On the Neutral Meson*

349 The neutral meson was introduced (1) in order to account for the far that the proton-proton (P-P) force and proton-neutron (P-N) force L1 nuclear forces are equal. Since then, attempts have been made to disco ,r neutral mesons in cosmic-raysY) These experiments consist in placihg counters above and below various substances and examining the transformation of charged mesons into neutral mesons inside matter. At first affirmative results were obtained, but later, it became clear, due to researches by Rossi and the Nishina Laboratory,CR) that these results do not imply the existence of neutral mesons but are due to the scattering and shower of charged mesons, thus giving the attempts to discover phenomena relating to neutral mesons within cosmic-rays a negative conclusion. Simultaneously , it was found by Sakata and Tanikawa(4) that the neutral meson has an extremely short lifetime , 10-16 sec., and decays into a photon, thus explaining why neutral mesons are not to be observed in cosmic-rays. However, it has become possible to discuss the existence and nature of neutral mesons on the ground of other experimentsY) This is connected with the change in the way of conceiving the structure of cosmic-rays, following the experiments of Schein and his co-workers,c6) They made balloon observations in regions near the top of the atmosphere (pressure 2 cm. Hg; less than 1 radiation unit), and obtained the following results: i. e., the number of charged particles vertically penetrating lead layers of thicknesses 4.-18cm. does not depend on the thickness of the layer, and moreover these particles produce almost no showers within the lead (less than a few percent). This leads to the conclusion that electrons with energy * This article is the manuscript to a lecture delivered at the Meson Symposium held in Tokyo in September, 1943_ Discussions based on subsequent data will be published shortly. Though some data used in this article is out-of-date, I have decided to publish it because I consider that the methods employed will be of some references. Thanks are due to Mr. Tatsuoka for his rendering it into English.