Light Hypernuclei Research Articles

Some possibilities for unusual light hypernuclei

Some possibilities are considered concerning the stability, formation, and identification of λ-hypernuclei corresponding to core nuclei (generally withI 3/2) away from the stability valley. The identification of such hypernuclei may be rather informative concerning the existence and properties of such little-known nuclei. For neutron-rich species,e.g.8Hλ,8Heλ, etc., the possible confusions of their major decay. modes with (and their separation from) similar well-established modes of more abundant hypernuclei are discussed. The expectation that narrow resonance levels (with lifetimes ≿10-21 s) should exist for certain hypernuclear systems (e.g. the p-4HegL system) is also mentioned briefly, since the production and observation of such resonance states could be achieved with existing techniques.

The spins and lifetimes of the light hypernuclei

The spins and lifetimes of the light hypernuclei

Binding energies of Λ particles and the Λ-N interaction

The implications of Λ-binding energies for the Λ-nucleon interaction have been considered when three-body forces are included in addition to two-body Yukawa forces. In part I the light hypernuclei with baryon number ≦ 5 are considered. For ΛH 3 the effect of three-body forces is small and may be obtained by a perturbation procedure; the binding energy of ΛH 3 being almost entirely determined by the two-body forces. For ΛH 5 the He 4 core is assumed to be undistorted by the Λ. The binding energies of ΛH 4, ΛHe 4 do not seem capable of yielding significant information about the interaction strenghts. The two-body system ΛH 2 is not bound unless the three-body forces are very strongly attractive. In part II the potential felt by a Λ in nuclear matter, this being considered as a Fermi gas, is calculated using perturbation theory. Both direct and exchange two-body forces are considered. The second order contributions, the effect of nuclear pair correlations and the velocity dependence are all small. The nucleon exchange factor used for the three-body forces strongly suppresses their effect in nuclear matter. The implications of ‘empirical’ values of the Λ-potential depth are considered. In particular if the pion-baryon couplings predominate then strongly attractive three-body forces of not too large a range are favoured, corresponding to a larger singlet than triplet two-body volume integral.

ΛBinding in Hypernuclei by Nonlocal Interaction

The characteristics of the Λ-N interaction at low energy have been obtained assuming that the Λ-N potential is nonlocal but separable and similar to that suggested by Yamaguchi in the case of N-N potential. The unknown parameters entering in the proposed potential are determined on the basis of the global symmetry hypothesis of the strongly interacting particles. Our model predicts, in agreement with Dalitz and Downs' phenomenological findings of the nature of the Λ-N potential based on hyperfragment data, that there is no bound Λ-nucleon system and that the singlet Λ-N potential is stronger than the triplet potential, both being attractive. Binding energies of the Λ particle in light hypernuclei based on the present model are, however, much too high compared with the experimental data. It is further pointed out that although the global symmetry hypothesis (g Λπ =g Σπ =g Nπ ) supplemented by the Yamaguchi type nonlocal Λ-N potential is incompatible with the presently existing data, the restricted symmetry (g Λπ =g Σπ ≠g Nπ ) model is certainly admissible.

Hyperfragments produced by K− capture in nuclear emulsion: ΰ0 Decay modes

This is a systematic study of the π0-decays of light hypernuclei (A⩽5) produced by K− capture in nuclear emulsion. Approximately 3·104 K− stars were examined, yielding a total of 94 l-prong events satisfying the following conbitions:Rc⩾5 µm, 8 µm⩽R0⩽50 µm,θ⩾66°, whereRc is the range of the primary (connecting track),R0 that of the secondary (recoil) andθ the angle between their directions at their point of intersection. Of the 69 events which could be charge (Z) identified, 55 hadZ=1 and 14Z=2. In theZ=1 category the true π0-events could not be separated from 1-prong Σ−-events because of the overwhelming (≳ 90%) contribution of the latter. It is concluded that probably all the 14Z=2 events represent true HeΛ decays (∼ 104HeΛ and ∼ 45HeΛ). In addition, three 2-prong events were consistent with the interpretation as4HeΛ decays. One can define for the π0-decays of4HeΛ the ratio:R′(4HeΛ)=(two-body decays)/(all decays — «unobservable» ones); the «unobservable» ones being π0-neutron-recoil decays with recoil momenta <110 MeV/c and other modes with ⩾1 neutron. A valueR′(4HeΛ)=0.82−.18+.15 is found here. The analogous quantity for4HΛ was computed from the data of ref. (10) and yieldsR′(4HΛ)=0.88−0.06+0.07. These two numbers are equal within their experimental errors. This equality is consistent with the assumption |p_/s_|=|p0/s0| as predicted by the ΔT=1/2 rule. The ratio, ϱ, of two body π0-decays of4HeΛ to the total number of π−-decays of4HeΛ was determined to be 0.7±0.4. Accordingly as the spin of4HeΛ is 0 or 1, ϱ is a measure of the strength either of thes- or of thep-wave channel in the neutral mode of the free Λ decay.

Hypernuclear Binding Energies and theΛ-Nucleon Interaction

A phenomenological analysis is made of the binding-energy data for light hypernuclei in terms of a two-body $\ensuremath{\Lambda}$-nucleon interaction, in which account is taken of the empirical information available on the structure of light nuclei. It is found that the observed binding energies can be interpreted in terms of a spin-dependent $\ensuremath{\Lambda}$-nucleon interaction; the agreement obtained is adequate for any force-range between 0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm and 0.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm (for Yukawa shape) and any exchange character for this interaction. From observations on $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ decay it is argued that the $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$, $_{\ensuremath{\Lambda}}\mathrm{He}^{4}$ doublet has zero spin, which requires that the singlet $\ensuremath{\Lambda}$-nucleon interaction be more attractive than the triplet. The well-depth parameter for the singlet $\ensuremath{\Lambda}$-nucleon interaction has values from 0.90 to 0.85 for ranges between 0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm and 0.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm, the triplet interaction being also attractive but only one-half to one-third as strong. Spin values are assigned to the light hypernuclei, and the angular correlations which could provide a check on these values are discussed. The absorption of ${K}^{\ensuremath{-}}$ mesons in helium discussed; it appears that the selection rules for the production of $A=4$ hypernuclei in this reaction may be confused by the existence of an excited state $_{\ensuremath{\Lambda}}\mathrm{H}^{4*}$, $_{\ensuremath{\Lambda}}\mathrm{He}^{4*}$. A number of uncertainties in the phenomenological analysis of the heavier hypernuclei are pointed out; the present data do not disagree with the conclusions based on the analysis of the lighter and better-known hypernuclei with $Al~5$, but they add little weight to these conclusions.

General Clinics

The results of a compilation of the binding energy values of light hypernuclei (A ⩽ 16) obtained by the European K− Collaboration and the EFINS-North-western Collaboration are presented. The events were analysed by the same computer programme and uniform stringent selection criteria were applied.