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CONVERSI 1947

Conversi, M.; Pancini, E.; Piccioni, O.;
On the Disintegration of Negative Mesons
Phys. Rev. 71 (1947) 209;

Reprinted in
R. N. Cahn and G. Goldhaber, The Experimental Foundations of Particle Physics, Cambridge Univ. Press (1991) 37.
The Physical Review - the First Hundred Years, AIP Press (1995) 817.

Motivation
In a previous Letter to the Editor, we gave a first account of an investigation of the difference in behavior between positive and negative mesons stopped in dense materials. Tomonaga and Araki showed that, because of the Coulomb field of the nucleus, the capture probability for negative mesons at rest would be much greater than their decay probability, while for positive mesons the opposite should be the case. If this is true, then practically all the decay processes which one observes should be owing to positive mesons.
Several workers have measured the ratio η between the number of the disintegration electrons and the number of mesons stopped in dense materials. Using aluminum, brass, and iron, these workers found values of η close to 0.5 which, if one assumes that the primary radiation consists of approximately equal numbers of positive and negative mesons, support the above theoretical prediction. Auger, Maze, and Chaminade, on the contrary, found η to be close to 1.0, using aluminum as absorber.
Last year we succeeded in obtaining evidence of different behavior of positive and negative mesons stopped in 3 cm of iron as an absorber by using magnetized iron plates to concentrate mesons of the same sign while keeping away mesons of the opposite sign (at least for mesons of such energy that would be stopped in 3 cm of iron). We obtained results in agreement with the prediction of Tomonaga and Araki. After some improvements intended to increase the counting rate and improve our discrimination against the "mesons of the opposite sign,'' we continued the measurements using, successively, iron and carbon as absorbers. The recording equipment was one which two of us had previously used in a measurement of the meson's mean life. (Extracted from the introductory part of the paper.).

Accelerator COSM Detectors CNTR

Related references
More (earlier) information appears in
M. Conversi, E. Pancini, and O. Piccioni, Phys. Rev. 68 (1945) 232;
See also
S. Tomonaga and G. Araki, Phys. Rev. 58 (1940) 90;
Analyse data from
M. Conversi and O. Piccioni, Nuovo Cim. 2 (1944) 71;
M. Conversi and O. Piccioni, Phys. Rev. 70 (1946) 859;
M. Conversi and O. Piccioni, Phys. Rev. 70 (1946) 874;
P. Auger, R. Maze, and Chaminade, Compt. Ren. 213 (1941) 381;
Y. Nishina, M. Takeuchi, and T. Ichimiya, Phys. Rev. 55 (1939) 585;
H. Maier-Leibnitz, Z. Phys. 112 (1939) 569;
T. H. Johnson and P. Shutt, Phys. Rev. 61 (1942) 380;
H. Jones, Rev. of Mod. Phys. 11 (1939) 235;
D. J. Hughes, Phys. Rev. 57 (1940) 592;
G. Bernardini et al., Phys. Rev. 68 (1945) 109;
M. Conversi and O. Piccioni, Nuovo Cim. 2 (1944) 40;
B. Rossi and N. Nereson, Phys. Rev. 62 (1942) 417;
F. Rasetti, Phys. Rev. 60 (1941) 198;

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