250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



first sensed by the Japanese physicist Yukawa, as a result of efforts 

 to understand the nature of the forces that bind the particles of an 

 atomic nucleus together. From purely theoretical considerations, 

 Yukawa was able to calculate the mass and mean life of the particle 

 in question : about 300 times the mass of the electron and a mean life 

 of the order of %oo °^ a microsecond (a microsecond is a millionth of 

 a second). 



Not long after Yukawa's calculations were made, it was found that 

 the principal constituents that we observe as cosmic rays at sea level 

 are charged particles having a mass of the general order of magnitude 

 of that of Yukawa's predictions, but these particles have a mean life 

 of the order of 100 times that predicted by him. 



Of course, the discrepancy of 100 times in the mean life was em- 

 barrassing to the logic of the subject, but physicists were happy to 

 have experimental verification of Yukawa's work to the extent of the 

 actual existence of any kind of particle intermediate in mass between 

 the electron and the proton, and having any kind of a finite life 

 expectancy. It was hoped that time would clear up the discrepancy, 

 possibly by modifications of the theory. 



Time did, indeed, clear up the discrepancy, but not quite in the way 

 anticipated. The particle found in cosmic rays was not Yukawa's 

 mesotron, but rather a child of that mesotron. Before very long, 

 experiment revealed that there did indeed exist in nature a particle, 

 now called the pi-mesotron (^--mesotron), of mass about 300 times the 

 electron's mass and with a mean life of the order of a hundred 

 millionth of a second, as predicted by Yukawa. The pi-mesotrons are 

 usually brought into existence as the result of the bombardment of 

 atomic nuclei by high-energy charged particles such as the primary 

 cosmic rays which enter our atmosphere from outer space. This kind 

 of mesotron can have either a positive or negative charge. 



Experiment has further revealed the existence of uncharged meso- 

 trons of mass about 300 electron units; these are also called pi- 

 mesotrons. They are produced by bombardment of atomic nuclei by 

 high-energy particles, and have a life expectancy of the order of 

 10~ 13 second. A neutral pi-mesotron decays into two photons of high 

 energy — gamma rays. 



The child of the pi-mestron is called a mu-mesotron (/x-mesotron), 

 and it is the result of the death of a pi-mesotron, which is thought to 

 be accompanied also by the emission of a neutrino. The mu-mesotron 

 plays the most important role in cosmic-ray phenomena. Being a 

 charged particle, it, of course, ionizes like any other charged particle. 

 However, it has a rest mass 210 times as great as that of the electron, 

 and so an energy 210 times that of the electron for the same velocity. 

 Moreover, on account of its large mass, it is relieved of the duty of 



