Speaker
Description
Collisions of ultra-relativistic bubble walls during a cosmological first-order phase transition can act as a “cosmic collider,” producing particles far heavier than the transition scale. Previous treatments model the process as the off-shell decay of the scalar background. This approach parametrically overestimates hard particle production and yields results that depend on the gauge and on the choice of field-space coordinates. We propose a gauge-invariant alternative, analogous to the partonic description of high-energy collisions: in the ultra-relativistic limit, the colliding walls pass through each other almost freely, and heavy particles arise from on-shell scatterings among the quanta that make up the Lorentz-contracted walls. We discuss how this revised picture of heavy scalar, fermion and vector production changes the predictions for dark matter, leptogenesis and gravitational waves.