Abstract
We report the rate of cosmic ray air showers with multiplicities exceeding 15 muon tracks recorded in the NOvA Far Detector between May 2016 and May 2018. The detector is located on the surface under an overburden of 3.6 meters water equivalent. We observe a seasonal dependence in the rate of multiple-muon showers, which varies in magnitude with multiplicity and zenith angle. During this period, the effective atmospheric temperature and surface pressure ranged between 210 K to 230 K and 940mbar to 990mbar, respectively; the shower rates are anti-correlated with the variation in the effective temperature. The variations are about 30% larger for the highest multiplicities than the lowest multiplicities and 20% larger for showers near the horizon than vertical showers.
Highlights
We observed that the rate of multiple-muon cosmic ray air showers in a detector near the surface presents a seasonal variation with a peak rate in the winter
We showed that this effect is dependent on the primary cosmic ray energy by looking at two detector observables, the multiplicity and zenith angle
A stronger seasonal effect is seen for air showers with higher multiplicities or zenith angles near the horizon
Summary
During the summer months, when temperature is high and the density is lowest, meson decays are more probable, which leads to an expected peak in the muon rates This summer peak has been confirmed by many experiments in single-muon air showers [1,2,3,4] and is explained by existing models [5]. The MINOS experiment [8] observed a winter peak in two underground detectors, with minimum muon energies of 60 GeV and 700 GeV, and showed that at a depth of at least 225 meters water equivalent the effect from altitude differences suggested by DECOR was too small to fully explain the observation.
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