Characterization of the varying flux of atmospheric muons measured with the Large Volume Detector for 24 years

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Abstract

The Large Volume Detector, hosted in the INFN Laboratori Nazionali del Gran Sasso, is triggered by atmospheric muons at a rate of $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{Hz}$. The data collected over almost a quarter of a century are used to study the muon intensity underground. The $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ muon series, the longest ever exploited by an underground instrument, allows for the accurate long-term monitoring of the muon intensity underground. This is relevant as a study of the background in the Gran Sasso Laboratory, which hosts a variety of long-duration, low-background detectors. We describe the procedure to select muon-like events as well as the method used to compute the exposure. We report the value of the average muon flux measured from 1994 to 2017: ${I}_{\ensuremath{\mu}}^{0}=3.35\ifmmode\pm\else\textpm\fi{}0.000{5}^{\text{stat}}\ifmmode\pm\else\textpm\fi{}0.0{3}^{\text{sys}}\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}4}\text{ }\text{ }{\mathrm{m}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. We show that the intensity is modulated around this average value due to temperature variations in the stratosphere. We quantify such a correlation by using temperature data from the European Center for Medium-range Weather Forecasts: we find an effective temperature coefficient ${\ensuremath{\alpha}}_{T}=0.94\ifmmode\pm\else\textpm\fi{}0.0{1}^{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.0{1}^{\mathrm{sys}}$, in agreement with other measurements at the same depth. We scrutinize the spectral content of the time series of the muon intensity by means of the Lomb-Scargle analysis. This yields the evidence of a 1-year periodicity, as well as the indication of others, both shorter and longer, suggesting that the series is not a pure sinusoidal wave. Consequently, and for the first time, we characterize the observed modulation in terms of amplitude and position of the maximum and minimum on a year-by-year basis.

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