  Determining the dark matter (DM) mass is of paramount importance for
understanding dark matter. We present a novel parametrization of the DM speed
distribution which will allow the DM mass to be accurately measured using data
from Weakly Interacting Massive Particle (WIMP) direct detection experiments.
Specifically, we parametrize the natural logarithm of the speed distribution as
a polynomial in the speed v. We demonstrate, using mock data from upcoming
experiments, that by fitting the WIMP mass and interaction cross-section, along
with the polynomial coefficients, we can accurately reconstruct both the WIMP
mass and speed distribution. This new method is the first demonstration that an
accurate, unbiased reconstruction of the WIMP mass is possible without prior
assumptions about the distribution function. We anticipate that this technique
will be invaluable in the analysis of future experimental data.
