The Gravitational Wave Background as a Particle Detector

by Angus Spalding

Thursday 4 Jun 2026, 14:00 → 15:00 Europe/London

I will show that spectral features of a gravitational-wave background can directly determine the Lagrangian parameters of BSM 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, imprinting 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. 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. I will illustrate this framework for several well-motivated BSM particles, including right-handed neutrinos, gravitinos, moduli fields, and axion-like particles.