Nonmesonic decay of the Λ-hyperon in hypernuclei produced by p + Au collisions

Abstract

Abstract. The lifetime of the Λ-hyperon for the nonmesonic decay ΛN → NN has been determined by a measurement at COSY Jülich of the delayed fission of heavy hypernuclei produced in proton -Au collisions at Tp=1.9 GeV. It is found that heavy hypernuclei with mass numbers A≈(180±5) and atomic numbers Z≈(74±2) fission with a lifetime τΛ=130 ± 13(stat.) ± 15(syst.) ps. This value together with the results obtained for other heavy hypernuclei in previous investigations indicates -on the confidence level of 0.9 -a violation of the phenomenological ∆I=1/2 rule for the ΛN → NN transitions as known from the weak mesonic decays of kaons and hyperons. The free Λ-hyperon undergoes almost with 100% probability the mesonic decay, i.e. Λ → π − + p or Λ → π o + n. The energy release (≈ 40 MeV) and its sharing among pion and nucleon implies (due to momentum conservation) that the energy of the nucleon is much smaller than the Fermi energy of nucleons in the nucleus. Therefore, such a process is strongly suppressed in nuclei and a different type of the hyperon decay -nonmesonic decaydominates for all but the lightest hypernuclei. This decay can be induced by neutrons (n+Λ → n+n) or by protons (p+Λ → p+n) with an energy release much higher than in the mesonic decay (≈ 180 MeV). The higher nucleon energy, due to an equal sharing of the energy among both nucleons, implies that this process is not blocked by the Pauli principle.The nonmesonic Λ -decay represents an example for the nonleptonic weak interaction of baryons with a change of strangeness (∆S = 1) and isospin (∆I= 1/2 or 3/2). Thus it is an analogue of the weak nucleon -nucleon interaction, but it involves a new degree of freedom, i.e. strangeness. The strong and Coulomb interactions preserve strangeness and therefore the weak interaction responsible for the nonmesonic decay is not masked by contributions from these two interactions. Therefore the nonmesonic decay enables to study both parity violating and parity conserving amplitudes in contrast to the nucleonnucleon interaction, where the latter amplitudes are completely masked by strong and Coulomb forces [1].The standard model of electro-weak interactions favors neither ∆I= 1/2 transitions nor the ∆I= 3/2 transitions, but experimental investigations on the properties of mesonic decays of kaons and hyperons lead to the obvious dominance of the ∆I= 1/2 part (the so called ∆I= 1/2 rule) [2]. The question arises whether this is also the case for the nonmesonic decay of the Λ-hyperon. Data from the nonmesonic decay of light hypernuclei, which were used to test this hypothesis in the phenomenological model proposed by Block and Dalitz [3], are affected by too large errors to solve this problem unambigously [4]. Another possibility for testing the validity of the ∆I= 1/2 rule is an investigation of the dependence of the hyperon lifetime for the nonmesonic decay on the mass of the hypernucleus in which the hyperon is embedded [5]. Such a test requires, besides the knowledge of the lifetimes of light hypernuclei, also the precis...

Lifetime of heavy hypernuclei and its implications on the weak ΛN interaction

“…This reduces the load on the detectors in the prompt fission region for Au and Bi targets by about an order of magnitude relative to U . These expectations (calculations) were confirmed in the actual experiments at COSY-Jülich us-ing Bi [32], Au [45] and U targets [46], where similar cross sections could be observed experimentally.…”

“…Au130±13(stat.)±15(syst.) 130 ± 20[45] Bi 161 ±7(stat.)±14(syst.) 161 ± 16[32] U 138 ±6(stat.)±17(syst.)…”

Weak Decay of Λ–hypernuclei

“…Experiments of this kind are very difficult to perform (the produced hypernucleus cannot be unambiguously identified) and, as already mentioned, the lifetimes are generally measured with large errors. Only very recently [42,138,139], more accurate results have been obtained from delayed fission experiments. Nevertheless, there is a certain disagreement among these new data, and the saturation value of the lifetime for very heavy hypernuclei is not established with precision.…”

“…At Jülich (COSY) [42,139], by using proton-nucleus scattering processes, the total lifetime of very heavy hypernuclei (in the region of bismuth and uranium: A ≃ 200 ÷ 240) has been measured by delayed fission observations. By using the same techniques [138], again at COSY, the lifetime for hypernuclei with mass numbers A = 180±5, produced in p-Au collisions, has been obtained. At BNL AGS [141], the 12 Λ B hypernucleus has been produced by the (stopped K − , π 0 ) reaction, the Λ being created on a proton, with the final π 0 detected by a high energy resolution (less than 1 MeV FWHM) neutral meson spectrometer.…”

Production of Λ-hypernuclei in A(p,K + ) Λ B reactions

“…In recent years, with the developments of the high-resolution detector and the high luminosity proton beam, several laboratories initiated the relevant experiments of this kind. For example, the lifetimes of heavy hypernuclei produced in the Au, Bi and U target experiments were measured by detecting the delayed fission by the COSY-13 Collaboration [1][2][3] . The production and decay experiments of hypernuclei were carried out at Dubna [4] .…”