Seminarium "Teoria cząstek elementarnych i kosmologia"

I will show that spectral features of a gravitational-wave background can directly determine Lagrangian parameters of beyond-the-Standard-Model particles, independently of the gravitational-wave production mechanism. Long-lived particles generically induce a temporary period of early matter domination in the thermal history of the Universe, which imprints two characteristic frequencies in any primordial gravitational-wave background corresponding to the onset and end of this epoch. These frequencies are determined by the initial abundance, mass, and decay rate of the dominating species. I will show that once the underlying model is specified the observed spectral features directly determine the particle mass and decay rate, allowing gravitational-wave observations to probe the Lagrangian parameters of particle physics models. As illustrative examples, I will consider long-lived right-handed neutrinos, gravitinos, and ALPs to demonstrate the versatility and applicability of these results.