Session BJ: Mini-Symposium on Progress in Strangeness Physics I

Spectroscopy of S = -1 hypernuclei at KEK, BNL and J-PARC

The hypernuclear physics program at J-PARC will start soon. Taking this occasion, I will summarize what we have achieved in the spectroscopy of $\Lambda$ hypernuclei at KEK-PS and BNL-AGS using meson beams, where the SKS spectrometer and the Ge detector array, Hyperball, have played essential roles. The ($\pi^+$,$K^+$) reaction spectroscopy data in a wide mass range clearly demonstrated single-particle orbits of a $\Lambda$ even in a heavy nucleus and revealed properties of the $\Lambda$'s nuclear potential. Then almost full set of p-shell $\Lambda$ hypernuclear gamma-ray data provided the strengths of each of the $\Lambda$-N spin-dependent forces (spin-spin,spin-orbit, and tensor interactions). In addition, the ($\pi^-$,$K^+$) reaction was successfully introduced to observe neutron rich hypernuclei as well as to study $\Sigma$-nucleus interaction. Future perspectives at the J-PARC 50 GeV proton synchrotron are also discussed. At J-PARC, the K1.8 beam line and the SKS spectrometer are almost ready to get the first beam. We plan gamma-ray spectroscopy experiments covering a wide mass range, from $^4_{\Lambda}$He to sd-shell hypernuclei such as $^{19}_{\Lambda}$F, and then even heavier ones, using a newly-constructed Ge detector array, Hyperball-J. The ($\pi^-$,$K^+$) spectroscopy of neutron-rich hypernuclei will be also exploited. One of the physics motivations of these experiments is to investigate the three-body $\Lambda$$N$$N$ force caused by $\Sigma$$N$- $\Lambda$$N$ coupling. We also try to extend the hypernuclear chart toward the neutron drip line and to investigate possible modifications of deformation induced by a $\Lambda$. In future, more S=-1 hyperbnuclear experiments are also planned at the K1.1 beam line and the high-resolution pion beam line with the dispersion matching technique.   

$\gamma$-ray spectroscopy of $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C: Results of the KEK E566 experiment

Bound excited states of $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C were populated following the ($\pi^+$, $K^+$)$^{12}_{\Lambda}$C/ $^{11}_{\Lambda}$B+p reaction. The 12-GeV/c primary proton beam was provided by KEK-PS and the secondary $\pi^+$ beams of 1.05 GeV/c were produced and momentum analyzed by the K6 beam line bombarding a 18.6-g/cm$^2$ Carbon target. A momentum of the scattered K$^+$ was subsequently tracked by the SKS spectrometer system. $\gamma$ rays from hypernuclei produced were detected by the Ge detector array, Hyperbal2, which consisted of 14 standard closed end-type and 6 Clover-type detectors each surrounded by BGO background suppressing counters. Construction of missing mass spectrum identified the bound state as well as the proton emitting unbound excited states of $^{12}_{\Lambda}$C, the latter leading to $^{11}_{\Lambda}$B. $\gamma$ rays in coincidence with these regions were associated with the respective hypernucleus. Six $\gamma$-ray transitions, three of each belonging to $^{11}_{\Lambda}$B and $^{12}_{\Lambda}$C, were observed. Cascade decays of $^{11}_{\Lambda}$B($3/2^+ \rightarrow 1/2^+ \rightarrow 5/2^+$) and the ground state doublet spacing of $^{12}_{\Lambda}$C were newly identified from the present analysis. In addition, a 157-keV $\gamma$ ray from a hyperfragment of $^{12}_{\Lambda}$C was observed. From the energy level spacings we check the consistency of the strength of spin-spin ($\Delta$) and nuclear-spin orbit (S$_N$) in the effective $\Lambda$N interaction of the $p$-shell hypernuclei. Results of the analysis and the discussion will be presented.   

Shell model study of typical sd-shell hypernuclei

Theoretical and experimental studies of $s$- and $p$-shell hypernuclei have been performed and, in the future, experiments of $\mathit{sd}$-shell hypernuclei will be carried out at \mbox{J-PARC}. The level structures of $\mathit{sd}$-shell nuclei are richer and more complex than those of $p$-shell nuclei. For example, the states of ${}_{}^{19}\mathrm{F}$ have the following structures; (i) the energy difference between the $1/2_{}^{+}$ ground state and the $1/2_{}^{-}$ first-excited state is only $0.110$ $\mathrm{MeV}$ and (ii) a rotational band is seen in the energy spectrum. Thus we are interested in effects of the $\Lambda N$ interaction on the parity doublet and the rotational band in the $\mathit{sd}$-shell hypernuclei. Also, an effects of a positive pairing correlation in the $\Lambda N$ interaction may reveal in the structures of the $\mathit{sd}$-shell hypernuclei because of the $0d_{5/2}^{}$ orbit with the higher spin. In this presentation, we will discuss the structures of ${}_{\;\Lambda}^{19}\mathrm{F}$ and ${}_{\;\Lambda}^{20}\mathrm{F}$ obtained by shell-model calculations with $0 \hbar \omega$ and $1 \hbar \omega$ model spaces.   

The Impurity Effects in sd-Shell $\Lambda $ Hypernuclei

In this talk we will report the impurity effects in \textit{sd}-shell $\Lambda $ hypernuclei based on the theoretical model of Antisymmetrized Molecular Dynamics (AMD). One of the unique aspects of hypernuclei is the structure change caused by hyperon as an impurity. In particular, we can expect the drastic structure change in \textit{sd}-shell $\Lambda $ hypernuclei, since several \textit{sd}-shell nuclei have the coexistence of various structures within very small excitation energy. To study such impurity effects in detail, we have extended AMD for hypernuclei. Since this extended AMD does not make any assumption on cluster structure, it makes possible to investigate the difference of the impurity effect for the shell and cluster structure. We have studied the structure change of \textit{sd}-shell hypernuclei by this model using YNG interaction. For example, in the case of $^{20}_{\Lambda }$Ne, we have found that the addition of $\Lambda $ has opposite effect to the preceding study. Such calculation has been performed for C, F, Ne hypernuclei and we will discuss the structure change and its dependence on the core state and the $\Lambda $ particle orbital.   

High Resolution Hypernuclear Spectroscopy by the (e,e'K$^{+}$) Reaction (JLab E01-011)

$\Lambda$ hypernuclear spectroscopy by the (e,e$'$K$^{+}$) reaction is a powerful tool to investigate $\Lambda$N interaction because this reaction excites various states up to deep inside of hypernucleus and sub-MeV resolution can be achieved thanks to the high quality primary electron beam from CEBAF at JLab. The second generation hypernuclear spectroscopy at JLab Hall C, E01-011, was successfully performed in the summer of 2005 introducing High resolution Kaon Spectrometer (HKS) and a new configuration for scattered electron spectrometer. These unique techniques significantly improved both energy resolution and hypernuclear tagging efficiency, and we succeeded to study various hypernuclei including $^{7}_{\Lambda}$He and $^{28}_{\Lambda}$Al with high resolution and sufficient statistics for the first time by this reaction. The analysis is now in the final stage and systematic errors of binding energy and cross section were estimated with a help of the detailed Monte Carlo simulation. The overview and recent result of E01-011 experiment will be presented in this talk.   

Strength Functions for Photoproduction of Medium-Mass Hypernuclei

Strength functions have been calculated for the photoproduction of $\Lambda$-hypernuclei by choosing typical medium-mass nuclear targets such as $^{28}$Si, $^{40}$Ca, and $^{52}$Cr. The DWIA framework has been adopted together with the modern amplitudes for the elementary $\gamma p\rightarrow \Lambda K^+$ process. For the targets with surface proton $jj$-closed orbit (or the similar situation), the unnatural parity high-spin states such as $4^-$, $5^+$, $6^-$ and $7^+$ are selectively excited due to the spin-flip dominant character of the elementary amplitudes. On the other hand, for the proton LS-closed target ($^{40}$Ca), natural parity high-spin states are excited as well. In both cases, it is important to obtain well-separated clear spectra. The nuclear level fragmentation caused by the one-proton annihilation is taken into account. The theoretical spectrum predicted for the first target ($^{28}$Si) proved to be in very good agreement with the result of recent analysis for the $^{28}$Si$(e,e'K^+)^{28}_{\Lambda}$Al experiment done at JLab. Thus predictions for the latter two targets seem to give the promising and reliable spectra to encourage further extention of the $(e,e'K^+)$ experiments. Novel aspects of medium-mass hypernuclear spectroscopy will be discussed.   

Mesonic decay of neutron-rich $\Lambda$ hypernuclei

Although the pionic decay of $\Lambda$ particle is suppressed in finite nuclei due to the Pauli principle, it still competes with the more dominant non-mesonic decay mode in light hypernuclei. In this contribution, we discuss the pionic decay of light neutron-rich $\Lambda$ hypernuclei. To this end, we describe the structure of hypernuclei with the Skyrme-Hartree-Fock method, and compute the decay rate with the single-particle wave function so obtained. We apply this method to carbon isotopes, from $^{13}_{\Lambda}$C to $^{23}_{\Lambda}$C. Our calculation indicates that the decay rate for the $\pi^{-}$ mode, $\Lambda \rightarrow$ p $+ \pi^{-}$, increases as a function of mass number, while that for the $\pi^{0}$ mode, $\Lambda \rightarrow$ n $+ \pi^{0}$, is largely suppressed as expected. This is due to the fact that the proton single-particle potential is deepened for neutron-rich nuclei because of a strong proton-neutron interaction. We will also discuss the effect of the final state interaction between $\pi$ meson and nuclei.   

Study of double-$\Lambda$ Hypernuclei at J-PARC (E07) experiment

To study double strangeness system such as double-$\Lambda$ hypernuclei and H-dibaryon, Hybrid-emulsion experiments with counter (E176) and scintillating-fiber (E373) have been performed for these twenty years. In the experiments, we have obtained nearly ten thousand events of $\Xi ^-$ hyperon capture at rest in nuclear emulsion, and observed 8 events of sequential decay of light double-$\Lambda$ hypernuclei and 5 events of twin hypernuclei. Recently, we succeeded to measure two $\Lambda$ binding energies of $6 \atop {\Lambda \Lambda}$He, $11 \atop {\Lambda \Lambda}$Be and $13 \atop {\Lambda \Lambda}$B. However, very little is known for double-strangeness system. In this talk, we present a quite improved experiment (E07 at J-PARC) with ten times' statistics of the previous experiments. A new-generation hybrid-emulsion method is applied to search for double-$\Lambda$ hypernuclei. In the experiment, we handle Double-sided Silicon Strip tracking Detector (DSSD) for precise detection of $\Xi ^-$ hyperon in the emulsion, and huge amount of emulsion gel (2.6 tons). We also develop speedy scanning system to complete scanning of 10$^6$ $\Xi ^-$ hyperons within a few years. It is expected that one million $\Xi ^-$ hyperons produce about 10$^2$ double-$\Lambda$ hypernuclear events in the emulsion. We will make a nuclear chart with double strangeness.   

Analysis method for double-$\Lambda$ hypernulear events

To study hyperon-hyperon interaction, the experiment E373 was carried out at KEK-PS. $\Xi^-$ hyperons are produced via the $p(K^-, K^+)\Xi^-$ reaction, and double-$\Lambda$ hypernucleus were produced at a $\Xi^-$ stopping point as a fragment in the nuclear emulsion. Double-$\Lambda$ hypernucleus sequentially decays via non-mesonic or mesonic weak interaction. So that, double-$\Lambda$ hypernuclear event has three vertices. Until now, we succesfully found 7 double-$\Lambda$ hypernuclear events. The recoiled hyperfragments had only few $\mu$m track lengths in the emulsion, then we need the high precision measurement. In this paper, we report detailed method of analysis and those results for two double-$\Lambda$ hypernuclear events which were successfully reconstructed as those ones. To reconstruct events we took pictures all tracks related the event every 0.1 $\mu$m depth, and obtained the brightness and position information. Using central values of brightness along the tracks, straight-line fitting fas been made. Thus, production and decay vertices have been measured as intersection of the lines. Accoding to the above analysis, one event was found to be as a $^{11}_{\Lambda \Lambda}$Be, and another one was uniquely identfied as a $^{6}_{\Lambda \Lambda}$He.   

Observation of cascade weak decays of double hypernuclei

A hybrid-emulsion experiment E373 has been carried out at the KEK 12 GeV proton synchrotron using a 1.66 GeV/$c$ $K^-$ meson beam. The purpose of this experiment is to study double-strangeness systems produced via $\Xi^-$ hyperon capture at rest with ten times as large statistics as past experiments. We have completed emulsion scanning and have successfully found several events which include a sequential weak decay of a double hypernucleus. Among them, the results of the reconstruction of two events, ``Demachiyanagi'' and ``Nagara'', were already reported. However, the recent change of the mass of a $\Xi^-$ hyperon in particle listings by Particle Data Group affects the reconstructed binding energies of these events. The event reconstruction of two new events have been made. One of them, named ``Mikage'', was uniquely identified as a cascade weak decay of a $^{~~6}_{\Lambda\Lambda}$He. The other event, ``Hida'', was reconstructed most likely as a decay of a $^{~11}_{\Lambda\Lambda}$Be. In this talk, we will give updated values of the binding energies of the first two events, and present the reconstructed results of two new events. The result of the reanalysis of the event found in the previous hybrid-emulsion experiment E176 will be also reported.   

Five-body structure of double $\Lambda$ hypernuclei

Recently, in KEK-E373 experiment, new double $\Lambda$ hypernucleus, 'hida' event, $^{11}_{\Lambda \Lambda}$Be was reported. This observation is important to get information on $\Lambda \Lambda$ interaction. For study of stucture of this $\Lambda$ hypernucleus, we perform five-body calculation of $\alpha +\alpha +n+\Lambda +\Lambda$ model. In this symposium, the level structure of this hypernucleus will be discussed.