Mesonic weak decays of Λ- and ΛΛ-hypernuclei

doi:10.1016/0375-9474(92)90717-X

Nuclear Physics A

Volume 547, Issues 1–2, 14 September 1992, Pages 115-126

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Abstract

Systematic estimates of pionic decays of light-to-heavy hypernuclei are presented with the discussion on the effects of shell-structure, Pauli blocking and pion distortion. For light Λ- and ΛΛ-hypernuclei, a practical method is proposed to calculate realistically the continuum pion spectra for three-body decay modes.

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Weak decay of Λ-hypernuclei
2002, Physics Report
In this review we discuss the present status of strange nuclear physics, with special attention to the weak decay of Λ-hypernuclei. The models proposed for the evaluation of the Λ decay widths are summarized and their results are compared with the data. The rates $Γ_{NM} =Γ_{n} +Γ_{p} (+Γ_{2})$ , Γ_π⁰ and Γ_π⁻ are well explained by several calculations. Despite the intensive investigations of the last years, the main open problem remains a sound theoretical interpretation of the large experimental values of the ratio Γ_n/Γ_p. However, the large uncertainties involved in the experimental determination of the ratio do not allow to reach any definitive conclusion. The Γ_n/Γ_p puzzle is strongly related to the so-called ΔI=1/2 rule on the isospin change in the non-mesonic decay, whose possible violation cannot be established at present, again due to the insufficient precision of the data. Although recent works offer a step forward in the solution of the puzzle, further efforts (especially on the experimental side) must be invested in order to understand the detailed dynamics of the non-mesonic decay. Even if, by means of single nucleon spectra measurements, the error bars on Γ_n/Γ_p have been considerably reduced very recently at KEK (however, with central data compatible with older experiments), a clean extraction of Γ_n/Γ_p is needed. What is missing at present, but planned for the next future, are measurements of (1) nucleon energy spectra in double coincidence and (2) nucleon angular correlations: such observations allow to disentangle the nucleons produced in one- and two-body induced decays and lead to a direct determination of Γ_n/Γ_p. Notably, the two-body component of the non-mesonic decay rates has not been measured yet, due to the too low counting rates expected for a coincidence experiment. For the asymmetric non-mesonic decay of polarized hypernuclei the situation is even more puzzling. Indeed, strong inconsistencies appear already among data. A recent experiment obtained a positive intrinsic Λ asymmetry parameter, a_Λ, for $_{Λ}^{5} H → e$ . This is in complete disagreement with a previous measurement, which obtained a large and negative a_Λ for p-shell hypernuclei, and with theory, which predicts a negative value moderately dependent on nuclear structure effects. Also in this case, improved experiment establishing with certainty the sign and magnitude of a_Λ for s- and p-shell hypernuclei will provide a guidance for a deeper understanding of hypernuclear dynamics and decay mechanisms.
Nonmesonic weak decay of light hypernuclei with coherent ∑ mixing
2002, Nuclear Physics A
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.
The π- mesonic decay rates on 12<inf>Λ</inf>C , 28<inf>Λ</inf>Si and <inf>Λ</inf>Fe
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π+ emission from hypernuclei and the week δI = 3/2 transitions
1999, Nuclear Physics A
Low energy π⁺ emission in hypernuclear weak decays is studied in the soft pion limit. It is found that the π⁺ decay amplitude is dominated by the $ΔI = 32$ part of non-mesonic weak decays according to the soft pion theorem. The ratios, $R(π^{+} soft π^{−})$ , for light hypernuclei are estimated in the direct quark mechanism, which predicts violation of the $ΔI = 12$ rule, and found to be about $13$ for _Λ⁴He.
Double-Λ hypernuclei and the nuclear medium effective ΛΛ interaction
1999, Nuclear Physics A
We fit the ¹S₀ ΛΛ interaction in the nuclear medium to the masses of the experimentally known double-Λ hypernuclei: _ΛΛ⁶He, _ΛΛ¹⁰Be and _ΛΛ¹³B. We derive this effective interaction from OBE Jülich ΛΛ-type potentials and using both Hartree-Fock and variational approaches. We find that the inclusion of ΛΛ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double-Λ hypernuclei to the ωΛΛ coupling and the form factor at the σΛΛ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double-Λ hypernuclei, not discovered yet. Finally, we discard the existence of ¹S₀ ΛΛ bound states provided the ΛΛ-ΞN coupling can be neglected.
Mesonic and non-mesonic decay widths of 4<inf>Λ</inf>H and 4<inf>Λ</inf>He
1998, Nuclear Physics A
Results are reported on precise measurements of the weak-decay processes of_Λ⁴ H and_Λ⁴ He hypernuclei formed via the⁴He (K_stopped⁻, π⁻) reaction. The lifetimes of_Λ⁴H (194₋₂₆⁺²⁴ ps) and_Λ⁴He (256 ± 27 ps) were obtained. The π⁰ and π⁻ decay branching ratios of_Λ⁴He (0.515 ± 0.035 and 0.322 ± 0.034, respectively) were precisely determined and they are compared with recent theoretical estimates and related experimental values. Our results favor a Λ-nucleus interaction with an inner repulsive core. The non-mesonic weak decay widths of_Λ⁴H and_Λ⁴He are reexamined on the basis of our measurements and updated knowledge of the mesonic decay processes. It is found that the non-mesonic decay of_Λ⁴He predominantly takes place via the Λ + p → n + p process.

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Nuclear Physics A

Abstract

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Cited by (22)

Weak decay of Λ-hypernuclei

Nonmesonic weak decay of light hypernuclei with coherent ∑ mixing

The π<sup>-</sup> mesonic decay rates on <sup>12</sup><inf>Λ</inf>C , <sup>28</sup><inf>Λ</inf>Si and <inf>Λ</inf>Fe

π<sup>+</sup> emission from hypernuclei and the week δI = 3/2 transitions

Double-Λ hypernuclei and the nuclear medium effective ΛΛ interaction

Mesonic and non-mesonic decay widths of <sup>4</sup><inf>Λ</inf>H and <sup>4</sup><inf>Λ</inf>He