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Nobel prize to C. F. Powell awarded in 1950 "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with his method''

LATTES 1947B

Lattes, C.M.G.; Occhialini, G.P.S.; Powell, C.F.;
Observation on the Tracks of Slow Mesons in Photographic Emulsions
Nature 160 (1947) 453;

Reprinted in
R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 41.

Introduction
In recent experiments, it has been shown that charged mesons, brought to rest in photographic emulsions, sometimes lead to the production of secondary mesons. We have now extended these observations by examining plates exposed in the Bolivian Andes at a height of 5,500 m, and have found, in all, forty examples of the process leading to the production of secondary mesons. In eleven of these, the secondary particle is brought to rest in the emulsion so that its range can be determined. In Part I of this article, the measurements made on these tracks are described, and it is shown that they provide evidence for the existence of mesons of different mass. In Part 2, we present further evidence on the production of mesons, which allows us to show that many of the observed mesons are locally generated in the "explosive'' disintegration of nuclei, and to discuss the relationship of the different types of mesons observed in photographic plates to the penetrating component of the cosmic radiation investigated in experiments with Wilson chambers and counters.
Our preliminary measurements appear to indicate, therefore, that the emission of the secondary meson cannot be regarded as due to a spontaneous decay of the primary particle, in which the momentum balance is provided by a photon, or by a particle of small rest-mass. On the other hand, the results are consistent with the view that a neutral particle of approximately the same rest-mass as the μ-meson is emitted. A final conclusion may become possible when further examples of the μ-decay, giving favorable conditions for grain-counts, have been discovered. (Extracted from the introductory part of the paper.).

Accelerator COSM Detectors EMUL

Related references
More (earlier) information appears in
G. P. S. Occhialini and C. F. Powell, Nature 159 (1947) 93;
G. P. S. Occhialini and C. F. Powell, Nature 159 (1947) 186;
C. M. G. Lattes et al., Nature 159 (1947) 694;
More (later) information appears in
C. M. G. Lattes, G. P. S. Occhialini, and C. F. Powell, Nature 160 (1947) 486;

Reactions

π± μ± neutral

cs

Particles studied

	π–	ex, pw
	π+	ex, pw

Record comments
Confirmation of the π^–. First evidence for pion decay π^± → μ^± neutrals.

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