A halo event observed with an emulsion chamber and air shower array at Mt Chacaltaya

A hybrid experiment to operate simultaneously an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt Chacaltaya (5200 m, Bolivia). An event with a halo, a blackened area of ∼1 cm on x-ray film of the emulsion chamber, was observed with the experiment. Information about the halo (E_halo = 850 TeV) and on high energy particles of electromagnetic and hadronic components outside the halo (Σ E _γ = 632.5 TeV and Σ E_h^(γ) = 278.8 TeV with a detection threshold of 2 TeV) was obtained from emulsion chamber data. Details about low energy hadrons were determined from the hadron calorimeter data, and characteristics of the accompanying air shower ( _eN = 7.0 × 107, s = 0.59) were determined by the air shower array. We reconstruct the event, based on the observed data. The event is compared with simulated events, which supports the conclusion that nuclear interactions change their characteristics in the high energy region so as to result in stronger energy subdivision.