Directional recoil detection
  Searches for dark matter-induced recoils have made impressive advances in the
last few years. Yet the field is confronted by several outstanding problems.
First, the inevitable background of solar neutrinos will soon inhibit the
conclusive identification of many dark matter models. Second, and more
fundamentally, current experiments have no practical way of confirming a
detected signal's galactic origin. The concept of directional detection
addresses both of these issues while offering opportunities to study novel dark
matter and neutrino-related physics. The concept remains experimentally
challenging, but gas time projection chambers are an increasingly attractive
option, and when properly configured, would allow directional measurements of
both nuclear and electron recoils. In this review, we reassess the required
detector performance and survey relevant technologies. Fortuitously, the
highly-segmented detectors required to achieve good directionality also enable
several fundamental and applied physics measurements. We comment on near-term
challenges and how the field could be advanced.
