Observation of energetic delayed hadrons in air showers—New massive particles?

An experiment to measure the arrival-time distribution of energetic hadrons with respect to air-shower fronts near shower cores was carried out at an altitude of 2900 m. Three events were observed with delays greater than 30 nsec and energies greater than about 45 GeV; these events constitute a fraction of 3.3×${10}^{-4\mathrm{}}$ of the events. Estimates of backgrounds and conventional processes failed to provide an interpretation for these events. These events could indicate the production of relatively stable particles with mass ≳5 GeV/${\mathit{c}}^2$.