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

From the late 1920s to the early 1950s, cosmic rays were the main instrument to investigate what we now call "high-energy physics". In approximately 25 years, an intense experimental and theoretical work brought particle physics from its childhood to its maturity. The data collected at that time played a crucial role in order to outline the quantum theories of fundamental interactions - electromagnetic, weak and strong - and they were an excellent training field for many young scientists. The knowledge acquired on cosmic rays constituted the basis of the extraordinary progresses that were to be achieved in fundamental physics with the introduction of particles accelerators. 
Many papers on this subject have already been published, both by historians and by the scientists who personally worked at that time. However, a complete historical reconstruction of those 25 years of cosmic-ray researches has not been written yet. In particular, an overall description of the researches carried out with the contributions of Italian physicists is still missing. The present paper tries to outline such a description, taking into account the fact that in those very years, for scientific and non scientific reasons, physics researches were getting organised within international collaborations. Without pretending to be exhaustive at all, we would like this paper to be a stimulus for further researches on the subject.

The Italian contributions to cosmic-ray physics from Bruno Rossi to the G-Stack. A new window into the inexhaustible wealth of nature

Results on heavy mesons and hyperons from cloud chamber ionization measurements

The properties of the heavy unstable particles

The double production of a Λ∘ and aK μ +

After a description of the methods used in the measurements of momentum and range, the data on 46 S-events obtained with a double cloud-chamber experiment are examined. Momentum-range measurements on 22 positive S’s give a mass consistent with a single value at (928 ± 13) me. The analysis of the ranges of the secondaries shows, however, that not all S-events can be explained by a single decay mode. A large proportion of the secondaries correspond to a light meson of unique range (75.7 ± 1.7) g·cm−2 Cu. The comparison of this range with the measured masses of the primary particles proves that these secondaries are μ-mesons. The accompanying neutral secondary is shown at the same time to be a particle of zero mass. The absence of cascade showers associated with S-events with long-range secondaries shows that the neutral secondary is a neutrino. The decay mode Kμ → μ + ν previously proposed is more firmly established. Two independent mass estimates are obtained for the Kμ-particle. One—from momentum-range measurements—gives directly (906 ± 27) me. Another—from the mode of decay and the range of the μ-secondary—gives indirectly (941 ± 11) me. A final best mass of (935 ± 15) me is obtained by the combination of the two estimates after taking into account possible additional errors. The Kμ-particle is then shown to be essentially positive and to constitute (66 ± 18)% of the S’s which passed through the top chamber. No precise information can be obtained as to the mode of decay of S-particles which are known not to belong to the Kμ-decay mode.

Further discussion of the K μ decay modedecay mode

Results of the study of s-events in the double cloud-chamber of the école polytechnique

Among 35 000 pictures of the double cloud chamber a t the Pic du Midi [1], we have found five posi t ive and four negat ive z-mesons decaying in flight in the naomentum chamber . All five posi t ive events are fully measurable , bu t only one of the negatives. The three o ther negat ive events are mos t p robab ly decays in flight wi th three charged secondaries and are assumed to be z-> 3~ decays. We have analyzed all the four-prong events and found two which are not z-meson decays. One identified as n chance coincidence of a negat ive t rack and the decay point of a 0~ and one which is mos t p robab ly a cascade decay of a. negaMve hyperon, ~ccording to the scheme ~ -* A ~ A ~ --> p+rc . The distance be tween the two decay points is so small t h a t the event looks like a three-body decay. Table I resumes the da ta on five posi t ive and one negat ive event : The fi~:st 9 lines of Table I give the measured values: P is the m o m e n t u m in MeV/e. A refers to the pr imary , B, C and D to the secondaries. The meaning of the angles can be seen in Fig. 1. The following lines of Table I give the calculated quant i t ies: ~ _P~ is the sum of the longitudinal m o m e n t a and E P~ the vec tor sum of the t ransverse m o m e n t a of t h e three secondaries wi th respect to the direction of the pr imary . P~ adj. is the weighted mean value of the m o m e n t u m of the p r imury and ~ P~.

τ-Meson in the momentum cloud chamber of the école polytechnique

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Further discussion of the K μ decay modedecay mode

Results of the study of s-events in the double cloud-chamber of the école polytechnique

τ-Meson in the momentum cloud chamber of the école polytechnique