Narrow Muon Bundles below Ground

抄録

The observation of muon bundles caused by cosmic rays was carried out for 17,000 hours by means of spark chamber at 30 m.w.e. below ground. The muon bundles in a narrow region of 20∼30 cm in diameter were observed over the expectation from the fluctuation of spatial distribution or muons by the Monte Carlo method. Their frequency was about 2×10⁻⁴ m⁻² h⁻¹. The frequency of these muon bundles cannot be explained as known interaction processes without extreme assumptions.