Explanation of the seasonal variation of cosmic multiple muon events observed with the NOvA Near Detector

TL;DR

This study analyzes seasonal variations in cosmic muon events detected by NOvA, revealing that atmospheric temperature changes influence muon production height, explaining the observed anticorrelation between muon rate and temperature.

Contribution

It provides the first detailed explanation linking atmospheric altitude effects to seasonal muon rate variations using CORSIKA simulations.

Findings

Confirmed anticorrelation between muon rate and atmospheric temperature

Identified altitude-geometry effects as the cause of seasonal variation

Resolved discrepancy in seasonal phases of single and multiple muon events

Abstract

The flux of cosmic ray muons at the Earth's surface exhibits seasonal variations due to changes in the temperature of the atmosphere affecting the production and decay of mesons in the upper atmosphere. Using 1473 live days of data collected by the NuMI Off-axis $\nu_e$ Appearance (NOvA) Near Detector during 2018--2022, we studied the seasonal pattern in the multiple-muon event rate. The data confirm an anticorrelation between the multiple-muon event rate and effective atmospheric temperature, consistent across all the years of data. Previous analyses from MINOS and NOvA saw a similar anticorrelation but did not include an explanation. We find that this anticorrelation is driven by altitude--geometry effects as the average muon production height changes with the season. This has been studied with a CORSIKA cosmic ray simulation package by varying atmospheric parameters, and provides an explanation to a longstanding discrepancy between the seasonal phases of single and multiple-muon events.