Modulations of the cosmic muon signal in ten years of Borexino data

摘要

We have measured the flux of cosmic muons in the Laboratori Nazionali del Gran Sasso at 3800 m w.e. to be (3.432 ± 0.003) · 10~(-4)m~(-2)s~(-1) based on ten years of Borexino data acquired between May 2007 and May 2017. A seasonal modulation with a period of (366.3 ± 0.6) d and a relative amplitude of (1.36 ± 0.04)% is observed. The phase is measured to be (181.7 ± 0.4) d, corresponding to a maximum at the 1~(st) of July. Using data inferred from global atmospheric models, we show the muon flux to be positively correlated with the atmospheric temperature and measure the effective temperature coefficient αT = 0.90 ± 0.02. The origin of cosmic muons from pion and kaon decays in the atmosphere allows to interpret the effective temperature coefficient as an indirect measurement of the atmospheric kaon-to-pion production ratio r_(K/π) = 0.11_(-0.07) ~(+0.11) for primary energies above 18 TeV. We find evidence for a long-term modulation of the muon flux with a period of ～ 3000 d and a maximum in June 2012 that is not present in the atmospheric temperature data. A possible correlation between this modulation and the solar activity is investigated. The cosmogenic neutron production rate is found to show a seasonal modulation in phase with the cosmic muon flux but with an increased amplitude of (2.6 ± 0.4)%.