ar X iv : n uc l - th / 0 11 20 36 v 1 1 2 D ec 2 00 1 Weak decay of Λ – hypernuclei

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 ΓNM = Γn+Γp (+Γ2), Γπ0 and Γπ− 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 Γn/Γp. However, the large uncertainties involved in the experimental determination of the ratio do not allow to reach any definitive conclusion. The Γn/Γp puzzle is strongly related to the so–called ∆I = 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 Γn/Γp have been considerably reduced very recently at KEK (however, with central data compatible with older experiments), a clean extraction of Γn/Γp 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 Γn/Γp. 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, aΛ, for 5 Λ ~ He. This is in complete disagreement with a previous measurement, which obtained a large and negative aΛ for p–shell hypernuclei, and with theory, which predicts a negative value moderately dependent on nuclear structure effects. Also in this case, improved experiments establishing with certainty the sign and magnitude of aΛ for s– and p–shell hypernuclei will provide a guidance for a deeper understanding of hypernuclear dynamics and decay mechanisms.