Strange Quark Stars as Probe of Dark Matter
  We demonstrate that the observation of old strange quark stars (SQSs) can set
important limits on the scattering cross sections $\sigma_q$ between the light
quarks and the non-interacting scalar dark matter (DM). By analyzing a set of
1403 of solitary pulsarlike compact stars in the Milky Way, we find the old
solitary pulsar PSR J1801-0857D can set the most stringent upper limits on
$\sigma_q$ or the DM-proton scattering cross sections $\sigma_p$. By converting
$\sigma_q$ into $\sigma_p$ based on effective operator analyses, we show the
resulting $\sigma_p$ limit by assuming PSR J1801-0857D to be a SQS could be
comparable with that of the current direct detection experiments but much
weaker (by several orders of magnitude) than that obtained by assuming PSR
J1801-0857D to be a neutron star (NS), which requires an extremely small
$\sigma_p$ far beyond the limits of direct detection experiments. Our findings
imply that the old pulsars are favored to be SQSs rather than NSs if the scalar
DM were observed by future terrestrial experiments.
