Session DH: Mesic Nuclei and Hypernuclei

Formation of Mesonic Atoms and Mesonic Nuclei

Meson properties at finite nuclear densities are believed to have close connection to the fundamental theory, QCD, and have been studied for a long time both theoretically and experimentally. In this context, Mesonic atoms and Mesonic nuclei are interesting systems. In this talk, we study the recent research activities in this field and consider mainly formation reactions of Mesonic atoms and Mesonic nuclei theoretically. We report the new possibilities of the spectroscopic study of the pionic atoms using the (d,$^3$He) reactions. We consider the (d,$^3$He) reaction at finite angles to produce the atomic states with different angular momenta and on the odd-neutron nuclear target to produce the pionic states in the even-even nucleus which has a well-known neutron distribution. As for the $\eta$(958) mesonic nuclei, we summarize the recent research activities on the $\eta$(958) meson property in nucleus and report the possible formation of the $\eta$(958) mesonic nuclei by the (p,d) reactions in detail.   

Search for $\eta '$ mesic nuclei with (p,d) reaction at GSI

We report a spectroscopy experiment of the ($p$,$d$) reaction searching for $\eta '$ mesic nuclei. The large mass of the $\eta '$ meson is explained by the axial anomaly effect associated with spontaneous breaking of chiral symmetry. In nuclear medium, due to restoration of chiral symmetry, mass of the $\eta '$ meson may be reduced, and $\eta '$ meson-nucleus bound states may exist. To search for such $\eta '$ mesic nuclei, we plan an inclusive measurement of the ${}^{12}$C ($p$,$d$) reaction around the $\eta '$ emission threshold using a 2.5 GeV proton beam at GSI. For momentum analysis of the ejectile deuteron, the FRS (Fragment Separator) is used as a spectrometer. The first experiment will be carried out in July-August 2014. The status of the experiment and preliminary results will be reported.   

The search for the eta' mesic nuclei in the BGOegg experiment

It is predicted that the mass of eta' mesons decreases in nuclei due to the partial restoration of the chiral symmetry. If the eta' mass decreases, the eta' meson and the nuclear form a bound state. We search for the eta' bound state in the BGOegg experiment at the LEPS2 beam line at SPring-8. Using the GeV gamma ray, the eta' meson is produced in 12C (gamma, p) eta' 11B reaction. The eta' production is identified by detecting eta meson from eta' decay using the BGOegg calorimeter. The energy of protons is measured by the RPC-TOF. From the missing mass spectrum, the eta' bound state is searched. This experiment started on May 2014. In this talk, the feasibility of this experiment and the performance of the detectors are described.   

An analysis of the 12C(p,d) reaction at eta'(958) meson production region by microscopic transport model (JAM)

We study theoretically the $^{12}$C($p,d$) reaction for the formation of the $\eta$' mesonic nucleus to optimize the experiments at GSI and FAIR, where the missing mass spectroscopy of the $^{12}$C($p,d$) reaction is adopted to measure $\eta$' meson bound states in $^{11}$C. This method was proposed in Ref. [1] and the peak structures are expected in the inclusive spectra of the deuteron in case that the discrete states exist. The semi-exclusive measurements are also considered at FAIR to reduce the background, where protons / charged pions are measured in coincidence with the deuteron. We present the theoretical distributions of the emitted charged particle in the ($p,d$) reaction. The charged particles produced by the $\eta$' absorption are expected to have uniform angular distribution with the specific energy of the absorption process, while those by the background distribute in the forward directions. Thus, we can reduce the background largely by the differences of the charged particle distributions from these processes. We use the microscopic transport model [2] and we report the advantages of the semi-exclusive measurements.\\[4pt] [1] K. Itahashi et al., PTP128, 601(12), H. Nagahiro et al., PRC87, 045201(13).\\[0pt] [2] Y. Nara et al., Phys. Rev. C61, 024901(00).   

Future plans for missing-mass spectroscopy experiment of $\eta'$ mesic nuclei at FAIR

The mass of an $\eta'$ meson at finite density is expected to decrease from the in-vacuum one, due to partial restoration of chiral symmetry. If this is the case, a bound state of an $\eta'$ meson and a nucleus may exist. In order to investigate this system, we are performing a missing-mass spectroscopy experiment at GSI-SIS, making use of the $(p,d)$ reaction. Both high-statistics and high-resolution measurement can be realized by taking advantage of an intense primary proton beam supplied by the SIS-18 synchrotron and the fragment separator (FRS) used as a spectrometer. The first experiment (GSI S-437) will take place in July-August, 2014. Furthermore, we will proceed with a series of experiments at FAIR, which is under construction. Compared with the GSI experiment, a semi-exclusive measurement of the $(p,dp)$ reaction, in which protons from the decay of $\eta'$ mesic nuclei will be detected simultaneously so as to improve the signal-to-noise ratio, will be enabled by installing an additional counter system near the target. In addition, we will accumulate much higher statistics in an inclusive measurement with an upgraded DAQ. In this contribution, we will discuss these future plans, based on very preliminary results of the first GSI experiment.   

The $\eta'$-optical potential in the nuclear medium based on the $\eta'$N interaction from a chiral effective model

In this talk, we discuss the $\eta'$ optical potential based on the $\eta'$N two body interaction obtained from a chiral effective model. The $\eta'$ mass reduction inside the nuclear medium is expected by the degeneracy of the pseudoscalar-singlet and octet mesons in the chiral restored phase in the chiral limit. The observation of the $\eta'$-nucleus bound state is planned experimentally. Here, we estimate the $\eta'$ optical potential using the $\eta'$N interaction obtained from linear sigma model. The $\eta'$N interaction in the linear sigma model comes from the scalar meson exchange and UA(1) symmetry breaking, and it is found to be fairly strong attraction. This strongly attractive two body interaction leads to a deep and attractive optical potential. Moreover, the transition to $\eta$N channel is included in our calculation, so the $\eta'$ optical potential have imaginary part. The imaginary part is relatively small compared to the real part in our estimation. Such a strongly attractive and the small absorptive $\eta'$-optical potential in the nuclear medium gives narrow bound states in $\eta'$ and nucleus systems.   

Search for the $\overline{K}NN$ bound states via photon-induced reactions

The existence of the $\overline{K}NN$ bound states has not been established experimentally. Search experiments in various reaction channels are awaited. The multi-GeV photon induced reaction is one of the promising channels to search for exotic systems with strangeness -1. We adopted the $d(\gamma, K^+\pi^-)X$, $d(\gamma, K^+)X$ and $d(\gamma, K^+\pi^+)X$ reactions with photon energies from 1.5 to 2.4 GeV to search for the $K^-pp$, $K^-pn$ and $K^-nn$ bound states in the LEPS/Spring-8 experiment. The symmetrical experimental setup for positive and negative charged particles enables us to search for three different isospin states simultaneously. We searched for a peak structure in the mass region from 2.22 to 2.36 GeV/$c^2$ in the inclusive missing mass spectrum for each reaction. In addition, we have studied the exclusive missing mass spectrum for the $d(\gamma, K^+)X$ reaction. $\Sigma\pi$ final states were selected and the structure in the $\Lambda(1405)/\Sigma(1385)$ region was investigated precisely. The line shape in the spectra will be discussed with emphasis on the final state interaction effect. The search results and the details of the analysis will be presented in this talk.   

Spectroscopy of $S=-2$ hypernuclei at J-PARC with a new spectrometer S-2S

The study of $S=-2$ hypernuclei is important for understanding baryon-baryon interaction and strange nuclear matter. However, experimental data of $S=-2$ systems are very limited. We will obtain the spectroscopic information of $S=-2$ hypernuclei using the $(K^-,K^+)$ reaction. As a first step, We plan to perform the J-PARC E05 experiment by using the $^{12}$C$(K^-,K^+)$ reaction. Following this experiment, we plan to carry out further studies on $\Xi$- and double $\Lambda$-hypernuclei with various targets. We will utilize the high intensity $K^-$ beam and high resolution beam spectrometer at J-PARC K1.8 beam line. To achieve both enough statistics and better resolution, a new spectrometer S-2S for scattered $K^+$ is under construction. It consists of a QQD-type configuration. It is designed to have a momentum resolution of better than $5\times10^{-4}$ (FWHM), which corresponds to energy resolution of 1.5 MeV. The construction of Q1 and Q2 has already been finished. We measured magnetic field of Q1, and obtained a field gradient of 8.7 T/m enough to achieve an acceptance of 60 msr at 1.3 GeV/$c$. We are also developing detectors, especially a water Cherenkov counter for on-line $K/p$ separation. The magnets and the detectors will be ready for installation in the next year.   

Microscopic particle-rotor model for low-lying spectrum of $\Lambda$ hypernuclei

We will propose a novel microscopic particle-rotor model for the low-lying states of single-$\Lambda$ hypernuclei. The novel feature is that we combine the motion of $\Lambda$ particle with the core nucleus states, which are described by the state-of-the-art covariant density functional approach, that is, the generator coordinate method (GCM) based on the relativistic mean-field (RMF) approach supplemented with the particle number and the angular momentum projections. We will apply this model to $^9_\Lambda$Be employing a point-coupling version of the relativistic mean-field Lagrangian. We will show that a reasonable agreement with the experimental data for the low-spin spectrum is achieved using the $\Lambda N$ coupling strengths determined to reproduce the binding energy of the $\Lambda$ particle. We emphasize that, using this method, a spectrum of hypernuclei is calculated for the first time based on a density functional approach.   

Transition from $\Lambda \Lambda $ to H-dibaryon in the imaginary time method

Transition rate of thermal $\Lambda \Lambda $ to H-dibaryon is calculated by the use of imaginary time method. We consider the initial states, $\Lambda \Lambda (L=1, S=1)$, which mixes with $N\Xi (L=1, S=0)$ and $N\Xi (L=1, S=1)$, and the final state, H-dibaryon $(L=0, S=0)$. In imaginary time method, all initial (scattering) states are summed up and included in the transition operator, which is evaluated in the final state so that we use only the final state wave function. The algorithm of imaginary time method is as follows; firstly we prepare the final-state wave function at zero temperature. Next we evolve the wave function along imaginary time axis, and then we can evaluate the sum over the wave functions at finite temperature. Finally we take the overlap with the final-state wave function again and get the transition rate. Results will be reported in the presentation.   

Search for H-dibaryon with Hyperon spectrometer at J-PARC

Recent Lattice QCD calculations have suggested a possible existence of H-dibaryon near the $\Lambda \Lambda $ threshold. In the previous experiments at KEK-PS, a hint of peak structure was observed in the $\Lambda \Lambda $ invariant mass spectrum at just above the $\Lambda \Lambda $ threshold. An experiment to search for H-dibaryon through $\Lambda \Lambda $ and $\Lambda \pi $-p mass spectra was proposed at J-PARC (E42). Much better statistics and mass resolution than the previous data are expected. For this experiment, we have been constructing a novel hyperon spectrometer with use of a GEM-TPC and a superconducting Helmholtz magnet. The expected performance of the spectrometer and the proposed experiment is reported based on the R{\&}D results of the TPC and simulations. Possible physics with the hyperon spectrometer other than the H-dibaryon is also discussed.