Constraints on superheavy dark matter decaying into h, Z and W

Abstract Dark matter particles could be superheavy (mass MX 10⁹~ M X ≳ 10 9 GeV) provided that their lifetime X τ X is extremely long, i. e. greater than 10^22~ ≃ 10 22 yr. Such stringent constraints on X τ X are generally obtained by limiting the prompt emission of ultrahigh energy (10⁹~ ≳ 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 MX 10^13~ M X ≳ 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 h ν and gauge-induced Z /W Z ν / W ℓ 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 M δ M, which must satisfy approximately M 2 10^-17/MX/ (10⁹~GeV) ^0. 5 δ M ≲ 2 × 10 - 17 / M X / (10 9 GeV) 0. 5 GeV for MX 10⁹ M X ≳ 10 9 GeV. Some implications in the context of inflationary cosmologies are discussed.