A window for cosmic strings

Abstract

Particle emission, in addition to gravitational radiation from cosmic string loops, affects the resulting loop distribution and hence the corresponding observational consequences of cosmic strings. Here we focus on two models in which loops of length $\ell$ are produced from the infinite string network with a given power-law. For both models we find that, due to particle production, the Stochastic Gravitational Wave Background (SGWB) is cut off outside the region of parameter space probed by any current or planned GW experiment. Therefore the present constraints from the LIGO-Virgo-Kagra (LVK) collaboration still hold. However for one of these models, if a fraction $\gtrsim O(10^{-3})$ of these particles cascades into $\gamma$-rays, and if the gravitational backreaction scale follows the Polchinski-Rocha model, then the string tension is tightly constrained from below by measurements of the Diffuse $\gamma$-Ray Background, and from above by the SGWB. With reasonable assumptions, the joint constraint on the string tension set by these two possible observables reduces the available parameter space of this cosmic string model to a narrow band. Future upgrades to LVK will either rule out this model or detect strings.