Heavy Hypernuclei Research Articles

Future prospects of spectroscopic study of Lambda hypernuclei at JLab and J-PARC HIHR

The (e, e′K+) reaction spectroscopy opened a door to high resolution spectroscopy of Λ hypernuclei at JLab and it is currently the only technique to give sub-MeV energy resolution for reaction spectroscopy of wide-mass range Λ hypernuclei. New experiments from light to heavy hypernuclei are under preparation at JLab to solve hypertriton puzzle, to clarify charge symmetry breaking of Λ hypernuclei and to give a clue to solve the hyperon puzzle or the puzzle of heavy neutron stars. As a hypernuclear precision spectroscopy experiment with other than electron beams, there is a newly proposed experiment using the (π+, K+) reaction at the new HIHR beamline, which is a key facility in the J-PARC hadron experimental facility extension project. The HIHR beamline adopts the momentum dispersion match technique and will enable us to perform sub-MeV resolution spectroscopy for isospin partners to the Λ hypernuclei studied with the electron beams at JLab.

Investigation of the Products of Heavy Hypernuclei

Evaporation of hypernuclei is investigated similar to evaporation of normal nuclei taking place in deep-inelastic nuclear collisions. Final cold hypernuclei can be obtained in such processes and offer a new direction for investigation. This may concern the production of exotic states which can exist due to the presence of a hyperon. It will be difficult to obtain such exotic states in other reactions since there are practical limitations on the use of radioactive targets in experiments. We have investigated the evaporation mechanism of some heavy hypernuclei for emission of light hypernuclei which take place from the target and projectile residues. Theoretical calculations are carried out by using computer codes developed for the statistical multifragmentation model generalized to hypernuclei.

Constraining the Λ -nucleus potential within the Liège intranuclear cascade model

The new version of the Li\`ege intranuclear-cascade model (INCL), recently extended to the production of strange particles and hypernuclei, is used to investigate the $\mathrm{\ensuremath{\Lambda}}$-nucleus potential in a broad range of nuclear masses from Si to Pb. The combination of the INCL calculation results with experimental cross sections of $\mathrm{\ensuremath{\Lambda}}$ hypernuclei, obtained from (${\ensuremath{\pi}}^{+},{K}^{+}$) reaction studies, allows us to constrain the $\mathrm{\ensuremath{\Lambda}}$-nucleus potential depth with an accuracy of about 0.9 MeV. Our results show that the potential depth increases with the mass number ($A$) from 28 MeV in the region of medium-mass hypernuclei up to a maximum of 39.6 MeV in the region of heavy hypernuclei around $A=208$. This deviation could be related to the nucleon-isospin dependence of the three-body $\mathrm{\ensuremath{\Lambda}}$-nucleon-nucleon force in asymmetry matter whose contribution is more relevant in hyperneutron matter due to a strong contribution from $\mathrm{\ensuremath{\Lambda}}nn$ interactions.

Direct measurements of the lifetime of medium-heavy hypernuclei

The lifetime of a Λ particle embedded in a nucleus (hypernucleus) decreases from that of free Λ decay mainly due to the opening of the ΛN→NN weak decay channel. However, it is generally believed that the lifetime of a hypernucleus attains a constant value (saturation) for medium to heavy hypernuclear masses, yet this hypothesis has been difficult to verify. This paper presents a direct measurement of the lifetime of medium-heavy hypernuclei that were hyper-fragments produced by fission or break-up from heavy hypernuclei initially produced with a 2.34 GeV photon-beam incident on thin Fe, Cu, Ag, and Bi target foils. For each event, fragments were detected in coincident pairs by a low-pressure multi-wire proportional chamber system. The lifetime was extracted from decay time spectrum formed by the difference of the time zeros between the pairs. The measured lifetime from each target is actually a statistical average over a range of mass with mean about 1/2 of the target mass and appears to be a constant of about 200 ps. Although this result cannot exclude unexpected shorter or longer lifetimes for some specific hypernuclei or hypernuclear states, it shows that a systematic decrease in lifetime as hypernuclear mass increases is not a general feature for hypernuclei with mean mass up to A≈130. On the other hand, the success of this experiment and its technique shows that the time delayed fissions observed and used by all the lifetime measurements done so far on heavy hypernuclei could likely have originated from hyper-fragments lighter than the assumed masses.

Exotic structure of medium-heavy hypernuclei in the Skyrme Hartree-Fock model

We study exotic structure of medium heavy hypernuclei with core nuclei of Ar isotopes and $^{40}\mathrm{Ca}$. Particulary, the superdeformed (SD) states and the corresponding hypernuclei are calculated in the frame of the Skyrme Hartree-Fock (SHF) + BCS model together with a microscopic $\mathrm{\ensuremath{\Lambda}}N$ interaction. We found that the $\mathrm{\ensuremath{\Lambda}}$ separation energy of the ground state is larger than that of the SD state. The result is consistent with the antisymmetrized molecular dynamics (AMD) calculation, but inconsistent with that of relativistic mean field (RMF). The similarity between SHF and AMD and the difference of $\mathrm{\ensuremath{\Lambda}}$ separation energy between SHF and RMF both come from the density distribution in the core nuclei and the corresponding hypernuclei.

Relativistic ion collisions as the source of hypernuclei

We shortly review the theory of hypernuclei production in relativistic ion collisions, that is adequate to future experiments at BM@N, NICA, and FAIR. Within a hybrid approach we use transport, coalescence and statistical models to describe the whole process. We demonstrate that the origin of hypernuclei can be explained by typical baryon interactions, that is similar to the production of conventional nuclei. In particular, heavy hypernuclei are coming mostly from projectile and target residues, whereas light hypernuclei can be produced at all rapidities. The yields of hypernuclei increase considerably above the energy threshold for $ \Lambda$ hyperon production, and there is a tendency to saturation of yields of hypernuclei with increasing the beam energy up to few TeV. There are unique opportunities in relativistic ion collisions which are difficult to realize in traditional hypernuclear experiments: The produced hypernuclei have a broad distribution in masses and isospin. They can even reach beyond the neutron and proton drip-lines and that opens a chance to investigate properties of exotic hypernuclei. One finds also the abundant production of multi-strange nuclei, of bound and unbound hypernuclear states with new decay modes. In addition, we can directly get an information on the hypermatter both at high and low temperatures.

Skyrme force for light and heavy hypernuclei

We present a lambda-nucleon Skyrme force suitable for reproducing the observed binding energies of the whole range of known light and heavy single-lambda hypernuclei with $A=5,...,208$. Notable exceptions such as ${}_{\ensuremath{\Lambda}}^{9}\mathrm{Be}$, featuring a well-developed cluster structure, are identified and examined in comparison with a cluster approach for the hypernuclear structure.

Position reconstruction in fission fragment detection using the low pressure MWPC technique for the JLab experiment E02-017

When a Λ hyperon is embedded in a nucleus it can form a hypernucleus. The lifetime and its mass dependence of stable hypernuclei provide information about the ΛN interaction in the nuclear medium. This work will introduce the Jefferson Lab experiment (E02-017), which aims to study the lifetime of the heavy hypernuclei using a specially developed fission fragment detection technique: a multi-wire proportional chamber operating under low gas pressure (LPMWPC). The trajectory of the detected fragment is reconstructed and used to find the fission point on the target foil, the position resolution is less than 1 mm, which meets the original design, the separation of target materials and events mixture percentage in different regions are verified by Monte Carlo simulation.

Non-mesonic weak decay of hypernuclei with effective field theory

The ΛN → NN interaction, the main responsible for the decay of heavy hypernuclei, is described within the effective field theory (EFT) framework. The EFT is developed up to (q⃗2), where q⃗ is the transferred momentum between the interacting baryons. Numerical values for the leading order low-energy constants are obtained by two means. First, from a fit to the hypernuclear decay observables, and second, by comparing the EFT to the one-meson-exchange model describing the same interaction. All the two-pion exchange diagrams entering at the next-to-leading in the EFT have been calculated.

Pi-Mesonic Weak Decay Rates of Light-to-Heavy Hypernuclei

Pionic decays of light-to-heavy A-hypernuclei are summarized including a review and the updated estimates which are based on the pion distorted waves generated by a sophisticated full-form optical potential. For heavy hypernuclei, the pion distortion effects yield a remark­ able enhancement as large as two orders of magnitudes in comparison with the estimate without any distortion, where the Coulomb distortion is found to play an important role. The systematic calculations for all the sd-shell hypernuclei (including the updated P-shell results) predict that the total 7r0 and 1r- decay rates vary strikingly depending on the structures of individual hypernucleus and final nucleus. It is also shown by presenting the excitation functions that the partial pionic decay is selective enough to be utilized as a hypernuclear spectroscopic tool.

Effects of the two-body and three-body hyperon-nucleon interactions inΛhypernuclei

Background: The calculation of the hyperon binding energy in hypernuclei is crucial to understanding the interaction between hyperons and nucleons. Purpose: We assess the relative importance of two- and three-body hyperon-nucleon force by studying the effect of the hyperon-nucleon-nucleon interaction in closed shell \Lambda-hypernuclei from A=5 to 91. Methods: The \Lambda-binding energy has been calculated using the auxiliary field diffusion Monte Carlo method for the first time, to study light and heavy hypernuclei within the same model. Results: Our results show that including a three-body component in the hyperon-nucleon interaction leads to a saturation of the \Lambda-binding energy remarkably close to the experimental data. In contrast, the two-body force alone gives an unphysical limit for the binding energy. Conclusions: The repulsive contribution of the three-body hyperon-nucleon-nucleon force is essential to reproduce, even qualitatively, the binding energy of the hypernuclei in the mass range considered.

Description of single-Λhypernuclei with a relativistic point-coupling model

We extend the relativistic point-coupling model to single-$\ensuremath{\Lambda}$ hypernuclei. For this purpose, we add $N$-$\ensuremath{\Lambda}$ effective contact couplings to the model Lagrangian and determine the parameters by fitting to the experimental data for $\ensuremath{\Lambda}$ binding energies. Our model well reproduces the data over a wide range of mass region although some of our interactions yield the reverse ordering of the spin-orbit partners from that of nucleons for heavy hypernuclei. The consistency of the interaction with the quark model predictions is also discussed.

Role of ground-state correlations in hypernuclear nonmesonic weak decay

The contribution of ground state correlations (GSC) to the non--mesonic weak decay of $^{12}_\Lambda$C and other medium to heavy hypernuclei is studied within a nuclear matter formalism implemented in a local density approximation. We adopt a weak transition potential including the exchange of the complete octets of pseudoscalar and vector mesons as well as a residual strong interaction modeled on the Bonn potential. Leading GSC contributions, at first order in the residual strong interaction, are introduced on the same footing for all isospin channels of one-- and two--nucleon induced decays. Together with fermion antisymmetrization, GSC turn out to be important for an accurate determination of the decay widths. Besides opening the two--nucleon stimulated decay channels, for $^{12}_\Lambda$C GSC are responsible for 14% of the rate $\Gamma_1$ while increasing the $\Gamma_{n}/\Gamma_{p}$ ratio by 4%. Our final results for $^{12}_\Lambda$C are: $\Gamma_{\rm NM}=0.98$, $\Gamma_{n}/\Gamma_{p}=0.34$ and $\Gamma_2/\Gamma_{\rm NM}=0.26$. The saturation property of $\Gamma_{\rm NM}$ with increasing hypernuclear mass number is clearly observed. The agreement with data of our predictions for $\Gamma_{\rm NM}$, $\Gamma_n/\Gamma_p$ and $\Gamma_2$ is rather good.

Hypernuclear Weak Decays

The recent theoretical studies of Λ-hypernuclear weak decays of the nonmesonic and π-mesonic ones are developed with the aim to disclose the link between the experimental decay observables and the underlying basic weak decay interactions and the weak decay mechanisms. The expressions of the nonmesonic decay rates Γnm and the decay asymmetry parameter α1 of protons from the polarized hypernuclei are presented in the shell model framework. We then introduce the meson theoretical ΛN → NN interactions which include the one-meson exchanges, the correlated-2π exchanges, and the chiral-pair-meson exchanges. The features of meson exchange potentials and their roles on the nonmesonic decays are discussed. With the adoption of the π + 2π/ρ + 2π/σ + ω + K + ρπ/a1 + σπ/a1 exchange potentials, we have carried out the systematic calculations of the nonmesonic decay observables for light-to-heavy hypernuclei. The present model can account for the available experimental data of the decay rates, Γn/Γp ratios, and the intrinsic asymmetry parameters αΛ (αΛ is related to α1) of emitted protons well and consistently within the error bars. The hypernuclear lifetimes are evaluated by converting the total weak decay rates Γtot = Γπ+Γnm to τ , which exhibit saturation property for the hypernuclear mass A ≥ 30 and agree grossly well with experimental data for the mass range from light to heavy hypernuclei except for the very light ones. Future extensions of the model and the remaining problems are also mentioned. The π-mesonic weak processes are briefly surveyed, and the calculations and predictions are compared and confirmed by the recent high precision FINUDA π-mesonic decay data. This shows that the theoretical basis seems to be firmly grounded.

Λ spin–orbit splitting in heavy hypernuclei as deduced from DWIA analyses of the [formula omitted] reaction

By taking nuclear core excitations into account, detailed structure calculations of 89 ΛY have been performed as a function of the Λ spin–orbit splitting. The obtained wave functions have been used to estimate the (π+,K+) reaction cross sections within the distorted-wave impulse approximation (DWIA). A theoretical explanation is given quantitatively for the first time of how to understand the doublet substructure of each major peak observed in medium-heavy hypernuclear production. A small Λ spin–orbit splitting of around δ(0fΛ)=0.2 MeV is deduced, a value consistent with the small ΛN two-body spin–orbit interactions deduced from γ-ray measurements in three light hypernuclei.

Strange and heavy flavoured hypernuclei in chiral soliton models

The extension of the chiral soliton approach to hypernuclei --strange or heavy flavoured-- becomes more reliable due to the success in describing other properties of nuclei, e.g. the symmetry energy of nuclei with atomic numbers up to ∼ 30 . The binding energies of the ground states of light hypernuclei with S = - 1 have been described in qualitative agreement with data. The existence of charmed or beautiful hypernuclei and Theta-hypernuclei (strange, charmed or beautiful) with large binding energy is expected within same approach.

ΛΛ 6 He and ΛΛ interaction

A phenomenological potential of ΛΛ interaction is found on the basis of a treatment of ΛΛ6He as a six-particle system. The semirealistic ΛN and NN potentials used in this treatment ensure an adequate description of the binding energies of 1s-shell hypernuclei and Λp scattering, as well as of the binding energies and dimensions of the nuclear cores. The reliability and a high precision of the calculations are achieved owing to determining both an upper and a lower bound on the energies in question. A cluster model involving the Λα potential matched with the results of precise calculations for the 4He, Λ5He, and ΛΛ6He systems is used to calculate ΛΛ10Be and other double heavy hypernuclei and to analyze the possibility of a consistent description of all double hypernuclei.

Binding energies of hypernuclei and ΛN interaction

A central ΛN potential VΛN is found on the basis of an analysis of the binding energies of 1s-shell hypernuclei and Λp scattering. Within the exprerimental errors, this potential makes it possible to reproduce the binding energies of three-, four-, and five-particle ground and excited states of hypernuclei and the angular and energy dependences of the cross sections for Λp scattering. The accuracy and reliability of the calculation of three-, four-, and five-particle hypernuclear and the respective nuclear systems is ensured by determining both upper and lower bounds on the energy. Within the Λ plus core model, the potential VΛN is matched with binding energies of heavy hypernuclei.

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 τΛ = (145±11) ps. This value for τΛ 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.