Seasonal modulation in the Borexino cosmic muon signal

摘要

Borexino is an organic liquid scintillator detector located in the underground Gran Sasso National Laboratory(Italy). It is devoted mainly to the real time spectroscopy of low energy solar neutrinos via the elastic scattering onelectrons in the target mass. The data taking campaign started in 2007 and led to key measurements of 7 Be and 8 Bsolar neutrinos as well as antineutrinos from the earth (geo-neutrinos) and from nuclear power reactors. Borexino isalso a powerful tool for the study of cosmic muons that penetrate the Gran Sasso rock coverage and thereby inducedsignals such as neutrons and radioactive isotopes which are today of critical importance for upcoming dark matter andneutrino physics experiments. Having reached 4y of continuous data taking we analyze here the muon signal and its4 m 2 s 1 . A modulation of this signal with a yearly period is observed with an amplitude of (1.29 ± 0.07)% and a phase of (179 ± 6) d, corresponding to June 28th .Muon rate fluctuations are compared to fluctuations in the atmospheric temperature on a daily base, exploiting the mostcomplete atmospheric data and models available. The distributions are shown to be positively correlated and the effectivetemperature coefficient is measured to be T = 0.93 ± 0.04. This result is in good agreement with the expectations ofthe kaon-inclusive model at the laboratory site and represents an improvement over previous measurements performed atthe same depth.