Seasonal Variation of Multiple-Muon Cosmic Ray Air Showers Observed in the NOvA Detector on the Surface

TL;DR

This study analyzes seasonal variations in high-multiplicity cosmic ray muon showers detected at the surface, revealing anti-correlations with atmospheric temperature and dependencies on multiplicity and zenith angle.

Contribution

It provides the first detailed measurement of seasonal effects on multiple-muon cosmic ray air showers at the surface, highlighting multiplicity and angular dependencies.

Findings

Muon shower rates vary seasonally with about 30% amplitude for high multiplicities.

Anti-correlation observed between shower rates and atmospheric temperature.

Higher multiplicity and near-horizon showers show larger seasonal variations.

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.