Snowmass2021 Cosmic Frontier White Paper: Ultraheavy particle dark
  matter
  We outline the unique opportunities and challenges in the search for
"ultraheavy" dark matter candidates with masses between roughly $10~{\rm TeV}$
and the Planck scale $m_{\rm pl} \approx 10^{16}~{\rm TeV}$. This mass range
presents a wide and relatively unexplored dark matter parameter space, with a
rich space of possible models and cosmic histories. We emphasize that both
current detectors and new, targeted search techniques, via both direct and
indirect detection, are poised to contribute to searches for ultraheavy
particle dark matter in the coming decade. We highlight the need for new
developments in this space, including new analyses of current and imminent
direct and indirect experiments targeting ultraheavy dark matter and
development of new, ultra-sensitive detector technologies like next-generation
liquid noble detectors, neutrino experiments, and specialized quantum sensing
techniques.
