Search for heavy long-lived particles in high-energy cosmic rays

We present the results of an experimental search for energetic particles which arrive at sea level delayed with respect to the shower front, with an order of magnitude greater exposure than previous experiments. The experiment was sensitive to showers from cosmic rays between 105 and 107 GeV per nucleus. Events with signals greater than 20 equivalent particles and delayed more than 20 ns were observed and studied. A four-dimensional Monte Carlo simulation of cosmic-ray showers combined with accelerator calibration data showed that the observed events as well as previously reported observations of this type can be accounted for by rare fluctuations in signals from low-energy hadrons in the air shower. No evidence for the existence of heavy long-lived particles in air showers was found. We set an upper limit to the flux of these particles at the 90% confidence level of 1.4×10-12 cm-2sr-1s-1.