New-particle search in very-high-energy nuclear interactions of cosmic rays

Systematic analyses for new-particle search are carried out by using cosmic-ray data ($E_0\sim {10}^5$ GeV) obtained from the Brazil-Japan two-story emulsion-chamber experiment exposed on Mt. Chacaltaya. Twelve events, selected by two different ways from 127 carbon-jet (C-jet) events with $\Sigma E_{\gamma }\ge 5\times {10}^3$ GeV and $N_{\gamma }\ge 4$, are measured by geometrical arrival-direction measurement. Five abnormal cores that are not emitted directly from C jets are observed. An estimation on $X$-particle production as a possible origin of abnormal cores gives very high production frequencies. Various sources for possible backgrounds are discussed and their rates are found to be negligibly small.