{MeV}$ and width $\sim 67\,\mathrm{MeV}$ was produced by kaon absorption, and decayed into a $\Lambda$ and a proton. However, there are alternative interpretations on the $\Lambda$-$p$ invariant-mass spectrum, such as the effect of final state interaction, pointed out by Magas \textit{et al.} [Phys. Rev. C 74, 025206 (2006)] In order to distinguish them experimentally, we analyzed three kinds of back-to-back hyperon-nucleon pairs ($\Lambda$-$p$, $\Lambda$-$n$, $\Sigma^-$-$p$) with about one order of magnitude more statistics taken in 2006--2007. We observed a large difference, especially between the $\Lambda$-$p$ and $\Lambda$-$n$ pairs, with regard to the distribution near the threshold. It may originate from a strong isospin dependence of $\overline{K}N$ interaction, and reinforce the assumption of the $K^-pp$ production in kaon absorption. In this talk, the current status of the analysis on hyperon-nucleon pairs will be presented.   

Exclusive Hyperon Production at CLAS

This talk will provide an overview of the exclusive hyperon production experiments being carried out by the CLAS Collaboration at Jefferson Laboratory. The program is designed to measure cross sections and a complete set of beam, target, and recoil hyperon polarization observables for both $K\Lambda$ and $K\Sigma$ final states with beam energies up to 6~GeV. These data will span a broad kinematic range in momentum transfer $Q^2$ and invariant energy $W$, and nearly the full center-of-mass angular range of the kaon. Analyses of data on both proton and neutron targets are underway or planned for the near future involving polarized beams (longitudinally polarized electrons, circularly and linearly polarized photons) and polarized targets (longitudinally and transversely polarized). The main goal of this series of measurements is to provide precision data needed to disentangle the resonant and non-resonant amplitudes in the intermediate state to uncover baryon resonances that couple to the strangeness channels ($N^* \to K\Lambda$, $K\Sigma$, $\Delta^* \to K\Sigma$). This will improve our understanding of the nucleon excitation spectrum, and hence, probe the effective degrees of freedom of the nucleon. These data will also serve to improve our understanding of the dynamics underlying strangeness production in general. Over the next several years it is expected that a full coupled-channels analysis, including hadroproduction data, as well as data with both real and virtual photons, will be completed by several groups, including the Excited Baryon Analysis Center (EBAC) at Jefferson Laboratory. The precision data from CLAS will be a crucial input for these analyses. Within the next five years, the CLAS facility will be significantly upgraded for use with the higher-energy electron beams available with the 12-GeV Jefferson Laboratory energy upgrade. The future of the strangeness physics program with the new CLAS12 facility will be highlighted.