Eur. Phys. J. C (2025) 85: 985
https://doi.org/10.1140/epjc/s10052-025-14736-3

Regular Article - Theoretical Physics

Constraints on superheavy dark matter decaying into $h\nu$, $Z\nu$ and $W\ell$

Benchmark example within an extended seesaw framework

Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJCLab) CNRS/IN2P3, Université Paris-Saclay, Orsay, France

^a deligny@ijclab.in2p3.fr

Received: 11 June 2025
Accepted: 3 September 2025
Published online: 14 September 2025

Abstract

Dark matter particles could be superheavy (mass $M_X\gtrsim {10}^9$GeV) provided that their lifetime ${\tau }_X$ is extremely long, i.e. greater than $\simeq {10}^{22}$yr. Such stringent constraints on ${\tau }_X$ are generally obtained by limiting the prompt emission of ultrahigh energy ($\gtrsim {10}^9$GeV) gamma rays and neutrinos from the decay processes to below the corresponding flux upper bounds. In this paper, we show that even more severe bounds can be obtained for $M_X\gtrsim {10}^{13}$GeV from the synchrotron radiation of electron decay byproducts in the Galaxy. We illustrate the power of these constraints using generic Higgs-induced $h\nu$ and gauge-induced $Z\nu /W\ell$ decay channels, motivated by particle-physics setups invoking right-handed neutrinos. As a concrete benchmark, we consider a superheavy dark-matter candidate within an extended type-I seesaw framework and show that the lower bounds on lifetime can be translated into upper bounds on a mass-mixing parameter $\delta M$, which must satisfy approximately $\delta M\lesssim 2\times {10}^{-17}/{[M_X/({10}^9\text{GeV})]}^{0.5}$ GeV for $M_X\gtrsim {10}^9$ GeV. Some implications in the context of inflationary cosmologies are discussed.