Gravitational Waves from Nnaturalness

TL;DR

This paper explores how future gravitational wave detectors could test the Nnaturalness solution to the electroweak hierarchy problem by detecting signals from phase transitions in a multiverse-like scenario with multiple Standard Model copies.

Contribution

It introduces the potential for gravitational wave observatories to probe Nnaturalness, especially through signals from first-order phase transitions in lighter quark sectors, expanding the experimental reach beyond traditional methods.

Findings

Gravitational wave signals from phase transitions could be detectable by upcoming observatories.

Certain parameter regions allow for energy transfer to sectors with light quarks, consistent with existing bounds.

Multiple future experiments can explore new parts of the Nnaturalness parameter space.

Abstract

We study the prospects for probing the Nnaturalness solution to the electroweak hierarchy problem with future gravitational wave observatories. Nnaturalness, in its simplest incarnation, predicts $N$ copies of the Standard Model with varying Higgs mass parameters. We show that in certain parameter regions the scalar reheaton transfers a substantial energy density to the sector with the smallest positive Higgs squared mass while remaining consistent with bounds on additional effective relativistic species. In this sector, all six quarks are much lighter than the corresponding QCD confinement scale, allowing for the possibility of a first-order chiral symmetry-breaking phase transition and an associated stochastic gravitational wave signal. We consider several scenarios characterizing the strongly-coupled phase transition dynamics and estimate the gravitational wave spectrum for each. Pulsar timing arrays (SKA), spaced-based interferometers (BBO, Ultimate-DECIGO, $\mu$Ares, asteroid ranging), and astrometric measurements (THEIA) all have the potential to explore new regions of Nnaturalness parameter space, complementing probes from next generation cosmic microwave background radiation experiments.