PrefaceThe mechanism of multiple particle production a t ext.remely high energythis is undoubtedly one of the most challenging topics in contemporary nuclear physics. (By the word "extremely high" we mean here the energy region above lo3 Gev.) The problem remains still quite unsettled, though it has been intensively attacked both by experimental and theoretical workersl). Various theories and models have so far been proposed almost independently of each other and with little interrelation to each other; and we cannot tell for certain which of them, if any, reproduces the main aspects of the experiments. This situation is partly due to the incompetence of the current field theories a t this new domain of physics, and partly due to the lack of systematic knowledge on the characteristic features of the phenomena. l) Excellent review articles on this problem have been published by LEWIS [ I ] and by ROZENTAL' and CERNAVSKIJ [Z]. The latter, in particular, has given a systematic and detailed account of experimental and theoretical investigations, available at the time of its publication (1954). 1 Zeitschrift ,.Fortschritte der Physik"Z. KOBA and S. TAKAGI With the remarkable improvement of experimental techniques in recent years, available data have been and are being steadily accumulated, and rapid progress is expected also in the field of theoretical investigations. It would be sensible, therefore, for us to write a critical report on our present knowledge and understanding concerning this problem, in order to orientate ourselves for further development.I n Part I the empirical information on extremely high energy nuclear phenomena will be described, and in Part I1 a survey of some of the existing theories and models will be given. I n Part 111, more recent developments will be discussed, including the foundations and interrelations of various theories, models proposed recently, general reconsideration of theories and models, and the limit of applicability of the current field theories.A typical example at present would be something like the following: Giant stacks, containing 1 -10 1 emulsion (for instance, 200 plates of 30 cm x 40 cm X 0.6 mm) are flown up by a ballcon to an altitude of 20 -30 km and exposed during 6 -8 hours.Events of 5 lo5 Gev can be studied by such an experiment.The Texas project in 1958 is expected t o fly 200 1 emulsion (1,000 plates of 60 cmx 80 cm x 0.6 mm) and t o supply us with data of 5 lo6 Gev events. An ideal case, barely possible with the present technique, would be, according to~OCCON1, several days exposure of 1 m3 emulsion, which would catch lo7 Gev events [ 5 ] .There are also attempts to make the emulsion stack stilllarger at the sacrifice of altitude, by making use of airplanes or simply putting it on the top of a mountain.
1'1.3. Estimation of the primary energy 1.3.1. We make the assumption: The velocity of secondary relativistic particles, vi, (i = 1,2, . . ., n,) in the c. m. s.~), and the velocity of the c. m. s. to the 1. s .~) , vc, are very close t o each other, i. e., ...