シェイク　リアサト Riasat Sheikh

Dark matter makes up about 26% of the energy density of the universe, yet everything we know about it comes from its gravity. My research asks what it is, what keeps it stable, how much of it the early universe left behind, and how it interacts with ordinary matter, however faintly. It could be a single particle or a whole family of hidden particles. Underground detectors have not yet seen a confirmed signal, which narrows the possibilities and points to signals these searches were not designed to look for.

Much of my work uses symmetry to build models of dark matter. In one family of models, called pseudo-Nambu-Goldstone boson (pNGB) dark matter, symmetry makes dark matter bump into ordinary atoms only very rarely, which would explain why detectors have missed it, while still letting the early universe end up with the right amount of it. I have also proposed a reason this protection holds, based on a recently studied kind of symmetry rule. Some of these models contain more than one kind of dark particle, and in some, part of the dark matter ends up moving much faster than the rest. I have studied whether large neutrino detectors could see this fast-moving part. Other models have a slightly heavier version of the dark matter particle that can survive from the early universe until today. When this heavier version hits an atom in a detector, it can drop to the lighter state and give the atom an extra kick, which could explain an unusually energetic event recently reported by the underground LZ experiment.

I also work on less conventional ways to measure dark matter. Even without electric charge, dark matter could still respond weakly to electric and magnetic fields. Its collisions would leave microscopic damage tracks in minerals, and rocks keep these tracks for up to a billion years, so paleo-detection in a small sample can record far more collisions than a laboratory detector running for a few years. On the cosmological side, I study where the cosmic magnetic fields seen in galaxies, and possibly in the near-empty space between them, come from. One possibility I am exploring is that an oscillating field from a hidden force strengthened them.