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

TL;DR

This paper explores how high-energy flavour symmetry breaking in the early universe could produce gravitational waves detectable by future observatories, linking particle physics models to observable cosmological signals.

Contribution

It demonstrates that flavoured SU(2) symmetry breaking at high scales can generate observable gravitational waves, expanding the potential for testing flavour theories through cosmological signals.

Findings

First-order phase transitions require order-one gauge couplings.

Adding leptoquarks broadens the detectable parameter space.

Future detectors like the Einstein Telescope can observe these signals.

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.