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

We initiate the study of `bubbletrons’, by which we mean ultra-high-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 $M_{DM}$ that bubbletrons can produce in a $U(1)$ gauge PT, finding $M_{DM}\sim {10}^5\mathrm{/}{10}^{11}\mathrm{/}{10}^{15}$ GeV for PT scales $v_{\phi }\sim {10}^{-2}\mathrm{/}{10}^3\mathrm{/}{10}^9$ GeV. Bubbletrons realise a novel link between ultra-high-energy phenomena and gravitational waves (GW) sourced at the PT, from nanohertz to megahertz frequencies.