The best evidence we have for dark matter is from large-scale gravitational and cosmological measurements. Combining these with precise measurements of the movement of stars near the sun, we infer that there must be dark matter right here in the vicinity of the Earth! Not a lot of it: the mass equivalent of one hydrogen atom per 3 cubic centimeters, which is slightly less dense than the near-vacuum of space. But it raises the exciting possibility of direct detection of dark matter: observing astrophysical dark matter using measurements taken on Earth.

One standard technique is to bury a giant tank of stuff (for example, silicon, germanium, or liquid xenon) deep underground and wait for a dark matter particle to bump into one of the constituent atoms. For dark matter heavier than a GeV, scattering off atomic nuclei provides the best signal, but for MeV-scale dark matter, the nucleus doesn’t recoil enough to deposit significant energy (think of a ping-pong ball hitting a bowling ball). For this lighter dark matter, atomic electrons are a better target. A clever choice of target atom or molecule, informed by chemistry, can allow dark matter to excite electronic transitions of energy as low as a few eV, resulting in a scintillation photon or an ionized electron which can be picked up by a suitable detector. In a fun, interdisciplinary collaboration involving chemists at MIT, materials scientists at Northwestern, condensed matter physicists at UIUC, and dark matter experimentalists at UChicago and Fermilab, I am developing a prototype detector based on organic scintillator crystals which could be sensitive to the daily modulation of the dark matter distribution as the Earth rotates over the course of a day.

For even lighter dark matter, my colleagues and I showed that we can use exotic materials known as Dirac semimetals as targets. These are materials where the electrons conspire to behave like relativistic electrons in empty space, but moving a thousand times slower. As long as the electrons in these materials are moving slower than the dark matter, they can be excited by scattering with an incoming dark matter particle as light as a few keV, or through absorption of dark matter as light as a few eV. This opens up a wide range of possibilities to detect dark matter which is very difficult to see with any other detection mechanism. In fact, the particular behavior of the electrons is not as important as the fact that the band gap (the energy cost to excite an electron in the material) is small. Along with colleagues in condensed matter here at UIUC and at Los Alamos National Laboratory, and detector experts at SLAC, I am currently investigating other narrow-gap semiconductors as target materials and developing a prototype of this experimental design, which could achieve the first sensitivity to DM lighter than the electron.

All of these detection methods for dark matter-electron scattering rely on the observation that electrons are not free particles: rather, they’re bound to atoms. Thinking more broadly, this means that the dark matter scattering rate depends on the details of the condensed matter system (usually a solid or liquid) in which the electrons are embedded. An understanding of condensed matter physics is therefore crucial, because many-body effects can drastically change the expected rate compared to modeling the electrons with single-particle wavefunctions. One example is the plasmon, a collective oscillation of electrons in semiconductors which may explain some tantalizing unexplained excesses in low-threshold direct detection experiments. Another example is the Migdal effect, where an electron can be ejected from an atom even if the dark matter particle only interacts with the nucleus. Even “ordinary” nuclear scattering has rich phenomenology in solid-state detectors: the recoiling nucleus may not even behave as a free particle at energies below the keV scale because of strong Coulomb interactions with the surrounding electrons. I’m actively investigating all of these phenomena — there is still much we don’t know about the behavior of existing dark matter detectors at low energies!