Weak Interactions (Second Section) Chapter 8: Decays of Charged K Mesons

Weak non-mesonic decays of Λ-hypernuclei

422 K- and 13 ∑--capture stars emitting mesic hyperfragments (MHF) have been studied in an attempt to understand the mechanism of hyperfragment production. 123 K- stars emitting8Li fragments were also considered. Several criteria were used to separate K- captures in C, N, O and Ag, Br. Lower limits for the proportion of MHF produced in C, N, O are 57%, 73%, 84%, and 94% for HΛ, HeΛ, LiΛ and MHF of Z ≥ 4. For8Li, this lower limit is 75%. An overall frequency of (1.05±0.05)·10-2 MHF/K--capture and (3.8±1.4) · 10-2 HF/∑--capture is obtained. The energy spectrum of the pions emitted in the capture stars indicates that a substantial fraction of the emitted bound Λ’s originates in ∑-conversion processes. Indication of bound-Λ production from multinucleon K--capture processes was obtained, important in the case of HΛ. The momentum spectra for different hypernuclear species are very similar and the spectra of HeΛ and LiΛ are, in turn, comparable with the momentum spectra of He and8Li respectively. Hyperfragments are emitted preferentially opposite to the direction of emission of an associated pion or fast proton. The backwards-forward (B/F) ratio is for π, MHF events, 1.8±0.4. Two models for the production of hyperfragments are proposed. According to the first (« trapped-Λ ») the Λ created in the elementary process is trapped in the nuclear potential well of the parent nucleus. The HF is then emitted in the process of nuclear de-excitation. In the second model (« prompt-HF ») the HF is emitted promptly following the K- or ∑- interaction with a group of nucleons. It is concluded that both mechanisms may contribute. The occurrence of both processes can explain essentially all the salient features of hyperfragment emission.

Mesic decays of hypernuclei from K−-capture

Hypernuclei produced by K− interactions in nuclear emulsions (II) general characteristics and non-mesic decays

We have studied the consequences of the ΔT=1/2 selection rule on the energy spectra of the pions in K+→3π, K 2 0 →3π decay modes. Without any dynamical assumptions one can conclude that the π+, π0 spectra in K+→π0+π0+π+ and the π0, π+ (or π−) spectra in K 2 0 →π++π−+π0 respectively should be the same. The effect of strong final state π-π interactions on the general structure of the decay amplitude is discussed and the possibility of detecting experimentally the presence of such strong interactions is suggested. The question of whether the experimental spectra provide a critical test of ΔT=1/2 rule is also considered.

Test for ΔT=1/2 in K+→3π and K 2 0 →3π decay modes

A survey is given for the possibility of accounting for strange particle decays by the addition of further terms of a weak interaction current J/sub mu / , but which do not conserve strangeness. Pionic decay modes for strange particles are described in terms of the current J/sub mu / and four-fermion coupling of the form V-A. The possibility of isotopic spin relationships in weak decay processes has been considered in connection with the isotopic spin character of the weak interaction currents. Leptonic modes of strange particle decay are discussed. (C.J.G.)

R.H. Dalitz

Papers

Strange particle decay processes and the fermi interaction