Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session BC: Mini-symposium on Hypernuclei |
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Sponsoring Units: DNP JPS Chair: Tomokazu Fukuda, Osaka Electro-Communication University Room: Ritz-Carlton Hotel Salon 3 |
Monday, September 19, 2005 7:00PM - 7:30PM |
BC.00001: Spectroscopy of $\Lambda$ Hypernuclei: Recent Progress and Future Prospects Invited Speaker: Recently, great progress has been made in spectroscopic studies of $\Lambda$ hypernuclei. In this talk, the present status and future plans of $\Lambda$ hypernuclear spectroscopy are presented, particularly on precision $\gamma$-ray spectroscopy. We have been investigating detailed structure of $\Lambda$ hypernuclei by means of the precision $\gamma$-ray spectroscopy technique with a large germanium detector array, Hyperball. We have studied $\gamma$ transitions in various p-shell $\Lambda$ hyerpnuclei ($^7_\Lambda$Li, $^9_\Lambda$Be, $^{10}_\Lambda$B, $^{11}_\Lambda$B, $^{15}_\Lambda$N, and $^{16}_\Lambda$O) via the $(\pi^+,K^+\gamma)$ reaction at KEK and the $(K^-,\pi^-\gamma)$ reaction at BNL. The precise level structure data of these hypernuclei allowed us to determine all the spin-dependent (spin-spin, spin-orbit, and tensor) $\Lambda$$N$ interaction strengths, which give stringent constraints to baryon-baryon interaction models. The upgraded apparatus (Hyperball2) is now ready for further studies, where one of the most important subjects is the study of magnetic moment of a $\Lambda$ in a nucleus through measurement of $B(M1)$ values for $\Lambda$-spin-flip $M1$ transitions in hypernuclei. In near future, hypernuclear $\gamma$ spectroscopy will be greatly developed by using strong beams from the 50 GeV proton synchrotron at J-PARC. In addition, recent progress and future plans are summarized for high-resolution $\Lambda$ hypernuclear spectroscopy via the $(e,e'K^+)$ reaction at Jlab and spectroscopy of neutron-rich $\Lambda$ hypernuclei via the $(\pi^-,K^+)$ reaction at KEK. [Preview Abstract] |
Monday, September 19, 2005 7:30PM - 7:45PM |
BC.00002: Spectroscopy of $^{11}_\Lambda$B John Millener Hypernuclear $\gamma$-ray transitions in $^7_\Lambda$Li, $^9_\Lambda$Be, $^{10}_\Lambda$B, $^{11}_\Lambda$B, $^{15}_\Lambda$N, and $^{16}_\Lambda$O have been studied with the Hyperball Ge array in a series of experiments at KEK and BNL. Comparison of the observed transition energies with shell-model calculations using a basis of $\Lambda$ and $\Sigma$ states with the hyperon in the lowest s orbit have led to a determination of the strengths of the spin-spin, spin-orbit, antisymmetric spin-orbit, and tensor components of the $\Lambda$N effective interaction from the data on $^7_\Lambda$Li, $^9_\Lambda$Be, and $^{16}_\Lambda$O. Here, the strength of the $\Lambda$-$\Sigma$ coupling (essentially an effective central $\Lambda$N-$\Sigma$N interaction) has been fixed by theory and other data but it could be varied. The observed transitions in other hypernuclei then serve as consistency checks on the parametrization of the YN interaction. Specifically, six transitions in $^{11}_\Lambda$B have been observed and their interpretation will be discussed. [Preview Abstract] |
Monday, September 19, 2005 7:45PM - 8:00PM |
BC.00003: $\gamma$-ray spectroscopy of $^{12}_{\Lambda}$C and $^{11}_{\Lambda}$B via the Hyperball2 array Sari Kinoshita Based on the success of a series of Hyperball experiments, an upgrading of Hyperball has been undertaken. The new array, Hyperball2, houses six Clover-type detectors, which are segmented into four Ge crystals, plus 14 single-crystal Ge detectors. Each detector is surrounded by BGO scintillator counters for a reduction of backgrounds in $\gamma$-ray spectrum by vetoing contaminated events. The photo-peak detection efficiency of Hyperball2 is nearly doubled from that of the previous array. We examined the performance at Tohoku Univ. CYRIC. The first experiment of Hypernuclei with this array, in conjunction with Superconducting Kaon Spectrometer (SKS), is scheduled at the KEK beam facility to perform $\gamma$-ray spectroscopy of $^{12}_{\Lambda}$C and $^{11}_{\Lambda}$B using the ($\pi^+$, K$^+$) reaction on $^{12}$C target. In this experiment, one of the main goals is to measure a lifetime of 7/2$^+$ state in $^{11}_{\Lambda}$B, which is selectively produced via one proton emission decay of $^{12}_{\Lambda}$C(2$^+$). The measured lifetime, by means of Doppler shift attenuation method, determines the reduced transition probability of the $^{11}_{\Lambda}$B(7/2$^+$$\rightarrow$5/2$^+$) $\Lambda$-spin-flip M1 transition (B(M1;7/2$^+$$\rightarrow$5/2$^+$)). Properties of a $\Lambda$ particle in free space may change in nuclear matter and the magnetic moment is one of them. The measurement of the B(M1;7/2$^+$$\rightarrow$5/2$^+$) value will provide a quantitative account for the change of the magnetic moment of a $\Lambda$ if at all. Equally as important,strengths of the $\Lambda$N interaction can be inferred from excited energy levels of $^{12}_{\Lambda}$C. [Preview Abstract] |
Monday, September 19, 2005 8:00PM - 8:15PM |
BC.00004: A new prediction for the binding energy of \textbf{$_\Lambda ^7$}He hypernucleus Vladimir Suslov, Igor Filikhin, Branislav Vlahovic P-shell A=7 hypernuclei are considered in the cluster$_\Lambda ^7 $He+N+N model. The folding procedure using the OBE simulating (NSC97f) model for $\Lambda $N potential and various $\alpha \Lambda $ potentials are applied to construct the $_\Lambda ^7 $He-N interaction. Configuration space Faddeev calculations are performed for hyperon binding energy of the $_\Lambda ^7 $He ($\frac{1}{2}^+)$ and $_\Lambda ^7 $Li ($\frac{1}{2}^+$and $\frac{3}{2}^+$, T=0) hypernuclei. A new predicted value for $B_\Lambda (_\Lambda ^7 $He) is 5.35 MeV. This value was obtained for the $_\Lambda ^6 $He(2$^-)$ excitation energy equal to 0.26 MeV. Since the 2$^-$ state of $_\Lambda ^6 $He has not yet been observed, the $_\Lambda ^6 $He(2$^-)$ excitation energy was chosen to reproduce the experimental value of the $_\Lambda ^7 $Li($\frac{3}{2}^+)$ excitation energy by the adjustment of the $_\Lambda ^6 $He-N effective potential. Our results are compared with those from [1]. 1. E. Hiyama et al. Phys. Rev. C53, 2075 (1996), Phys. Rev. C59, 2351 (1999). [Preview Abstract] |
Monday, September 19, 2005 8:15PM - 8:30PM |
BC.00005: $\Lambda$ spin-orbit splittings deduced from DWIA analysis of the $^{89}$Y$(\pi^+,K^+)^{89}_{\Lambda}$Y reaction Toshio Motoba, John Millener, Dmitry Lanskoy, Yasuo Yamamoto High resolution measurements of hypernuclear $\gamma$ rays from the $^9$Be$(\pi^+,K^+\gamma )^9_{\Lambda}$Be and $^{13}$C$(K^-,\pi^-\gamma )^{13}_{\Lambda}$C reactions have shown clearly that the $\Lambda N$ spin-orbit force is very small. In heavier cases such as $^{89}_{\Lambda}$Y, however, the $(\pi^+,K^+)$ experiments show a series of strong peaks having doublet-like substructure which apparently suggests sizable $\Lambda N$ spin-orbit splittings. In order to resolve this discrepancy, detailed structure calculations of $^{89}_{\Lambda}$Y have been performed by taking nuclear core excitation into account. The obtained wave functions have been used to estimate the $(\pi^+,K^+)$ reaction cross sections within DWIA. Based on a careful analysis of the peak structure, a theoretical explanation is given for the first time how to understand the doublet substructure in a series of observed major peaks in $^{89}_{\Lambda}$Y. In the discussion we conclude small $\Lambda$ spin-orbit force which is consistent with the two light hypernuclear cases. [Preview Abstract] |
Monday, September 19, 2005 8:30PM - 8:45PM |
BC.00006: Hypernuclear Spectroscopy in JLab's Hall~A: experimental aspects Franco Garibaldi With the addition of two superconducting septum magnets and a Ring Imaging Cerenkov detector (RICH), the high resolution spectrometer pair in Hall A at Jefferson Lab have become a powerful tool for high resolution Hypernuclear spectroscopy. With both spectrometers set at 6$^{o}$, the very good momentum resolution capability, the state of the art particle identification, and the very high quality CEBAF beam enable us to observe bound states in Lambda-hypernuclei with sub-MeV FWHM resolutions. The use of the proximity focusing RICH detector for kaon identification in the 2 GeV/c region of momenta reduced the large pion and proton backgrounds in the hypernuclear spectra to a negligible level. The basic parameters, the data analysis procedure, and the resulting performance of the RICH detector obtained during the hypernuclear spectroscopy experiment will be presented and compared with Monte Carlo simulations. Technical and optical features of the two septum magnets will also be discussed. [Preview Abstract] |
Monday, September 19, 2005 8:45PM - 9:00PM |
BC.00007: High Resolution 1p Shell Hypernuclear Spectroscopy at JLAB Mauro Iodice Hypernuclear physics is a unique tool for providing information on the nature of the potential between nucleons and strange baryons. Information on the nature of the force between nucleons and strange baryons, i.e. the Lambda-N interaction, can be accessed by studying the spectroscopy of nuclei in which a nucleon has been replaced by a Lambda particle to form a bound state. The experiment E94107 at Jefferson Lab, Hall A, using a pair of high resolution spectrometers coupled to a pair of septum magnets, together with a RICH detector for particle identification, completed its first measurements of hypernuclear electroproduction on carbon and beryllium targets in June 2004. Data on oxygen using a windowless waterfall target were recently taken during June 2005. Results on Carbon and Beryllium and preliminary data on oxygen will be presented and compared with theoretical models. [Preview Abstract] |
Monday, September 19, 2005 9:00PM - 9:15PM |
BC.00008: Spectroscopic study of hypernuclei via (e,e$'$K$^+$) reaction Yuichi Okayasu A new hypernuclear spectroscopy experiment by the (e,e$'$K$^+$) reaction will be carried out in June 2005 at Hall C, Jefferson Lab (E01-011). In order to improve the successful pioneering Jlab E89-009 experiment, we introduce a newly designed high resolution and large solid angle spectrometer (HKS) for the kaon arm, and employ ``Tilt method'' for the electron arm. In the ``tilt method,'' the electron spectrometer is vertically tilted by $\sim$8 degrees to reduce drastically the background electrons originating from Bremsstrahlung and M\o ller process. By adopting the new setup, we expected to : 1) increase hypernuclear yield by 50 times with higher beam current ($\sim$30 $\mu$A) and thicker target($\sim$100 mg/cm${}^2$), 2) improve to hypernuclear mass resolution twice better($\sim$400 keV[FWHM]). Under such condition, we plan to measure hypernuclear spectra for higher $Z$ targets {\it i.e.} ${}^{28}$Si, ${}^{51}$V and ${}^{89}$Y. We investigate $\Lambda$ single particle behavior deep inside the nucleus and try to understand the nature of $\Lambda$N interaction comparing with ordinary nuclei. In this report, the analysis procedure for kaon identification will be described with preliminary hypernuclear mass spectrum. [Preview Abstract] |
Monday, September 19, 2005 9:15PM - 9:30PM |
BC.00009: Direct Measurement of Lifetime of Heavy Hypernucleus by JLab E02-017 Lulin Yuan JLab E02-017 aims to measure hypernucleus $^{208}_\Lambda$Pb lifetime with less than 5\% precision, much better than previous measurements in this mass range. The experiment will be carried out in August, 2005. The hypernucleus is produced by (e,e'K$^+$) reaction. The experiment utilizes the fine time structure of CEBAF electron beam. Hypernuclear delayed fission products are detected by a LPMWPC in coincidence with scattered K$^+$ to positively identify hypernuclear production and its weak decay. These techniques result in much improved time resolution and reduced background. The experimental setup will be described and the preliminary fission time spectrum will be presented. [Preview Abstract] |
Monday, September 19, 2005 9:30PM - 9:45PM |
BC.00010: Strength of the $\Sigma$ mean field deduced from SCDW model analyses of $(\pi^-,K^+)$ inclusive spectra on $^{12}$C, $^{28}$Si, $^{58}$Ni and $^{209}$Bi M. Kohno, Y. Fujiwara, Y. Watanabe, K. Ogata, M. Kawai The semiclassical distorted wave model is developed to analyze $(\pi^-,K^+)$ $\Sigma$ formation inclusive spectra on various nuclear targets. We do not introduce a factorization approximation in terms of Fermi averaging of the elementary cross sections in nuclear medium. Confirming first that the model works well for the $(\pi^+,K^+)$ $\Lambda$ formation inclusive spectra, we apply it to the $\Sigma$ case. The shape and magnitude of the experimental spectra for various target nuclei taken at KEK [P.K. Saha {\it et al.}, Phys. Rev. C70, 044613 (2004)] are satisfactorily reproduced using a repulsive $\Sigma$-nucleus single-particle potential whose strength is of the order of 30$\sim$50 MeV. The isovector part of the $\Sigma$ single-particle potential is also determined from the data of heavier nuclei. [Preview Abstract] |
Monday, September 19, 2005 9:45PM - 10:00PM |
BC.00011: Roles of ${\rm A}_1$ meson exchange interaction in the hypernuclear nonmesonic weak decay observables Kazunori Itonaga, Toshio Motoba, Tamotsu Ueda, Th. A. Rijken The most puzzling and challenging problem in the nonmesonic weak decay at present is to resolve the large discrepancy between the theory and the data of asymmetry parameters of the proton emitted in the decay of $^5_\Lambda{\rm He}$ and $^{12}_\Lambda{\rm C}$. In our previous meson exchange model, the $1\pi$, $2\pi/\sigma$, $2\pi/\rho$, $1\omega$ and $1K$ exchange mesons are employed. The $\pi$ and $\sigma$ mesons are chiral partners and play role to bring the right magnitude of the decay rates. In the present study we introduce the $a_1$ meson exchange in the decay interactions, where the $a_1$ meson is $1^+$ and is a chiral partner of the $\rho$ meson. In our model, we consider the $a_1 \to \rho \pi$ decay process in the intermediate meson exchange and the $\pi$ is connected to the weak $\Lambda N \pi$ vertex. By choosing the appropriate coupling constants, we construct the $\rho \pi/a_1$ exchange potential $V_{\rho \pi/a_1}$. We will discuss on the roles of $\rho \pi/a_1$ exchange potential on the asymmetry parameter and other weak decay observables. [Preview Abstract] |
Monday, September 19, 2005 10:00PM - 10:15PM |
BC.00012: Double Strangeness Weak Interaction Kazuma Kakazawa In the E373(KEK-PS) experiment, several hundreds events caused by $\Xi^-$ hyperon capture at rest in nuclei were located in nuclear emeulsion to study $S=-2$ systems. Among those events, we detected seven candidate events with topologies showen by the production and decay of double-$\Lambda$ hypernucleus, and also found an event showing that a $\Sigma^-$ hyperon was probably emitted from $\Xi^-$ hyperon capture point, very recently. This kind of phenomenon, $\Xi$$N$ $\rightarrow$ $\Sigma$$N$ or $\Lambda$$\Lambda$ $\rightarrow$ $\Sigma$$N$, shall be quite usefull to know the weak interaction involving double strangeness. The result of the analysis wil be reported not only for the event, but also about the probability for such kind of the reaction caused by at rest capture of $\Xi^-$ hyperon in nucleus.\\ E373(KEK-PS)Collaboration : Gifu, Kyoto, Kobe, Toho, Wonkwang, Korea, New-Mexico., U.C. London, KEK, Nagoya, Chonnam, Gyeongsang, Konkuk, Carnegie-Mellon, BNL, Tohoku, Kyoto-Sangyo, Aichi, Tokyo, NIRS, Osaka City U., Osaka Pref. Edu. [Preview Abstract] |
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