$i$-incidental $N$-naturalness

TL;DR

This paper explores a generalized version of $N$-naturalness where the Standard Model can be a heavier sector, using resonant mixing to explain reheating and consistent with cosmological constraints, with potential gravitational wave signals.

Contribution

It introduces a broader class of $N$-naturalness models where the Standard Model is not necessarily the lightest sector, enabled by resonant mixing with the reheaton.

Findings

Resonant mixing causes preferential reheating of a specific sector.

The scenario aligns with current cosmological bounds.

Potential gravitational wave signals from multiple QCD phase transitions.

Abstract

$N$-naturalness is a novel solution to the electroweak hierarchy problem which posits $N$ copies of the Standard Model with varying Higgs mass-squared parameters. Reheating proceeds through a "reheaton" particle that deposits most of its energy density into the Standard Model and small but potentially measurable fractions into the other copies. Typically the sector with the lightest negative Higgs mass-squared is identified as the Standard Model. We demonstrate that $N$-naturalness admits a broader class of realizations in which the Standard Model is identified with a heavier sector, rather than being restricted to the lightest. This is made possible by resonant mixing between the reheaton and the Higgs, which generically causes one sector to be preferentially reheated and to acquire the largest share of the energy density, singling it out as the Standard Model. We demonstrate that this scenario is consistent with current cosmological bounds on new relativistic degrees of freedom and overclosure constraints from heavy stable relics, while future cosmic microwave background and high redshift surveys will probe significant portions of the remaining parameter space. Furthermore, we highlight the possibility of a novel stochastic gravitational wave spectrum from the many cosmological first order QCD phase transitions occurring across the other sectors.