  Primordial black holes (PBHs) have long been suggested as a candidate for
making up some or all of the dark matter in the Universe. Most of the
theoretically possible mass range for PBH dark matter has been ruled out with
various null observations of expected signatures of their interaction with
standard astrophysical objects. However, current constraints are significantly
less robust in the 20 M_sun < M_PBH < 100 M_sun mass window, which has received
much attention recently, following the detection of merging black holes with
estimated masses of ~30 M_sun by LIGO and the suggestion that these could be
black holes formed in the early Universe. We consider the potential of advanced
LIGO (aLIGO) operating at design sensitivity to probe this mass range by
looking for peaks in the mass spectrum of detected events. To quantify the
background, which is due to black holes that are formed from dying stars, we
model the shape of the stellar-black-hole mass function and calibrate its
amplitude to match the O1 results. Adopting very conservative assumptions about
the PBH and stellar-black-hole merger rates, we show that ~5 years of aLIGO
data can be used to detect a contribution of >20 M_sun PBHs to dark matter down
to f_PBH<0.5 at >99.9% confidence level. Combined with other probes that
already suggest tension with f_PBH=1, the obtainable independent limits from
aLIGO will thus enable a firm test of the scenario that PBHs make up all of
dark matter.
