With high spatial resolution, polarimetric imaging of a supermassive black hole, like $\mathrm{M}{87}^{\star }$ or Sgr ${\mathrm{A}}^{\star }$, by the Event Horizon Telescope can be used to probe the existence of ultralight bosonic particles, such as axions. Such particles can accumulate around a rotating black hole through the superradiance mechanism, forming an axion cloud. When linearly polarized photons are emitted from an accretion disk near the horizon, their position angles oscillate due to the birefringent effect when traveling through the axion background. In particular, the observations of supermassive black holes $\mathrm{M}{87}^{\star }$ (Sgr ${\mathrm{A}}^{\star }$) can probe the dimensionless axion-photon coupling $c=2\pi g_{a\gamma }{f}_a$ for axions with mass around $O({10}^{-20})\mathrm{eV}$ [$O({10}^{-17})\mathrm{eV}$] and decay constant $f_a<O({10}^{16})\mathrm{GeV}$, which is complimentary to other axion measurements.

We consider the use of quantum-limited mechanical force sensors to detect ultralight (sub-meV) dark matter (DM) candidates which are weakly coupled to the standard model. We show that mechanical sensors with masses around or below the milligram scale, operating around the standard quantum limit, would enable novel searches for DM with natural frequencies around the kHz scale. This would complement existing strategies based on torsion balances, atom interferometers, and atomic clock systems.

We propose boosted dark matter (BDM) as a possible explanation for the excess of keV electron recoil events observed by XENON1T. BDM particles have velocities much larger than those typical of virialized dark matter, and, as such, BDM-electron scattering can naturally produce keV electron recoils. We show that the required BDM-electron scattering cross sections can be easily realized in a simple model with a heavy vector mediator. Though these cross sections are too large for BDM to escape from the Sun, the BDM flux can originate from the Galactic Center or from halo dark matter annihilations. Furthermore, a daily modulation of the BDM signal will be present, which could not only be used to differentiate it from various backgrounds but would also provide important directional information for the BDM flux.

QCD axions or axionlike particles are among the most popular candidates for cold dark matter (DM) in the Universe. We proposed to detect axionlike DM, using linearly polarized pulsar light as a probe. Because of birefringence effect potentially caused by an oscillating galactic axion DM background, when pulsar light travels across the Galaxy, its linear polarization angle may vary with time. With a $\mathrm{soliton}+\mathrm{NFW}$ galactic DM halo profile, we show that this strategy can potentially probe an axion-photon coupling as small as $\sim {10}^{-13}{\mathrm{GeV}}^{-1}$ for axion mass $m_a\sim {10}^{-22}–{10}^{-20}\mathrm{eV}$, given the current measurement accuracy. An exclusion limit stronger than CAST ($\sim {10}^{-10}{\mathrm{GeV}}^{-1}$) and SN1987A ($\sim {10}^{-11}{\mathrm{GeV}}^{-1}$) could be extended up to $m_a\sim {10}^{-18}\mathrm{eV}$ and $\sim {10}^{-19}\mathrm{eV}$, respectively.

Boosted dark matter (BDM) is a well-motivated class of dark matter (DM) candidates in which a small component of DM is relativistic at the present time. We lay the foundation for BDM searches via hadronic interactions in large liquid-argon time-projection chambers (LArTPCs), such as the Deep Underground Neutrino Experiment (DUNE). We investigate BDM-nucleus scattering in detail by developing new event-generation techniques with a parameterized detector simulation. We study the discovery potential in a DUNE-like experiment using the low threshold and directionality of hadron detection in LArTPCs and compare with other experiments.