Direct Detection of Cosmic Walls with Paleo Detectors

Paleo detectors are emerging dark matter detection technology that exploits ancient minerals as passive, time-integrated detectors. Unlike conventional real-time experiments, they search for permanent damage tracks-typically tens of nanometers to micrometers long-left in crystal lattices by rare particle interactions, most notably dark matter induced nuclear recoils accumulated over millions to billions of years. In this paper I propose a direct detection strategy for cosmic walls-either bubble walls produced by a late-time first-order phase transition or domain walls in a scaling regime-using paleo detectors as the target medium. Because the cosmic wall is expected to traverse Earth at most $\mathcal{O}(1)$ time(s) in cosmic history, an ancient, continuously exposed detector is the only feasible way to observe it directly. By calculating the target recoils, I find that the smoking-gun signature would be parallel damage tracks found worldwide in minerals older than the wall-crossing epoch. I derive the limit on the wall-target coupling by assuming that a wall passed through the Earth within the last 0.5 Gyr. I also mention a novel indirect detection of ultra-relativistic cosmic walls by noting the induced cosmic rays.