  We discover that the mass of dark matter particles mDM is imprinted in
phase-correlations of the cosmic density field more significantly than in the
2-point correlation. In particular, phase-correlations trace mDM out to scales
about five times larger than the 2-point correlation. This result relies on a
new estimator l(r) of pure phase-information in Fourier space, which can be
interpreted as a parameter-free and scale-invariant tracer of filament-like
structure. Based on simulated density fields we show how mDM can, in principle,
be measured using l(r), given a suitably reconstructed density field.
