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Gravitational decays of secluded scalars and graviton dark radiation

Kazunori Nakayama1,2,*, Fuminobu Takahashi1,3,†, and Juntaro Wada1,4,‡

  • *Contact author: kazunori.nakayama.d3@tohoku.ac.jp
  • †Contact author: fumi@tohoku.ac.jp
  • ‡Contact author: juntaro.wada.e5@tohoku.ac.jp
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Phys. Rev. D 113, 056008 – Published 6 March, 2026

DOI: https://doi.org/10.1103/bqm7-479z

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 nonminimal 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 nonminimal 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.

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Corrections

23 September, 2026

Correction: The top panel of Fig. 5 contained an error and has been replaced.

Article Text

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Phys. Rev. D 113, 056008– Published 6 March, 2026
  • Received 10 December 2025
  • Accepted 13 February 2026
  • Corrected 23 September 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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