Cosmic ray composition studies through the Gerasimova-Zatsepin effects of heavy nuclei at LAAS

Extensive air showers (EASs) originated from pri- mary cosmic ray energies above 10 15 eV have been measured at multiple EAS observatories deployed in Japan since Sept. 1996. The typical EAS array has been located at the rooftop of the buildings in the university campus, and has GPS- disciplined 10 MHz oscillator to provide the UTC time stamp for each EAS event within a few µs accuracies. Searching for simultaneous and parallel EAS events at multiple EAS observatories due to Gerasimova-Zatsepin (GZ) effects have been carried out by comparing EAS arrival time stamps and directions detected by several baseline combinations of EAS arrays. The EAS pairs whose time difference and angular distance were less than 5 ms and less than 15 respectively, were se- lected and their angular distances from the solar direction and the lunar direction were examined. The data were com- pared with numerical GZ probability as a function of arrival directions of cosmic ray nuclei. Consequently, significant excesses of these events in the solar direction as expected in the numerical prediction of GZ effects were not found. We however found that the deficiencies of EAS pairs in the lunar direction, but its deviation is not significant.