Fog on the horizon: a new definition of the neutrino floor for direct
  dark matter searches
  The neutrino floor is a theoretical lower limit on WIMP-like dark matter
models that are discoverable in direct detection experiments. It is commonly
interpreted as the point at which dark matter signals become hidden underneath
a remarkably similar-looking background from neutrinos. However, it has been
known for some time that the neutrino floor is not a hard limit, but can be
pushed past with sufficient statistics. As a consequence, some have recently
advocated for calling it the "neutrino fog" instead. The downside of current
methods of deriving the neutrino floor are that they rely on arbitrary choices
of experimental exposure and energy threshold. Here we propose to define the
neutrino floor as the boundary of the neutrino fog, and develop a calculation
free from these assumptions. The technique is based on the derivative of a
hypothetical experimental discovery limit as a function of exposure, and leads
to a neutrino floor that is only influenced by the systematic uncertainties on
the neutrino flux normalisations. Our floor is broadly similar to those found
in the literature, but differs by almost an order of magnitude in the sub-GeV
range, and above 20~GeV.
