  The relation between the clustering properties of luminous matter in the form
of galaxies and the underlying dark matter distribution is of fundamental
importance for the interpretation of ongoing and upcoming galaxy surveys. The
so called local bias model, where galaxy density is a function of local matter
density, is frequently discussed as a means to infer the matter power spectrum
or correlation function from the measured galaxy correlation. However,
gravitational evolution generates a term quadratic in the tidal tensor and thus
non-local in the density field, even if this term is absent in the initial
conditions (Lagrangian space). Because the term is quadratic, it contributes as
a loop correction to the power spectrum, so the standard linear bias picture
still applies on large scales, however, it contributes at leading order to the
bispectrum for which it is significant on all scales. Such a term could also be
present in Lagrangian space if halo formation were influenced by the tidal
field. We measure the corresponding coupling strengths from the
matter-matter-halo bispectrum in numerical simulations and find a non-vanishing
coefficient for the tidal tensor term. We find no scale dependence of the bias
parameters up to k=0.1 h/Mpc and that the tidal effect is increasing with halo
mass. While the Lagrangian bias picture is a better description of our results
than the Eulerian bias picture, our results suggest that there might be a tidal
tensor bias already in the initial conditions. We also find that the
coefficients of the quadratic density term deviate quite strongly from the
theoretical predictions based on the spherical collapse model and a universal
mass function. Both quadratic density and tidal tensor bias terms must be
included in the modeling of galaxy clustering of current and future surveys if
one wants to achieve the high precision cosmology promise of these datasets.
