Constraints on superheavy dark matter decaying into $h\hat{I}½$, $Z\hat{I}½$ and $W\ell$ -- Benchmark example within an extended seesaw framework

Abstract

Dark matter particles could be superheavy (mass $M_X\gtrsim 10^9$GeV) provided that their lifetime $\ddot{I}{„}_X$ is extremely long, i.e. greater than $\simeq 10^{22}$yr. Such stringent constraints on $\ddot{I}{„}_X$ 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 $h\hat{I}½$ and gauge-induced $Z\hat{I}½/W\ell$ 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 $\hat{I}´M$, which must satisfy approximately $\hat{I}´M\lesssim 2\times 10^{-17}/[M_X/(10^9\mathrm{GeV}){]}^{0.5}$~GeV for $M_X\gtrsim 10^9$~GeV. Some implications in the context of inflationary cosmologies are discussed.

9 pages, 3 figures, submitted to EPJ C