Structure of neutron-rich Λ hypernuclei

Properties of light neutron-rich Λ hypernuclei (¹⁶ _ΛC, ¹² _ΛBe, and ¹¹ _ΛLi) are calculated within the Skyrme-Hartree-Fock approach. Interplay between hypernuclear interaction features and properties of these hypernuclei is studied. Response of weakly bound neutron states to hyperon addition depends generally on core distortion by hyperon, and it is essentially different for the different states. This response is especially sensitive to details of the ΛN interaction for 1p _1/2 states. Implications of the nuclear spin-orbit potential and nuclear incompressibility in the neutron-rich system properties are inferred. Dependence of the Λ binding energies in hypernuclei on Z at fixed A is discussed. Received: 16 December 1998     

Neutron halos in hypernuclei

Properties of single- Λ and double- Λ hypernuclei for even-N Ca isotopes ranging from the proton dripline to the neutron dripline are studied using the relativistic continuum Hartree-Bogoliubov theory with a zero-range pairing interaction. Compared with ordinary nuclei, the addition of one or two Λ-hyperons lowers the Fermi level. The predicted neutron dripline nuclei are, respectively, ⁷⁵ _ΛCa and ⁷⁶ _2ΛCa, as the additional attractive force provided by the Λ-N interaction shifts nuclei from outside to inside the dripline. Therefore, the last bound hypernuclei have two more neutrons than the corresponding ordinary nuclei. Based on the analysis of two-neutron separation energies, neutron single-particle energy levels, the contribution of continuum and nucleon density distribution, giant halo phenomena due to the pairing correlation, and the contribution from the continuum are suggested to exist in Ca hypernuclei similar to those that appear in ordinary Ca isotopes. Received: 21 October 2002 / Accepted: 11 January 2003 / Published online: 8 April 2003     

Studying of hypernuclei with nuclotron beams

The hypernuclear program at Dubna was started in 1988 with investigation of the light hypernuclei production and decay [1]. Nowadays, we are planning to measure lifetime and production cross section of _Λ⁶He and _Λ⁶H hypernuclei.     

Hypernuclei and baryon-baryon interaction

A central ΛN potential containing spin and charge dependences is constructed on the basis of an analysis of fundamental hypernuclear data (Λp scattering, the binding energies of the ground and excited states of hypernuclei). The lightest hypernuclei ( _Λ ³ H, _Λ ⁴ H, _Λ ⁴ He) are calculated as a system of three or four particles on the basis of the highly efficient variational method. Heavier hypernuclei are considered in terms of a twobody model (Λ + undeformed nucleon core) corrected by comparison with direct 3–5-particle calculations. By comparing the results of the calculations of single and double Λ hypernuclei and taking the considerations of meson theory into account, it is possible to estimate the binding energies of charmed and other (b- and t-quark) supernuclei. M. V. Lomonosov State University, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 19–27, October, 1997.     

Six quark cluster effects and Λ-binding energy difference between the mirror hypernuclei Λ 4 He and Λ 4 He

The lambda binding energy difference between two mirror hypernuclei _Λ ⁴ He and _Λ ⁴ He is studied by incorporating the quark structure of nucleons, specially the six quark cluster effects. A small contribution in the binding energy difference (59 keV in the non-relativistic quark model and 29 keV in the bag model) is obtained in the present work.     

ΛN Space-exchange effects in thes-shell hypernuclei and Λ 9 Be

Variational Monte Carlo calculations of the ground state separation energiesB _Λ of thes-shell hypernuclei and also of _Λ ⁹ Be have been made for an Urbana-type central space-exchange ΛN potential consistent with Λp scattering, and also including three-body ΛNN forces. Thes-shell hypernuclei are treated asA-body systems (A = baryon number), and _Λ ⁹ Be is analysed as a partially nine-body problem in the Λ — 2α model. The reduction ofB _Λ due to the space-exchange ΛN potential has been calculated for thes-shell hypernuclei for a range of interactions: both ΛN and ΛN + ΛNN forces. ForA = 3,4,5 the exchange energy is approximately, 0.04, 0.15 and 0.50 MeV, respectively. For _Λ ⁹ Be a much more limited study gives ≅ 1.3 MeV. These values are much larger than that for ‘soft’ ΛN +NN potentials when the correlations are weak. Preliminary results were presented at the DAE Symp. on ‘Nuclear Physics’ Vol. 32B (1989).     

Cluster model of s- and p-shell ΛΛ hypernuclei

The ΛΛ binding energy (B _ΛΛ) of the s- and p-shell hypernuclei are calculated variationally in the cluster model and multidimensional integrations are performed using Monte Carlo. A variety of phenomenological Λ-core potentials consistent with the Λ-core energies and a wide range of simulated s-state ΛΛ potentials are taken as input. The B _ΛΛ of _ΛΛ⁶He is explained and _ΛΛ⁵He and _ΛΛ⁵H are predicted to be particle stable in the ΛΛ-core model. The results for s-shell hypernuclei are in excellent agreement with those of non-VMC calculations. The _ΛΛ¹⁰Be in ΛΛαα model is overbound for combinations of ΛΛ and Λα potentials. A phenomenological dispersive three-body force, V _Λαα, consistent with the B _Λ of _Λ⁹Be in the Λαα model underbinds _ΛΛ¹⁰Be. The incremental ΔB _ΛΛ values for the s- and p-shell cannot be reconciled, consistent with the finding of earlier analyses.      

Further measurement of the 7Be(p,γ)8B cross section at low energies with the coulomb dissociation of 8B

We have measured the dissociation of ⁸B in the Coulomb field of ²⁰⁸Pb at E_in=51.9 MeV/nucleon and extracted the cross section of the ⁷Be(p,γ)⁸B reaction at 0.4 ≤ E_rel≤ 3 MeV, which is of importance for the ⁸B solar-neutrino production rate. The extracted astrophysical S₁₇ factors are consistent with our earlier Coulomb dissociation measurement, and agree with the values deduced from the direct capture measurements by Filippone et al., Vaughn et al. and Hammache et al. The S factor at zero energy was extracted to be 18.9±1.8 eV-b with the help of theoretical energy-dependence. Received: 15 August 1998     

ΛΛ-Ξ N coupling effects in light hypernuclei

The significance of ΛΛ-Ξ N coupling in double-Λ hypernuclei has been studied. The Pauli suppression effect due to this coupling in ⁶ _ΛΛ He has been found to be 0.43MeV for the coupling strength of the NSC97e potential. This indicates that the free-space ΛΛ interaction is stronger by an about 5^° phase shift than that deduced from the empirical data of ⁶ _ΛΛ He without including the Pauli suppression effect. In ⁵ _ΛΛ He and ⁵ _ΛΛ H, an attractive term arising from the ΛΛ-Ξ N conversion is enhanced by the formation of an alpha-particle in the intermediate Ξ states. According to this enhancement, we have found that the ΛΛ binding energy ( ΔB _ΛΛ) of ⁵ _ΛΛ He is about 0.27MeV larger than that of ⁶ _ΛΛ He for the NSC97e coupling strength. This finding deviates from the general picture that the heavier is the core nucleus, the larger is ΔB _ΛΛ.-1 Received: 17 July 2002 / Accepted: 27 September 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: akaishi@post.ket.jp Communicated by A. Molinari     

γ -ray spectroscopy study of 11 ΛB and 12 ΛC

In this paper, a preliminary result from the latest hypernuclear γ -ray spectroscopy experiment (KEK-E566) is presented together with a short discussion. The experiment was performed at the KEK-PS K6 beam line in 2005. In this experiment, the ¹²C(π⁺, K ⁺)¹² _ΛC reaction was employed to populate ¹² _ΛC/¹¹ _ΛB hypernuclei. A germanium detector array, Hyperball2, was constructed to detect γ -rays emitted from the hypernuclei produced. Three hypernuclear γ -ray peaks were observed and assigned.     

Study of ΛΛ dynamics and ground state structure of low and medium mass double Λ hypernuclei

We critically review the ΛΛ dynamics by examining Λ - Λ and Λ-nucleon phenomenological potentials in the study of the bound state properties of double-Λ hypernuclei _ΛΛ ⁶ He, _ΛΛ ¹⁰ Be, _ΛΛ ¹⁴ C, _ΛΛ ¹⁸ O, _ΛΛ ²² Ne, _ΛΛ ²⁶ Mg, _ΛΛ ³⁰ Si, _ΛΛ ³⁴ S, _ΛΛ ³⁸ Ar, _ΛΛ ⁴² Ca, _ΛΛ ⁴⁶ Ti, _ΛΛ ⁵⁰ Cr, _ΛΛ ⁵⁴ Fe, _ΛΛ ⁵⁸ Ni, _ΛΛ ⁶² Zn, _ΛΛ ⁶⁶ Ge, _ΛΛ ⁷⁰ Se, _ΛΛ ⁷⁴ Kr, _ΛΛ ⁷⁸ Sr, _ΛΛ ⁸² Zr, _ΛΛ ⁸⁶ Mo, _ΛΛ ⁹⁰ Ru, _ΛΛ ⁹⁴ Pd, _ΛΛ ⁹⁸ Cd, _ΛΛ ¹⁰² Sn in the frame work of (core+Λ+Λ) three body model. An effective ΛN potential is obtained by folding the phenomenological ΛN potential into the density distribution of the core nuclei. The former two cases (i.e. _ΛΛ ⁶ He and _ΛΛ ¹⁰ Be) are revisited to justify the correctness of the present potential model. Assuming the same potential model we predicted some of the structural properties of heavier doubly Λ-hypernuclei. The hyperspherical harmonics expansion method, which is an essentially exact method has been employed for the three body system. A convergence in binding energy up to 0.15% for K _max=20 has been achieved. In our calculation we have made no approximation in restricting the allowed l-values of the interacting pairs.     

Decay asymmetry in non-mesonic weak decay of light Λ -hypernuclei

The proton decay asymmetry, α^NM _p , of the polarized Λ -hypernuclei, ⁵ _ΛHe , ¹² _ΛC and ¹¹ _ΛB , has been investigated to understand the reaction mechanism of the non-mesonic weak-decay process. These Λ -hypernuclei were produced in the highest statistics ever via the (π⁺, K ⁺) reaction at 1.05GeV/c by using the SKS spectrometer at KEK 12GeV PS. The results show that the α^NM _p are very small for these s -shell and p -shell hypernuclei.     

Diffraction scattering of Λ 3 H and Λ 6 He hypernuclei on nuclei with allowance for edge diffuseness, the structure of hypernuclei, and multiple-scattering effects

General formulas for the amplitudes of the diffraction scattering of two-and three-cluster loosely bound nuclei on spherical nuclei for an arbitrary dependence of the profile functions on the impact parameter are represented in a form that is convenient for calculations. The differential cross sections for the elastic scattering of _Λ ³ H and _Λ ⁶ He hypernuclei and the total cross sections are obtained with allowance for the diffuseness of the target-nucleus edge, the binding energy of incident hypernuclei, the ranges of nuclear forces between the clusters in the hypernuclei, and multiple (double and triple) scattering. Changes in the behavior of the cross sections in response to a transition from the two-cluster to the three-cluster model of the aforementioned nuclei are revealed.     

Impurity nuclear physics

Using a germanium-detector array for hypernuclear γ spectroscopy (Hyperball), we measured B(E2) of the ⁷ _ΛLi hypernucleus and observed a significant shrinkage of the ⁶Li core induced by a Λ-particle. In this way, nuclear properties can be drastically changed by introducing a Λ-particle, which can be investigated by high-resolution hypernuclear γ spectroscopy. In the future neutron-rich hypernuclei will also be studied, where interesting modifications of nuclear structure by a Λ-particle are expected. Received: 1 May 2001 / Accepted: 4 December 2001     

Binding energies of hypernuclei and ΛN interaction

A central ΛN potential V _ΛN that describes, in the limits of experimental errors, the binding energies of three-, four-, and five-particle ground and excited states of hypernuclei, as well as energy and angular dependence of Λp scattering cross sections, is found on the basis of a conjoint analysis of binding energies of 1s-shell hypernuclei and Λp scattering. The reliability and accuracy of three-, four-, and five-particle calculations of hypernuclei and related nuclear systems are assured by the estimation of, an upper, as well as a lower bound of energy. In the framework of the model Λ+core, V _ΛN agrees with binding energies of heavy hypernuclei.     

Lifetime of heavy hypernuclei and its implications on the weak ΛN interaction

The lifetime of the Λ-hyperon in heavy hypernuclei measured in proton-Au, -Bi and -U collisions by the COSY-13 Collaboration at COSY-Jülich has been analyzed to yield τ_Λ = (145±11) ps. This value for τ_Λ is compatible with the lifetime extracted from antiproton annihilation on Bi and U targets, albeit much more accurate. Theoretical models based on the meson exchange picture and assuming the validity of the phenomenological ΔI = 1/2 rule predict the lifetime of heavy hypernuclei to be significantly larger (2-3 standard deviations). Such large differences indicate that at least one of the assumptions in these models is not fulfilled. A much better reproduction of the lifetimes of heavy hypernuclei is achieved in the phase space model, if the ΔI = 1/2 rule is discarded in the nonmesonic Λ decay. Received: 8 August 2002 / Accepted: 20 December 2002 / Published online: 25 March 2003 RID="a" ID="a"e-mail: h.stroeher@fz-juelich.de Communicated by Th. Walcher     

Muonic Molecules: Beyond the Coulomb Model

We report the latest numerical results for the binding energy of the weakly bound states of the muonic molecules (ddμ)₁₁ and (dtμ)₁₁ obtained with account of the leading QED, relativistic and nuclear structure effects, as well as of the interactions within the molecular complexes. The uncertainty of the theoretical value of the binding energy of (ddμ)₁₁ does not exceed 0.1 meV. This revised version was published online in August 2006 with corrections to the Cover Date.