High-precision search for dark photon dark matter with the Parkes Pulsar
  Timing Array
  The nature of dark matter remains obscure in spite of decades of experimental
efforts. The mass of dark matter candidates can span a wide range, and its
coupling with the Standard Model sector remains uncertain. All these unknowns
make the etection of dark matter extremely challenging. Ultralight dark matter,
with $m \sim10^{-22}$ eV, is proposed to reconcile the disagreements between
observations and predictions from simulations of small-scale structures in the
cold dark matter paradigm, while remaining consistent with other observations.
Because of its large de Broglie wavelength and large local occupation number
within galaxies, ultralight dark matter behaves like a coherently oscillating
background field with an oscillating frequency dependent on its mass. If the
dark matter particle is a spin-1 dark photon, such as the $U(1)_B$ or
$U(1)_{B-L}$ gauge boson, it can induce an external oscillating force and lead
to displacements of test masses. Such an effect would be observable in the form
of periodic variations in the arrival times of radio pulses from highly stable
millisecond pulsars. In this study, we search for evidence of ultralight dark
photon dark matter (DPDM) using 14-year high-precision observations of 26
pulsars collected with the Parkes Pulsar Timing Array. While no statistically
significant signal is found, we place constraints on coupling constants for the
$U(1)_B$ and $U(1)_{B-L}$ DPDM. Compared with other experiments, the limits on
the dimensionless coupling constant $\epsilon$ achieved in our study are
improved by up to two orders of magnitude when the dark photon mass is smaller
than $3\times10^{-22}$~eV ($10^{-22}$~eV) for the $U(1)_{B}$ ($U(1)_{B-L}$)
scenario.
