The Bino Variations: Effective Field Theory Methods for Dark Matter
  Direct Detection
  We apply effective field theory methods to compute bino-nucleon scattering,
in the case where tree-level interactions are suppressed and the leading
contribution is at loop order via heavy flavor squarks or sleptons. We find
that leading log corrections to fixed-order calculations can increase the bino
mass reach of direct detection experiments by a factor of two in some models.
These effects are particularly large for the bino-sbottom coannihilation
region, where bino dark matter as heavy as 5-10 TeV may be detected by near
future experiments. For the case of stop- and selectron-loop mediated
scattering, an experiment reaching the neutrino background will probe thermal
binos as heavy as 500 and 300 GeV, respectively. We present three key examples
that illustrate in detail the framework for determining weak scale
coefficients, and for mapping onto a low energy theory at hadronic scales,
through a sequence of effective theories and renormalization group evolution.
For the case of a squark degenerate with the bino, we extend the framework to
include a squark degree of freedom at low energies using heavy particle
effective theory, thus accounting for large logarithms through a "heavy-light
current." Benchmark predictions for scattering cross sections are evaluated,
including complete leading order matching onto quark and gluon operators, and a
systematic treatment of perturbative and hadronic uncertainties.
