  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
on electrons in the target mass. The data taking campaign started in 2007 and
led to key measurements of 7}Be and 8B solar neutrinos as well as antineutrinos
from the earth (geo-neutrinos) and from nuclear power reactors. Borexino is
also a powerful tool for the study of cosmic muons that penetrate the Gran
Sasso rock coverage and thereby induced signals such as neutrons and
radioactive isotopes which are today of critical importance for upcoming dark
matter and neutrino physics experiments. Having reached 4y of continuous data
taking we analyze here the muon signal and its possible modulation. The muon
flux is measured to be (3.41+-0.01)E-4/m2/s. 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
most complete atmospheric data and models available. The distributions are
shown to be positively correlated and the effective temperature coefficient is
measured to be alpha_T = 0.93 +- 0.04. This result is in good agreement with
the expectations of the kaon-inclusive model at the laboratory site and
represents an improvement over previous measurements performed at the same
depth.
