Measurement of the dipole in the cross-correlation function of galaxies

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 talk I will discuss the different types of effect that contribute to the dipole: relativistic effects, evolution effects and wide-angle effects. I will construct an optimal estimator to measure the dipole with multiple populations of galaxies. I will then present the first measurement of this dipole in the BOSS survey. I will show that the relativistic and evolution dipole are too small to be detected in this sample. However with the appropriate choice of kernel, the wide-angle effect can be significantly enhanced and measured with a signal-to-noise of 50.