Power Corrections to the Universal Heavy WIMP-Nucleon Cross Section
  WIMP-nucleon scattering is analyzed at order $1/M$ in Heavy WIMP Effective
Theory. The $1/M$ power corrections, where $M\gg m_W$ is the WIMP mass,
distinguish between different underlying UV models with the same universal
limit and their impact on direct detection rates can be enhanced relative to
naive expectations due to generic amplitude-level cancellations at leading
order. The necessary one- and two-loop matching calculations onto the
low-energy effective theory for WIMP interactions with Standard Model quarks
and gluons are performed for the case of an electroweak SU(2) triplet WIMP,
considering both the cases of elementary fermions and composite scalars. The
low-velocity WIMP-nucleon scattering cross section is evaluated and compared
with current experimental limits and projected future sensitivities. Our
results provide the most robust prediction for electroweak triplet Majorana
fermion dark matter direct detection rates; for this case, a cancellation
between two sources of power corrections yields a small total $1/M$ correction,
and a total cross section close to the universal limit for $M \gtrsim {\rm few}
\times 100\,{\rm GeV}$. For the SU(2) composite scalar, the $1/M$ corrections
introduce dependence on underlying strong dynamics. Using a leading chiral
logarithm evaluation, the total $1/M$ correction has a larger magnitude and
uncertainty than in the fermionic case, with a sign that further suppresses the
total cross section. These examples provide definite targets for future direct
detection experiments and motivate large scale detectors capable of probing to
the neutrino floor in the TeV mass regime.
