The Mass of Cosmic-Ray Mesotrons

The range of mesotrons in lead as a function of their momentum was determined in an experiment in which two simultaneously expanded counter-controlled cloud chambers were used. The upper chamber was in a magnetic field of 5300 gauss, while the lower, which was vertically in line, contained eight ½-inch thick lead plates. The range of a particle that stopped in one of the plates in the lower chamber, together with the $H\rho$ as measured in the upper chamber, suffice to determine the mass. Data for 26 particles have been obtained which indicate that, within the experimental error, the masses of mesotrons as measured in this experiment are all consistent with a unique rest mass of 202 times the rest mass of an electron. Statistical and systematic errors are analyzed.