Unusually penetrating particle detected by balloon-borne emulsion chamber

One example of an unusual particle track has been recorded in an emulsion chamber exposed to cosmic rays on a ballon at the atmospheric depth 11.7 g/cm2. The particle arrived at the chamber withZ/β=40±2 and β≳0.8. What is extraordinary with this particle is its arrival zenith angle, 87.4°, which amounts to a traversed atmospheric thickness ∼200 g/cm2. The anomalous nature of the present track “ET” (exotic track) is demonstrated through the difficulties in reconciling it with the explanation that it is due to an ordinary ultra-heavy cosmic-ray nucleus.