Gravitational Wave Emission from a Cosmic String Loop, I: Global Case

Abstract: We study the simultaneous decay of global string loops into scalar particles (massless and massive modes) and gravitational waves (GWs). Using field theory simulations in flat space-time of isolated loops with initial length $\sim 80-1700$ times their core width, we determine the power emitted into scalar particles, $P_{\varphi}$, and GWs, $P_{\rm GW}$, and characterize the loop decay timescale as a function of its initial length, energy and angular momentum. We quantify infrared and ultraviolet lattice dependencies of our results. For all type of loops and initial conditions considered, GW emission is always suppressed compared to particles as $P_{\rm GW}/P_{\varphi} \approx \mathcal{O}(10)(v/m_\text{p})^2\ll 1$, where $v$ is the vacuum expectation value associated with string formation. These conclusions are robust for the length-to-width ratios considered, with no indication they should change if the ratio is increased. The results suggest that the GW background from a global string network, such as in dark matter axion scenarios, will be suppressed compared to previous expectations.