Gravitational waves from cosmic strings in Froggatt-Nielsen flavour models

TL;DR

This paper explores how future gravitational wave experiments can test the origin of the Standard Model's flavour sector by detecting signals from cosmic strings predicted by Froggatt-Nielsen models, complementing low-energy flavour physics bounds.

Contribution

It demonstrates that gravitational wave observations can probe high-energy scales of flavour symmetry breaking, providing a novel way to test Froggatt-Nielsen models beyond current experimental limits.

Findings

GW experiments can set upper limits on symmetry-breaking scales up to 10^9 GeV.

Flavour physics and GW bounds are complementary in constraining model parameters.

Combined constraints can potentially exclude the entire parameter space of certain models.

Abstract

Gravitational waves (GW) are a powerful probe of the earliest moments in the Universe, enabling us to test fundamental interactions at energy scales beyond the reach of laboratory experiments. In this work, we assess the GW capability to probe the origin of the flavour sector of the Standard Model (SM). Within the context of Froggatt-Nielsen models of fermion masses and mixing based on a gauged $U(1)$ flavour symmetry, we investigate the formation of cosmic strings and the resulting stochastic GW background (GWB), estimating the sensitivity to the model's parameter space of future GW experiments. Comparing these results with the bounds from low-energy flavour observables, we find that these two types of experimental probes of the model are nicely complementary. Flavour physics observables can probe low to intermediate symmetry-breaking scales $v_\phi$, while future GW experiments are sensitive to the opposite regime, for which the string tension is large enough to yield sizeable GW signals, and in the long run can set an upper limit on the scale as stringent as $v_\phi \lesssim 10^9$ GeV. In certain scenarios, the combination of flavour constraints and future GW bounds can bring about a complete closure of the available parameter space, which illustrates how GWB searches can play an important role in testing the origin of the SM flavour sector even if that occurs at ultra-high energies.