Search for Massive Particles in Cosmic Rays

A search for heavy particles with a mass greater than that of a nucleon, the existence of which has been suggested by higher-symmetry schemes, was performed with an apparatus set up at Echo Lake, Colorado (elevation 10 600 ft). The search was sensitive to strongly interacting particles with masses in the range 5-15 BeV, with no restriction imposed on their electric charge. The method used was to measure the time interval between the arrival of strongly interacting particles and accompanying air shower particles. This information, coupled with information from a measurement of the particle's energy and range of absorption in a total absorption spectrometer, enabled a distinction to be made between massive elementary particles, nucleons, and nuclei. In an operating period of 1542 h and with an aperture of 0.78 ${\mathrm{m}}^2$ sr, one delayed event was found whose behavior in the total absorption spectrometer was atypical of a nucleon or nucleus. If one considers this event to represent the arrival of a massive particle, then its mass, calculated assuming that its production occurred 1 km above the apparatus, is approximately 6.5 BeV. This one event corresponds to a flux of the order of ${10}^{-10\mathrm{}}$ ${\left({\mathrm{cm}}^2\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{s}\mathrm{r}\right)}^{-1\mathrm{}}$, where a correction for detection efficiency has been included. As there is also an 8% probability that this event was a nucleon, we do not regard this as significant evidence for the existence of a massive elementary particle, but rather as setting an effective upper limit to the flux of such particles.