Cosmic-muon flux and annual modulation in Borexino at 3800 m water-equivalent depth

TL;DR

This study precisely measures the underground muon flux at Gran Sasso, observing a seasonal modulation correlated with atmospheric temperature, and provides detailed data over four years using Borexino.

Contribution

First detailed measurement of muon flux and its annual modulation at Gran Sasso with four years of data, confirming temperature correlation.

Findings

Muon flux: (3.41 ± 0.01) × 10⁻⁴ m⁻² s⁻¹

Modulation period: (366 ± 3) days, amplitude: (1.29 ± 0.07)%

Maximum muon flux on June 28th

Abstract

We have measured the muon flux at the underground Gran Sasso National Laboratory (3800 m w.e.) to be (3.41 \pm 0.01) \times 10-4m-2s-1 using four years of Borexino data. A modulation of this signal is observed with a period of (366\pm3) days and a relative amplitude of (1.29 \pm 0.07)%. The measured phase is (179 \pm 6) days, corresponding to a maximum on the 28th of June. Using the most complete atmospheric data models available, muon rate fluctuations are shown to be positively correlated with atmospheric temperature, with an effective coefficient {\alpha}T = 0.93 \pm 0.04. This result represents the most precise study of the muon flux modulation for this site and is in good agreement with expectations.