This article surveys our empirical knowledge of the production processes, the nuclear interactions and the decay processes of K particles and hyperons, as this appeared at the close of 1956, and interprets these phenomena within the framework of the charge-independent classification scheme proposed by Gell-Mann and by Nishijima for these `strange particles'. The assignment of a `strangeness' quantum number to each strongly interacting particle is a deep-lying aspect of this scheme, and the main qualitative features of the empirical data allow a direct interpretation in terms of a law of conservation of strangeness through all strong interactions. Where appropriate, more detailed analyses of the phenomena are made on the assumption that charge independence holds for these strong interactions. The evidence on decay processes for K particles and hyperons is discussed in some detail. These processes violate the strangeness conservation law and also involve changes in the total isotopic spin for those cases where this quantity is relevant for the final particles. The success of Gell-Mann's ?T = ? rule in accounting for certain aspects of the decay data is considered for each of the latter cases; this rule is given a simple origin in Schwinger's model for strange particle decay processes, which is discussed briefly. The implications of symmetry between particle and antiparticle, as pointed out by Gell-Mann and Pais for the neutral K particles and their decay modes, is also discussed in some detail, together with an account of the experimental data on long-lived K? particles and anomalous K? decay modes. Indications in the data which bear on the spin and parity values for K particles and hyperons are stressed, especially for the decay modes associated with ? and ? particles. Although it is shown that the products of ? and ? decay necessarily differ in spin or in parity, a great similarity has been found concerning all other properties of the ? and ? particles, and the proposals which have been made to account for this situation are surveyed. For the ?? particle, observations on bound ?? hypernuclei provide much information relevant to the ??-nucleon interaction and the ?? spin and parity, and this is also discussed in some detail. A section is devoted to discussion of `anomalous events' for which no interpretation is known in terms of the interactions of the well-established particles. These events and the possibility that heavier unstable particles or perhaps resonance states may exist have an obvious importance, of special relevance for attempts to construct a theoretical foundation leading naturally to the Gell-Mann-Nishijima scheme. A brief discussion of the preliminary attempts to establish such a foundation is given. An appendix has been added in proof to discuss the recent observations on the failure of parity conservation and of charge conjugation invariance in certain weak decay processes, and the effects which a failure of these symmetry properties also for strange particle decays would have on the arguments made and the conclusions reached in this survey.

K Mesons and Hyperons their strong and weak interactions

The systematic study of hyperfragments has been continued. A total of 32 000 cosmic-ray stars, 10 000 6-Bev proton stars, 32 000 3-Bev ${\ensuremath{\pi}}^{\ensuremath{-}}$-meson stars and 206 stars produced by stopped negative $K$ mesons were examined. Twenty hyperfragments were found in the cosmic-ray plates, 7 in the proton plates, 30 in the ${\ensuremath{\pi}}^{\ensuremath{-}}$-meson plates and 11 in the negative $K$-meson plates. The average charge of the hyperfragments is between 4 and 5. The average range of the hyperfragments is about 10 microns. Including previous data from this laboratory, only 2 mesonic decays have been found out of 98 events with $Z$ greater than 2. A total of 7 helium hyperfragments have been observed of which 4 decayed mesonically. Of 4 hydrogen hyperfragments, all decayed with the emission of a ${\ensuremath{\pi}}^{\ensuremath{-}}$ meson. In the following eight cases, it was possible to measure the total energy release in the hyperfragment disintegration, and hence to find the following values (in Mev) for the binding energy of the ${\ensuremath{\Lambda}}^{0}$ particle: $_{\ensuremath{\Lambda}}\mathrm{H}^{3}({B}_{\ensuremath{\Lambda}}=0.6\ifmmode\pm\else\textpm\fi{}0.6)$; $_{\ensuremath{\Lambda}}\mathrm{H}^{3}({B}_{\ensuremath{\Lambda}}=\ensuremath{-}0.5\ifmmode\pm\else\textpm\fi{}0.6)$; $_{\ensuremath{\Lambda}}\mathrm{H}^{3}({B}_{\ensuremath{\Lambda}}=0.4\ifmmode\pm\else\textpm\fi{}0.7)$; $_{\ensuremath{\Lambda}}\mathrm{H}^{4}({B}_{\ensuremath{\Lambda}}=0.5\ifmmode\pm\else\textpm\fi{}2.0 or 1.9\ifmmode\pm\else\textpm\fi{}2.0)$; $_{\ensuremath{\Lambda}}\mathrm{He}^{4}({B}_{\ensuremath{\Lambda}}=0.0\ifmmode\pm\else\textpm\fi{}2.0)$; $_{\ensuremath{\Lambda}}\mathrm{He}^{4}({B}_{\ensuremath{\Lambda}}=1.8\ifmmode\pm\else\textpm\fi{}0.6)$; $_{\ensuremath{\Lambda}}\mathrm{He}^{5}({B}_{\ensuremath{\Lambda}}=2.0\ifmmode\pm\else\textpm\fi{}0.6)$; $_{\ensuremath{\Lambda}}\mathrm{Be}^{9}({B}_{\ensuremath{\Lambda}}=6.5\ifmmode\pm\else\textpm\fi{}0.6)$. The binding energies tend to increase with increasing mass number. The fact that $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ and $_{\ensuremath{\Lambda}}\mathrm{He}^{5}$ hyperfragments exist, plus the fact that the binding energy of the ${\ensuremath{\Lambda}}^{0}$ particle in $_{\ensuremath{\Lambda}}\mathrm{Be}^{9}$ is greater than that of the last neutron in ${\mathrm{Be}}^{9}$, shows that the Pauli principle need not be considered for a ${\ensuremath{\Lambda}}^{0}$ particle bound in a nucleus. If the binding of the ${\ensuremath{\Lambda}}^{0}$ particle can be described in terms of a potential well the depth of the well is greater than 6.5 Mev, as indicated by ${B}_{\ensuremath{\Lambda}}$ for $_{\ensuremath{\Lambda}}\mathrm{Be}^{9}$. In light hyperfragments the low binding energies imply that the ${\ensuremath{\Lambda}}^{0}$ particle spends much of its time outside the nucleus. The momentum of the ${\ensuremath{\Lambda}}^{0}$ particle has been measured in several light hyperfragments where a ${\ensuremath{\pi}}^{\ensuremath{-}}$ meson and proton were emitted, by assuming it to be equal to the momentum of the ${\ensuremath{\pi}}^{\ensuremath{-}}$ meson and proton. In general the momentum values are quite low, which supports the hypothesis that the bound ${\ensuremath{\Lambda}}^{0}$ particle spends considerable time outside the nucleus. No further examples of energetic hyperfragments, as have been previously reported, were found in this work.

Disintegration of hyperfragments

A large emulsion stack, exposed at high altitude, has been used to study the decaymodes of K-particles which produce a single charged secondary. The 5 decay modes, K
 μ
 , χ, ϰ and τ′ have been recognized from the nature and energy of the charged secondary. The masses of the parent particles of the two-body decay modes, K
 μ
 and χ found from the mean range of the charged secondary are (976±7) me and (969±3) me respectively. Independent values derived from measurements of the scattering of the secondary track near the decay point are (954±17) me and (972±15) me respectively. The reliability of these mass values is discussed. An examination of the relative frequency of the decay modes in these experimental conditions indicates that the K
 μ
 -and χ-modes constitute respectively about 67 % and 20 % of all the K-particle decays: the Kβ, ϰ- and τ′-modes being present to about 9 %, 3 %, and 1 % respectively.

On the masses and modes of decay of heavy mesons produced by cosmic radiation

Charge Independence in Light Hyperfragments

Si presentano i risultati di una analisi sistematica sulle particelle pesanti instabili effettuata mediante pellicole di emulsione nucleare esposte alla radiazione cosmica durante la Spedizione Internazionale di Sardegna del 1953. Dopo una breve introduzione sui metodi sperimentali usati, vengono esposti dati riguardanti i mesoni pesanti, gli iperoni ed i frammenti instabili. Nelle conclusioni del lavoro sono presentate alcune considerazioni sulle frequenze relative dei vari eventi e sulla attendibilita degli schemi di decadimento proposti.

Particelle pesanti instabili in emulsioni nucleari

Observations on K-meson decays

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Particelle pesanti instabili in emulsioni nucleari

Observations on K-meson decays