  Cosmic ray antiprotons provide an important probe for the study of cosmic-ray
propagation in the interstellar space and to investigate the existence of
Galactic dark matter. Cosmic rays are hampered by the Moon, therefore a deficit
of cosmic rays in its direction is expected (the so-called "Moon shadow"). The
Earth-Moon system acts as a magnetic spectrometer. In fact, due to the
geomagnetic field the center of the Moon shifts westward by an amount depending
on the primary cosmic ray energy. Paths of primary antiprotons are therefore
deflected in an opposite sense in their way to the Earth. This effect allows,
in principle, the search of antiparticles in the opposite direction of the
observed Moon shadow.
  The ARGO-YBJ experiment, in stable data taking since November 2007 with an
energy threshold of a few hundreds of GeV, is observing the Moon shadow with
high statistical significance. Using about 1 year data, an upper limit of the
antip/p flux ratio in the few-TeV energy region is set to a few percent with a
confidence level of 90%.
