Direct detection of cosmic walls with paleodetectors

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi mathvariant="script">O</a:mi> <a:mo stretchy="false">(</a:mo> <a:mn>1</a:mn> <a:mo stretchy="false">)</a:mo> </a:math> 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 ultrarelativistic cosmic walls by noting the induced cosmic rays.