Gravitational waves from flavoured SU(2) early-universe phase transitions

Abstract

Flavourful extensions of the Standard Model aimed at explaining its fermionic mass structure typically rely on symmetries, broken at high-energy scales far beyond the reach of foreseeable direct collider searches. We illustrate, using a $SU(2)$ flavour gauge group, that the breaking of these symmetries up to scales as high as $10^7$ GeV could generate a gravitational-wave signal potentially observable by future observatories. We use dimensional reduction techniques to obtain the finite-temperature effective potential and study the possible first-order phase transitions. We match these transitions to steady-state hydrodynamical solutions in order to determine the corresponding gravitational-wave spectra. We observe that order-one gauge couplings are always required for a first-order phase transition to occur. On the other hand, adding leptoquarks (as an example of particles that are typically present in a complete flavour theory) significantly extends the testable parameter space. We find excellent prospects at the Einstein Telescope for future gravitational-wave detection of flavoured $SU(2)$ early-universe phase transitions.