Light Hypernuclei Research Articles

Existence ofΛΛ4H: A variational Monte Carlo search

A variational Monte Carlo (VMC) calculation for the binding energy B{sub {lambda}}{sub {lambda}} of the lightest hypernucleus {sub {lambda}}{sub {lambda}}{sup 4}H has been performed in the four-body {lambda}{lambda}pn model. A range of input {lambda}{lambda} potentials of moderate strength produce a particle-stable {sub {lambda}}{sub {lambda}}{sup 4}H for the simulated NSC97e and f {lambda}N potentials, whereas the phenomenological Minnesota {lambda}N potential needs a much stronger {lambda}{lambda} potential to bind. The VMC results for B{sub {lambda}}{sub {lambda}} agree with the prediction of the stochastic variational model but contradict the recent Faddeev-Yakubovsky calculation. As reported earlier, B{sub {lambda}}{sub {lambda}} is sensitive to the triplet {lambda}N channel for a given {lambda}{lambda} potential. The B{sub {lambda}}{sub {lambda}} of {sub {lambda}}{sub {lambda}}{sup 4}H in the three-body {lambda}{lambda}d cluster model is consistent with but slightly lower than the Faddeev calculation. The VMC method predicts a stable {sub {lambda}}{sub {lambda}}{sup 4}H system in both models and thus offers the possibility of identifying {sub {lambda}}{sub {lambda}}{sup 4}H in a future extension of E906 or of a related experiment at KEK, provided the simulated potentials are true representations of realistic Nijmegen potentials.

An EFT for the weak ΛN interaction

The nonleptonic weak | Δ S | = 1 ΛN interaction, responsible for the dominant, nonmesonic decay of all but the lightest hypernuclei, is studied in the framework of an effective field theory. The long-range physics is described through tree-level exchange of the SU(3) Goldstone bosons ( π and K), while the short-range potential is parametrized in terms of lowest-order contact terms obtained from the most general non-derivative local four-fermion interaction. Fitting to available weak hypernuclear decay rates for 5 Λ He, 11 Λ B and 12 Λ C yields reasonable values for the low-energy constants.

ΛN→NNweak interaction in effective-field theory

The nonleptonic weak $|\Delta S|=1$ $\Lambda N$ interaction, responsible for the dominant nonmesonic decay of all but the lightest hypernuclei, is studied in the framework of an effective field theory. The long-range physics is described through tree-level exchange of the SU(3) Goldstone bosons, while the short-range potential is parametrized in terms of the lowest-order contact terms. We obtain reasonable fits to available weak hypernuclear decay rates and quote the values for the parity-violating asymmetry as predicted by the present effective field theory.

Few-body aspects of light hypernuclei

Three- and four-body structure calculations of single Λ hypernuclei such as Λ7He, Λ9Be and Λ13C and double Λ hypernuclei such as ΛΛ4H, ΛΛ7He, ΛΛ7Li, ΛΛ8Li, ΛΛ9Li, ΛΛ9Be and ΛΛ10Be are discussed. The energy levels of Λ7He whose core is a famous halo nucleus, Λ6He, are predicted based on Λ5He + 2N three-body model. In the microscopic 2α + Λ(3α + Λ) model for Λ9Be(Λ13C), we predict the spin-orbit splittings of 5/21+−3/21+ in Λ9Be and 3/21−−1/21− in Λ13C to be 0.08–0.16 MeV and 0.39–0.78 MeV, respectively, with the use of OBE-model NSC97a ∼f. On the other hand quark-model ΛN spin-orbit force gives rise to half of the splittings of the smallest OBE-model prediction. The experimental data were reported to be 31.4−3.6+2.5 keV for Λ9Be and to be 152 ± 54 ± 36 keV for Λ13C. By performing four-body calculation of ΛΛ4H with both NNΛΛ and NNNΞ channels, we show that the ΛΛ − ΞN coupling potential plays an important role in making ΛΛ4H. Energy levels of the double-Λ hypernuclei, ΛΛ7He, ΛΛ7Li, ΛΛ8Li, ΛΛ9Li, ΛΛ9Be and ΛΛ10Be are predicted on the basis of an α + x + Λ + Λ four-body model, where x = n,p,d,t 3He and α, respectively.

Few-body aspects of light hypernuclei

Three- and four-body structure calculations of single Λ hypernuclei such as Λ 7He, Λ 9Be and Λ 13C and double Λ hypernuclei such as ΛΛ 4H, ΛΛ 7He, ΛΛ 7Li, ΛΛ 8Li, ΛΛ 9Li, ΛΛ 9Be and ΛΛ 10Be are discussed. The energy levels of Λ 7He whose core is a famous halo nucleus, Λ 6He, are predicted based on Λ 5He + 2 N three-body model. In the microscopic 2α + Λ(3α + Λ) model for Λ 9Be( Λ 13C), we predict the spin-orbit splittings of 5/2 1 +−3/2 1 + in Λ 9Be and 3/2 1 −−1/2 1 − in Λ 13C to be 0.08–0.16 MeV and 0.39–0.78 MeV, respectively, with the use of OBE-model NSC97a ∼f. On the other hand quark-model Λ N spin-orbit force gives rise to half of the splittings of the smallest OBE-model prediction. The experimental data were reported to be 31.4 −3.6 +2.5 keV for Λ 9Be and to be 152 ± 54 ± 36 keV for Λ 13C. By performing four-body calculation of ΛΛ 4H with both NNΛΛ and NNNΞ channels, we show that the ΛΛ − Ξ N coupling potential plays an important role in making ΛΛ 4H. Energy levels of the double-Λ hypernuclei, ΛΛ 7He, ΛΛ 7Li, ΛΛ 8Li, ΛΛ 9Li, ΛΛ 9Be and ΛΛ 10Be are predicted on the basis of an α + x + Λ + Λ four-body model, where x = n, p, d, t 3He and α, respectively.

Weakly Bound States in Light Hypernuclei

From the viewpoint of “critical stability”, we discuss the three- and four-body structure of 6ΛHe, 7ΛHe, 4ΛHe, and 4ΛΛH. With the α + Λ + N three-body model, 6ΛHe is found to have a three-layer structure of the matter distribution: α core, a Λ skin and neutron halo. Also the level structure of 7ΛHe with the three-body model of 5ΛHe + n + n is predicted. This stimulates a new study of neutron-rich and proton-rich hypernuclei. By performing a four-body calculation with both NNNΛ and NNNΣ channels and with both NNΛΛ and NNNΞ channels, we show that the ΛN-ΣN and ΛΛ-ΞN couplings are very important in critical stability of few-body hypernuclear systems.

Brueckner rearrangement effects inΛ5HeandΛΛ6He

Rearrangement effects in light hypernuclei are investigated in the framework of the Brueckner theory. We can estimate without detailed numerical calculations that the energy of the $\alpha$-core is reduced by more than 2.5 MeV when the $\Lambda$ adheres to $^4$He to form $^5_\Lambda$He. Similar assessment of rearrangement contributions is essential to deduce the strength of $\Lambda\Lambda$ interaction from experimentally observed $\Delta B_{\Lambda\Lambda}$. The recently observed experimental value of $\sim$ 1 MeV for the $\Delta B_{\Lambda\Lambda}$ of \hll suggests that the matrix element of $<\Lambda \Lambda |v|\Lambda \Lambda>$ in \hll is around -2 MeV.

Light hypernuclei as chiral solitons

The binding energies of light strange hypernuclei calculated within the chiral soliton approach are in qualitative agreement with data. Charmed or beautiful hypernuclei are predicted to be bound stronger than strange hypernuclei.

NONMESONIC DECAY OF Λ AND ΛΛ HYPERNUCLEI

Nonmesonic weak decays of single and double hypernuclei are studied on the basis of two-body baryon-baryon interactions, ΛN → NN and ΛΛ → YN transitions. The two-body weak transition is described by the combination of two processes, i.e., meson exchange and direct quark processes. The result indicates the importance of the direct quark(DQ) contribution. It largely enhances both the total decay rate and Γnn/Γpn ratio, and its property is different from the heavy meson exchanges (i.e. η, ρ, ω, and K*). The quark-meson hybrid picture, such as π+K+ DQ , reproduces the current experimental data except for the asymmetry parameter for light hypernuclei. The ΛΛ → YN transition in ΛΛ hypernucleus is complement of the ΛN → NN transition because it occurs only in the J = 0 channel which is hidden by J = 1 transition in the ΛN → NN case.

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

Nonmesonic decay of the Λ hyperon in heavy hypernuclei

The nonmesonic decay of Λ hyperons has been investigated by the observation of delayed fission of heavy hypernuclei produced in proton–U collisions at Tp = 1.9 GeV. The lifetime of heavy hypernuclei with masses A ≈ 220 of the present work, τΛ = (138 ± 6 (stat) ± 17 (syst)) ps, is the most accurate result obtained in proton and antiproton induced collisions on a U target so far. This result together with the values of the lifetime measured previously by the COSY-13 collaboration, for p+Bi (161 ± 7(stat) ± 14(syst)) ps and p+Au (130 ± 13(stat) ± 15(syst)) ps, gives an average value for the lifetime of heavy hypernuclei with masses A > 180: τΛ = (145 ± 11) ps. This value was obtained by averaging the above lifetimes with weights taken as reciprocals of the sum of squares of statistical and systematic errors. By comparing the lifetimes of light and heavy hypernuclei we found—on the confidence level of 0.98—a violation of the phenomenological ΔI = 1/2 rule known from the weak mesonic decays of strange particles.

Nonmesonic weak decay of light hypernuclei with coherent Σ mixing

Nonmesonic weak decays of the A=4, and 5 hypernuclei are studied. The short range parts of the hyperon–nucleon weak interactions are described by the direct quark (DQ) weak transition potential, while the longer range interactions are given by the π and K meson exchange processes. Virtual Σ mixings of the coherent type are found to give significant effects on the decay rates of 4 Λ He. A large violation of the ΔI=1/2 rule is predicted in the J=0 transition amplitudes.

Electroproduction of hypernuclei: an experimental challenge

The hypernuclear spectroscopy program planned for Hall A of Thomas Jefferson National Accelerator Facility as well as the two fundamental changes to the Hall A experimental apparatus needed for this program are described. The program involves a precise study of various light hypernuclei {sup 7}Li{sub {Lambda}}, {sup 8}Be{sub {Lambda}}, {sup 12}C{sub {Lambda}} and {sup 16}O{sub {Lambda}}. The two changes consist of the addition of two septum magnets to the Hall A spectrometers, allowing measurements at angles as far forward as 6{phi}, and the installation of a Ring Imaging Cerenkov detector (RICH) in the detector package, greatly enhancing the particle identification capability.

Non-mesonic weak decay of light hypernuclei

Non-mesonic weak decay of light hypernuclei

Electroproduction of strange nuclei

The advent of high-energy, CW-beams of electrons now allows electro-production and precision studies of nuclei containing hyperons. Previously, the injection of strangeness into a nucleus was accomplished using secondary beams of mesons, where beam quality and target thickness limited the missing mass resolution. We review here the theoretical description of the (e, e'K+) reaction mechanism, and discuss the first experiment demonstrating that this reaction can be used to precisely study the spectra of light hypernuclei. Future experiments based on similar techniques, are expected to attain even better resolutions and rates.

Shell-model description of Λ hypernuclei

Empirical data on the spectra of light hypernuclei, especially the data from recent gamma-ray experiments, is used to constrain the parameters which govern the p(nucleon)s(Lambda) and p(nucleon)p(Lambda) two-body matrix elements that enter into shell-model calculations.

Role of deformation in the nonmesonic decay of light hypernuclei

We discuss the nonmesonic decay of deformed p-shell hypernuclei. The Nilsson model with angular momentum projection is employed in order to take into account the deformation effects. The nonmesonic decay rate and the intrinsic $\Lambda$ asymmetry parameter decrease as a function of the deformation parameter, while the ratio of the neutron- to proton-induced decay rates increases. We find that the deformation effects change these observables by about 10 % for $^9_{\Lambda}$Be from the spherical limit.

Electroproduction of kaons and light hypernuclei

A detailed investigation of the basic hyperon-nucleon interactions in nuclei is one of the aims of Experiment 91-016, approved with high priority at CEBAF, to study the electroproduction of kaons on targets of deuterium, {sup 3}He, and {sup 4}He. Inasmuch as both the electron and K{sup +} are particles that interact relatively weakly with nucleons, electroproduction of light hypernuclei provides a low-distortion method for investigating the fundamental interactions between nucleons, {Alpha}`s, and {Epsilon}`s in few-body systems. In particular, the (e,e`K{sup +}) reactions on cryogenic targets of D, {sup 3}He, and {sup 4}He will be studied at incident electron energies near 3 GeV with coincident detection of the emergent e and K{sup +} in the HMS and SOS magnetic spectrometers in Hall C. Construction of the He target, operating at {approximately}10 atm, {approximately}50 K and capable of dissipating {approximately}30 W, is expected to be complete prior to commencement of production runs in Hall C. The first data runs for E91-016, expected to begin late in FY 1995, will also be the basis for a doctoral thesis at Hampton University. In addition to providing new information on the phases of hyperon-nucleon interactions, measurements of cross sections for hypernuclear formation, and interference phenomena,more » the data may provide evidence for the presence of bound {Epsilon}`s and strange di-baryonic states that are the subject of considerable theoretical discussion.« less

Three- and four-body structure of light hypernuclei

Hypernuclear physics has become very exciting owing to new epoch-making experimental data. Recent progress in theoretical and experimental studies of hypernuclei and future developments in this field are discussed.

Strangelet and hypernucleus production in 11.5 A GeV c-1 heavy-ioncollisions

We report on the latest results for charged and neutral strangelets forExperiment 864. Approximately, 30×109 10% most central collisionswere sampled and no strangelets with A<100 and lifetimes greater than50 ns were observed. To understand the coalescence mechanism forstrangeness better, light hypernuclei production has beenstudied. In E864 we looked for 4ΛHand 3ΛH that may be formed inhigh-energy, heavy-ion collisions through coalescence ofnuclear fragments and hyperons and can be detectedthrough their mesonic decay products (4He,π-) and (3He, π-). Thepreliminary results from 13.3×109 10% mostcentral Au-Pt collisions at 11.5 A GeV c-1 arepresented.