Physical Review D
We report the first observation of seasonal modulations in the rates of cosmic ray multiple-muon events at two underground sites, the MINOS Near Detector with an overburden of 225 mwe, and the MINOS Far Detector site at 2100 mwe. At the deeper site, multiple-muon events with muons separated by more than 8 m exhibit a seasonal rate that peaks during the summer, similar to that of single-muon events. In contrast and unexpectedly, the rate of multiple-muon events with muons separated by less than 5-8 m, and the rate of multiple-muon events in the smaller, shallower Near Detector, exhibit a seasonal rate modulation that peaks in the winter.
Adamson, P., Anghel, I., Aurisano, A., Barr, G., Bishai, M., Blake, A., ... & Childress, S. (2015). Observation of seasonal variation of atmospheric multiple-muon events in the MINOS Near and Far Detectors. Physical Review D, 91(11), 112006.