Seasonal modulations of the underground cosmic-ray muon energy
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, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
Seasonal Modulations of the Underground CosmicRay Muon Energy A. S. Malgin* Institute for Nuclear Research, Russian Academy of Sciences, pr. 60letiya Oktyabrya 7a, Moscow, 117312 Russia *email: [email protected] Received December 3, 2014
Abstract—The parameters of the seasonal modulations in the intensity of muons and cosmogenic neutrons generated by them at a mean muon energy of 280 GeV have been determined in the LVD (Large Volume Detector) experiment. The modulations of muons and neutrons are caused by a temperature effect, the sea sonal temperature and density variations of the upper atmospheric layers. The analysis performed here leads to the conclusion that the variations in the mean energy of the muon flux are the main source of underground cosmogenic neutron variations, because the energy of muons is more sensitive to the temperature effect than their intensity. The parameters of the seasonal modulations in the mean energy of muons and the flux of cos mogenic neutrons at the LVD depth have been determined from the data obtained over seven years of LVD operation. DOI: 10.1134/S1063776115080191
1. INTRODUCTION At present, the correlation between the annual modulations in the counting rate of events observed in dark matter particle search experiments [1–4] and the seasonal variations in the underground muon flux is being actively discussed. The muon intensity varia tions at great depths are considered as a possible source of the seasonal modulations of events in low background detectors. It is assumed that the modula tions can be produced by cosmogenic neutrons whose flux is linearly related to the varying muon flux. The cause of the seasonal muon variations at sea level and underground is well known [5–7]. This is a temperature effect that leads to a change in the density of the terrestrial atmosphere and its height as a result of its heating in summer and cooling in winter. On the one hand, a decrease in the density of the upper atmo spheric layers (stratosphere) through expansion when heated leads to an increase in the probability of the π μ decays of first generation charged pions from extensive air showers (EASs) and to a corresponding decrease in the number of pions (and the number of their decays π μ) in the last generations. On the other hand, the atmosphere expansion increases the e decays of low energy muons probability of the μ on their way to the Earth. The first fact, an increase in the probability of the decays π μ, gives a positive temperature effect observed in the high energy muon flux. The last two facts associated with low energy muons lead to a negative temperature effect, a reduc tion in the muon intensity at sea level, where the mean muon energy is ~4 GeV. The observed muon variations are determined by the combined action of the negative
and positive effects. The negative component domi nates approximately down to 20 meters water equiva lent (m w.e.). Its contribution is decreased with a depth increasing and becomes neglig
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