Production of heavy mesons by protons of energy between 2 and 3000 GeV

The masses of the secondary particles emitted from disintegrations produced by cosmic-ray protons of energy greater than 2 GeV have been determined by observations on the grain density and scattering of their tracks in photographic emulsions. It has thus been possible to determine the yield of the heavy charged mesons with mass between 900 and 1400 m e , as a function of the energy of the primary particles. Among 325 secondary particles which produce tracks with grain density between 1⋅07 and 2⋅0 times the minimum value, twenty heavy mesons have been identified. The statistical distribution in the measured values of their mass is consistent with a unique value of 1210±40 m e . The total observed proper time of flight of these particles is 2⋅9 x 10 -10 s; no example of decay 'in flight’ has yet been observed. The rate of production of these heavy mesons, referred to as K -particles, has been compared with the frequency with which heavy mesons are observed to decay at rest; the ratio is consistent with the estimated lifetime of the K -particles. In the disintegrations produced by primary particles of energy greater than ~ 20 GeV, approximately equal energy goes into production of π-and K -particles. The yield of K -particles at a primary energy of 2 to 3 GeV is estimated to be a few per cent per primary interaction. The results suggest th at not only π-mesons but also K -mesons are ‘heavy quanta’ associated with the nuclear field. If charged mesons of mass between 276 and 900 m e are created directly in showers, they occur with a frequency < 1 % of the π-particles.