Physics Colloquium

Recent cosmological data have established that roughly one third of the energy budget in the present cosmic epoch is in material form. However, various observations indicate that the "visible" atomic matter of everyday life makes up only about one fifth of all matter in the universe, with the rest being of a kind that has feeble interactions with atoms and light, and hence referred to as "dark matter." While visible matter is mostly composed of protons and neutrons, i.e. baryons, dark matter is of an unknown origin. A number of theoretical ideas, invoked to address other unresolved mysteries in particle physics, can lead to dark matter and have motivated much of the experimental activity for its direct detection.
The signals are so far either inconclusive or subject to debate. A different approach to the question of dark matter can arise from the empirical observation that dark and visible matter have similar energy densities, hinting at a common origin for both. We will outline a recent model based on this point of view, where the notion of baryon number is enlarged to include all cosmic matter. In this scenario, dubbed "hylogenesis", the asymmetry in atomic matter is balanced by an equal and opposite asymmetry in dark matter, leaving a zero net baryon number in the universe. An important signature of this setup is the annihilation of ordinary matter by dark matter, via inelastic scattering processes. This "induced nucleon decay" can yield distinct signals in proton decay experiments, making them a novel avenue for direct detection of dark matter. We will briefly discuss this and other potential signals of hylogenesis.