Gravitational Decays of Secluded Scalars and Graviton Dark Radiation

TL;DR

This paper investigates how secluded scalar fields decaying into gravitons can produce dark radiation, affecting the universe's thermal history, with a focus on the role of non-minimal gravity couplings and the resulting gravitational-wave signals.

Contribution

It analyzes the decay channels of a secluded scalar into gravitons and Standard Model particles, highlighting the suppression of graviton production via non-minimal couplings and predicting gravitational-wave spectra.

Findings

Decays into SM are dominated by Higgs and gluon channels.

Large Higgs non-minimal coupling suppresses graviton dark radiation.

Dark glueball domination leads to a specific gravitational-wave spectrum.

Abstract

We discuss graviton dark radiation produced by the decay of a secluded scalar field that couples to the Standard Model (SM) only through gravity. Such scalar fields are long-lived, and their decay channels generically include gravitons. If such particles existed and dominated the early universe, a sizable branching ratio into gravitons would yield non-negligible dark radiation that significantly alters the subsequent thermal history of the universe. In this work, we focus on the dark glueball as a representative secluded hidden scalar and compare the decay rates into SM particles via a non-minimal coupling to gravity with those into gravitons, paying attention to how the breaking of conformal invariance affects the amount of graviton dark radiation. We find that decays into the SM are dominated by two-body decay channels into Higgs bosons and gluons. In particular, when the Higgs field has a large non-minimal coupling to gravity, the production of graviton dark radiation is naturally suppressed in the metric formalism, and the SM sector is preferentially reheated and energy transfer to other hidden sectors is suppressed. Finally, we present the expected gravitational-wave spectrum resulting from dark glueball domination.