Establishing the Non-Primordial Origin of Black Hole-Neutron Star
  Mergers
  Primordial black holes (PBHs) from the early Universe constitute an
attractive dark matter candidate. First detections of black hole-neutron star
(BH-NS) candidate gravitational wave events by the LIGO/Virgo collaboration,
GW200105 and GW200115, already prompted speculations about non-astrophysical
origin. We analyze, for the first time, the total volumetric merger rates of
PBH-NS binaries, finding them to be subdominant to the astrophysical BH-NS
rates. In contrast to binary black holes, a significant fraction of which can
be of primordial origin, either formed in dark matter halos or in the early
Universe, PBH-NS rates cannot be significantly enhanced by contributions
preceding star formation. Our findings imply that the identified BH-NS events
are of astrophysical origin, even when PBH-PBH events significantly contribute
to the GW observations.
