Observation of seasonal variation of atmospheric multiple-muon events in the MINOS Near and Far Detectors

TL;DR

This study reports the first observation of seasonal variations in underground cosmic ray multiple-muon event rates at two sites, revealing different seasonal patterns based on muon separation and detector depth.

Contribution

It provides novel measurements of seasonal modulations in multiple-muon events at different depths, highlighting unexpected seasonal behavior based on muon separation.

Findings

Deep site multiple-muon rates peak in summer.

Shallower site and close muon separation rates peak in winter.

Different seasonal patterns depend on muon separation and detector depth.

Abstract

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.