  It is usually assumed that in the linear regime the two-point correlation
function of galaxies contains only a monopole, quadrupole and hexadecapole.
Looking at cross-correlations between different populations of galaxies, this
turns out not to be the case. In particular, the cross-correlations between a
bright and a faint population of galaxies contain also a dipole. In this paper
we present the first attempt to measure this dipole. We discuss the four types
of effects that contribute to the dipole: relativistic distortions, evolution
effect, wide-angle effect and large-angle effect. We show that the first three
contributions are intrinsic anti-symmetric contributions that do not depend on
the choice of angle used to measure the dipole. On the other hand the
large-angle effect appears only if the angle chosen to extract the dipole
breaks the symmetry of the problem. We show that the relativistic distortions,
the evolution effect and the wide-angle effect are too small to be detected in
the LOWz and CMASS sample of the BOSS survey. On the other hand with a specific
combination of angles we are able to measure the large-angle effect with high
significance. We emphasise that this large-angle dipole does not contain new
physical information, since it is just a geometrical combination of the
monopole and the quadrupole. However this measurement, which is in excellent
agreement with theoretical predictions, validates our method for extracting the
dipole from the two-point correlation function and it opens the way to the
detection of relativistic effects in future surveys like e.g. DESI.
