Constraints on the cosmic string loop collapse fraction from Primordial Black Holes

Abstract: A small fraction, $f$, of cosmic string loops can collapse to form Primordial Black Holes (PBHs). Constraints on the abundance of PBHs can therefore be used to constrain $f$. We update these calculations, taking into account the PBH extended mass function, and find $f < 10^{-31} (G \mu/ c^2)^{-3/2}$. This is roughly two orders of magnitude tighter than previous constraints. The improvement from the tighter constraints on the abundance of PBHs is partly off-set by refinements to the theoretical calculation of the cosmic string loop formation rate.