Ultrahigh-Energy Particle Collisions and Heavy Dark Matter at Phase Transitions

We initiate the study of "bubbletrons,"by which we mean ultrahigh-energy collisions of the particle shells that generically form at the walls of relativistic bubbles in cosmological first-order phase transitions (PT). As an application, we calculate the maximal dark matter mass MDM that bubbletrons can produce in a U(1) gauge PT, finding MDM∼105/1011/1015 GeV for PT scales vφ∼10-2/103/109 GeV. Bubbletrons realize a novel link between ultrahigh-energy phenomena and gravitational waves sourced at the PT, from nanohertz to megahertz frequencies.