Evidence for strongly interacting heavy shower particles at 70 m w.e. underground

Abstract

An array consisting of a telescope of four scintillation counters separated by layers of 15 cm of lead, and six two-scintillator extension trays of a total area of 5.5 m2 and with 5 cm lead absorbers, was operated at the Mt. Cappuccini station in Torino, 70 m w.e. underground. The pulse heights of the four telescope scintillators were displayed on an oscilloscope and photographed, so that the traversals of single particles could be distinguished from showers incident from the rock above or produced in the lead plates. The delays between the pulses in telescope and extension trays were also determined from the oscilloscope records. In the first part of the run events were recorded whenever a pulse in any detector of the array was followed, within 500 ns, by the passage of another particle through the telescope. In the second part all coincidences within ± 250 ns were accepted. During the 1590 h of the first run, 34 particles with delays between about 40 and 120 ns were observed, of which 22 showed interactions in the lead plates of the telescope, indicating the presence of a strongly interacting component among the shower particles. In the following 1037 h of operation mostly short «negative» and «positive» delays were registered. At negative delays the nuclear-active component comprised some 55% of all shower particles. Its interaction mean free path was (165±25) g/cm2. A similar fraction of interacting particles was found for positive delays, but its interaction mean free path was substantially larger (≥ 375 g/cm2), and its angular and lateral distributions significantly different. Measurement with increased lead absorbers confirmed that the nuclear-active component at negative delays was strongly attenuated, while that present at positive delays showed little attenuation losses. The interacting particles at negative delays can be identified as pions originating from nuclear interactions of single muons in the rock above the array. The most likely interpretation of the events observed at positive delays is that they are due to particles of a mass about 10÷15 times larger than that of a nucleon, possibly heavy triplet particles of single charge. Their total flux is of the order 10−7/cm2 sr s. The apparent discrepancy between these results and those of earlier experimenters who gave upper flux limits of the order 10−10/cm2 sr s for the flux of massive particles can be explained by the assumption that these particles are created in catastrophic processes involving dissociation of the colliding primaries, with subsequent decay of the unstable triplet particles into muons. This would leave little or no energy for the production of an air shower. If that is so, the cross-section for this process must be of the order of a few percent of the total inelastic cross-section.