  Many low-energy, particle-physics experiments seek to reveal new fundamental
physics by searching for very rare scattering events on atomic nuclei. The
interpretation of their results requires quantifying the non-linear effects of
the strong interaction on the spin-independent couplings of this new physics to
protons and neutrons. Here we present a fully-controlled, ab-initio calculation
of these couplings to the quarks within those constituents of nuclei. We use
lattice quantum chromodynamics computations for the four lightest species of
quarks and heavy-quark expansions for the remaining two. We determine each of
the six quark contributions with an accuracy better than 15%. Our results are
especially important for guiding and interpreting experimental searches for our
universe's dark matter.
