Microlensing constraints on primordial black holes with the Subaru/HSC
  Andromeda observation
  Primordial black holes (PBHs) have long been suggested as a viable candidate
for the elusive dark matter (DM). The abundance of such PBHs has been
constrained using a number of astrophysical observations, except for a hitherto
unexplored mass window of $M_{\rm PBH}=[10^{-14},10^{-9}]M_\odot$. Here we
carry out a dense-cadence (2~min sampling rate), 7 hour-long observation of the
Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam to search for
microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way
(MW) and M31. Given our simultaneous monitoring of tens of millions of stars in
M31, if such light PBHs make up a significant fraction of DM, we expect to find
many microlensing events for the PBH DM scenario. However, we identify only a
single candidate event, which translates into the most stringent upper bounds
on the abundance of PBHs in the mass range $M_{\rm PBH}\simeq [10^{-11},
10^{-6}]M_\odot$.
