Measurements of decoherence in small sea-level extensive air showers

A study is made of the progressive ‘decoherence’ of cosmic ray extensive air shower particle detector signals in small air showers through measurements of coincidence rates for pairs of detectors versus the detector separation. Measurements are made both when only the two separated detectors themselves trigger in coincidence, and when that coincidence trigger also requires the detection of a local air shower by a small external air shower array. The addition of the explicit air shower trigger ensures that the latter data correspond to showers of a larger particle size, and triggering by very localised shower cores is then unlikely. When including a shower trigger, the decoherence results appear substantially different in form. The coincidence rate between two detectors only can be approximated by a power law variation with separation distance. When triggering involves an air shower array, the variation becomes close to an exponential form with characteristic exponent distances varying systematically with increasing detector and air shower size thresholds. A result is that one can see that small air showers will exhibit clear non-Poissonian density fluctuations near their cores out to distances of ~5m, or at shower energies below ~0.05 PeV. These ideas can be helpful in understanding the statistical properties of signals when using large detectors in air shower arrays.