Production of heavy unstable particles by 1.37-bev pions

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

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

A number of independent accurate measurements of the masses of light mesons are critically reviewed. Assuming the positive-and negative-charged mesons to have equal masses, best values for the pion and muon masses can be calculated. It is found that the negative π- and μ-mesic X-ray energies measured by K absorption edges can be combined with the mass difference between the positive pion and muon to give a result with a standard deviation approximately one third of the assigned errors of the best previous measurement. All the results are in satisfactory agreement and can be combined in the usual way. The mass results are as follows:Mμ=(206.86±0.11) me;Mπ==(273.27±0.11) me. Experimental data on the mean lives, spins, parities, and decay modes are also reviewed and the results are given. The measurements of the masses of the various K-mesons are obtained in several ways:MKπ2=(966.0±1.5) me;MKπ2=(965.3±1.9) me;MΣ=(966.8±0.4) me. There is at present no evidence for a difference in the masses to a precision of 1 ÷ 2 me. Results on the Σ+-Σ− and τ+ masses and lifetimes and the analysis of τ+ decay are discussed. At present the experimental equality of these particles seems to be contradictory to assigning a spin of less than 2. The cosmic-ray data on the ϑ0 lifetime are reviewed and recent results on artificially-produced ϑ0’s are discussed. The hyperon mass data are reviewed.MΔδ=(2181.74±0.35) me;MΣδ=(2 327.4±1.0) me. The latest data on a Σ+ − Σ− mass difference and their mean lives are presented.

The masses of light mesons, K-mesons and hypersons in 1956

Relations and inequalities are given-which can be verified by experiment-between the transition probabilities of different decaymodes of new particles and light hyperfragments. In particular resstrictions are found for the relative probabilities of the different decay modes of charged ɛ-particles, for the relative probability of the Τ and of the ǵt-decays, and for the different mesonic decay modes of the lightest λ0-nuclei. These restrictions follow from the hypothesis that the weak decay interactions transform in charge space as the component, which satisfies charge conservation, of a spherical tensor of rank 1/2.

Charge properties of the weak decay interactions of the new particles

A photograph obtained in a Wilson cloud chamber, operated in a magnetic field, shows the following unusual event: A positive particle, produced in an interaction above the cloud chamber, decays in flight into a positive particle less massive than a $K$ meson. Four other lightly ionizing particles also originate from the decay point; these appear in the form of two small-angle pairs, each pair consisting of one positive and one negative particle. The observed momenta and ionizations are consistent with three of these four particles being either electrons or mesons; the fourth must be an electron.The event may be interpreted as the decay: ${K}^{+}\ensuremath{\rightarrow}\left({{\ensuremath{\pi}}^{+}}{{\ensuremath{\mu}}^{+}}\right)+{\ensuremath{\pi}}^{0}+Q \mathrm{Mev},$ followed by the decay of the ${\ensuremath{\pi}}^{0}$ meson into four electrons (a possible but hitherto unobserved mode of decay): ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}+{e}^{\ensuremath{-}}+{e}^{+}+{e}^{\ensuremath{-}}.$This interpretation leads to a remarkable internal consistency of the data and is supported by the following experimental facts: (i) There is good overall transverse-momentum balance in two mutually perpendicular planes.(ii) If we assume that all the particles in the above group of four are electrons resulting from the decay of a neutral particle the mass of the latter, determined from energy-momentum balance, is $({{255}_{\ensuremath{-}10}}^{+15}){m}_{e}$, in good agreement with the known mass of the ${\ensuremath{\pi}}^{0}$ meson.(iii) A transformation to the rest system of the ${\ensuremath{\pi}}^{0}$ meson shows that in this frame the four electrons come off as two small-angle pairs traveling in opposite directions. This is the most probable configuration in the four-electron decay of a ${\ensuremath{\pi}}^{0}$ meson.The $Q$ values calculated for the ${K}^{+}$ decay are: $Q({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{0})=({{213}_{\ensuremath{-}10}}^{+15})$ Mev and $Q({\ensuremath{\mu}}^{+}, {\ensuremath{\pi}}^{0})=({{207}_{\ensuremath{-}10}}^{+15})$ Mev. Comparison of the first $Q$ values with that for the ${\ensuremath{\theta}}^{0}$ particle $Q({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{\ensuremath{-}})=214\ifmmode\pm\else\textpm\fi{}5$ Mev, suggests that the unstable meson observed in this event and designated phenomenologically above as a ${K}^{+}$ meson may be a charged counterpart of the ${\ensuremath{\theta}}^{0}$ particle.This interpretation may explain at least some of the cases, observed by other workers, in which $\ensuremath{\gamma}$ rays appear to be associated with $S$ particles.Other possible interpretations of the event are considered.

Cloud-Chamber Evidence for a Charged Counterpart of the θ 0 Particle

Cloud-Chamber Observations on ChargedVParticles

Example of the cascade decay of a negative hyperon

Recent cloud chamber results on masses of heavy mesons and hyperons

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Cloud-Chamber Evidence for a Charged Counterpart of the θ 0 Particle

Cloud-Chamber Observations on ChargedVParticles

Example of the cascade decay of a negative hyperon

Recent cloud chamber results on masses of heavy mesons and hyperons