String scale interacting dark matter from π₁

We show that in a wide class of string derived models of particle physics, heavy string modes with masses around the GUT scale can serve as a viable dark matter candidate. These heavy string modes wind around specific cycles in the extra-dimensional space, closely related to the fundamental group $\pi_1$. As a consequence of a non-trivial $\pi_1$, there is an exact discrete symmetry that stabilizes such winding strings. The dark matter candidate couples to the Standard Model via gravity and via the exchange of heavy string states. We find that, for reasonable values of the string coupling, our dark matter candidate can be produced in sizable amounts via freeze-in. Our scheme applies to many string constructions, including Calabi-Yau compactifications, and can be tested against constraints from the CMB.