Boosted Dark Matter
If dark matter is too light, many direct detection experiments cannot search for it, because it would carry too little energy. But dark matter that can scatter with a nucleus or an electron inside a detector can also scatter with cosmic rays, being upscattered to relativistic energies and becoming much easier to detect. In fact, such relativistic dark matter could even be seen in neutrino experiments, which are much larger than dark matter detectors but have energy thresholds that are typically much too high to detect dark matter. This opens up the possibility of using detectors such as Super-Kamiokande, DUNE, or even IceCube as enormous direct detection experiments, while also probing extremely low dark matter masses.
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Several other astrophysical processes can accomplish the same thing. For example, when a supernova explodes, it ejects stellar material at speeds that can approach 10% of the speed of light. If nuclei in this ejecta collide with dark matter, they can boost it up to comparable speeds, a factor of 10-100 higher than typical dark matter velocities in the Milky Way. At the same time, the dense plasma inside a supernova can be hot enough to actually produce low-mass dark matter particles, which would escape from the supernova at relativistic or semi-relativistic speeds.
Cosmic rays colliding with dark matter can boost it to high energies.
Bounds on dark matter-electron interactions from cosmic ray boosted dark matter.
Astrophysical Signatures
Dark matter interactions with Standard Model particles can leave distinctive signatures in astrophysical observables. If cosmic rays scatter with dark matter, this would not only boost the dark matter to high energy, but also cause the cosmic rays to lose energy, altering the observed cosmic ray spectrum in calculable ways. Similarly, the production of dark matter in a supernova would cool the supernova, altering the observed flux of supernova neutrinos. In fact, the density of neutrinos inside a supernova is so large that this effect can be used to search for signs of dark matter-neutrino interactions, which are otherwise difficult to probe.​​
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I am also interested in how different models of dark matter can be distinguished, using their gamma ray or X-ray signals. Many dark matter models are predicted to produce X-rays or gamma rays, typically via annihilation or decay. These can be searched for with current instruments such as Fermi-LAT and INTEGRAL, or future missions such as COSI and AMEGO-X, and such indirect detection holds great promise for detecting dark matter. But these instruments may have the ability to not just detect dark matter, but also to identify it. Different dark matter models would produce signals with different spatial morphologies or energy spectra, and with sufficient energy resolution and observation time, it would be possible to distinguish these models from one another.
Parameter space for dark matter-neutrino interactions ruled out based on supernova cooling by dark matter production.
Dark Matter in the Earth
Most direct detection experiments are located deep underground, in order to shield them from cosmic rays and cosmic ray air showers, while still allowing weakly interacting dark matter particles to reach them. But if dark matter interacts too strongly, the Earth could block it from reaching these detectors as well, making most detectors effectively blind to the very thing they are searching for. I have worked extensively on modeling dark matter scattering and propagation in the Earth, developing both numerical and semi-analytic codes to determine detectors' sensitivity to strongly interacting dark matter. I have also explored how, for a class of model known as inelastic dark matter, scattering in the Earth can actually make dark matter easier to detect.
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When dark matter scatters in the Earth, it can lose enough energy to be gravitationally captured. If this happens, dark matter can build up in the Earth and begin to annihilate, producing anomalous heating in the Earth's core. I recently modeled the flow of this heat through the core, and showed that enough dark matter heating could actually melt portions of the inner core. This is a novel signature of dark matter interactions, which could be extended to other planets that are more efficient at capturing dark matter.
Velocity distribution of dark matter after scattering n times in the Earth.
Schematic diagram of dark matter heat flow in the Earth.