Observation of the Moon Shadow in Deep Underground Muon Flux

TL;DR

This paper reports the detection of the moon's shadow in underground muon flux at a significant level, confirming detector alignment and stability over a decade using the Soudan 2 detector at substantial depth.

Contribution

First observation of the moon shadow in underground muon flux with high statistical significance and verified detector alignment over ten years.

Findings

Moon shadow observed at 5σ significance

Detector angular resolution σ ≤ 0.3°

Stable detector alignment over 10 years

Abstract

A shadow of the moon, with a statistical significance of $5\sigma$, has been observed in the underground muon flux at a depth of 2090 mwe using the Soudan 2 detector. The angular resolution of the detector is well described by a Gaussian with $\sigma \le 0.3^{\circ}$. The position of the shadow confirms the alignment of the detector to better than $0.15^{\circ}$. This alignment has remained stable during 10 years of data taking from 1989 through 1998.